CVE-2021-46957

In the Linux kernel, the following vulnerability has been resolved:
riscv/kprobe: fix kernel panic when invoking sys_read traced by kprobe The
execution of sys_read end up hitting a BUG_ON() in __find_get_block after
installing kprobe at sys_read, the BUG message like the following: [
65.708663] ------------[ cut here ]------------ [ 65.709987] kernel BUG at
fs/buffer.c:1251! [ 65.711283] Kernel BUG [#1] [ 65.712032] Modules linked
in: [ 65.712925] CPU: 0 PID: 51 Comm: sh Not tainted 5.12.0-rc4 #1 [
65.714407] Hardware name: riscv-virtio,qemu (DT) [ 65.715696] epc :
__find_get_block+0x218/0x2c8 [ 65.716835] ra : __getblk_gfp+0x1c/0x4a [
65.717831] epc : ffffffe00019f11e ra : ffffffe00019f56a sp :
ffffffe002437930 [ 65.719553] gp : ffffffe000f06030 tp : ffffffe0015abc00
t0 : ffffffe00191e038 [ 65.721290] t1 : ffffffe00191e038 t2 :
000000000000000a s0 : ffffffe002437960 [ 65.723051] s1 : ffffffe00160ad00
a0 : ffffffe00160ad00 a1 : 000000000000012a [ 65.724772] a2 :
0000000000000400 a3 : 0000000000000008 a4 : 0000000000000040 [ 65.726545]
a5 : 0000000000000000 a6 : ffffffe00191e000 a7 : 0000000000000000 [
65.728308] s2 : 000000000000012a s3 : 0000000000000400 s4 :
0000000000000008 [ 65.730049] s5 : 000000000000006c s6 : ffffffe00240f800
s7 : ffffffe000f080a8 [ 65.731802] s8 : 0000000000000001 s9 :
000000000000012a s10: 0000000000000008 [ 65.733516] s11: 0000000000000008
t3 : 00000000000003ff t4 : 000000000000000f [ 65.734434] t5 :
00000000000003ff t6 : 0000000000040000 [ 65.734613] status:
0000000000000100 badaddr: 0000000000000000 cause: 0000000000000003 [
65.734901] Call Trace: [ 65.735076] [<ffffffe00019f11e>]
__find_get_block+0x218/0x2c8 [ 65.735417] [<ffffffe00020017a>]
__ext4_get_inode_loc+0xb2/0x2f6 [ 65.735618] [<ffffffe000201b6c>]
ext4_get_inode_loc+0x3a/0x8a [ 65.735802] [<ffffffe000203380>]
ext4_reserve_inode_write+0x2e/0x8c [ 65.735999] [<ffffffe00020357a>]
__ext4_mark_inode_dirty+0x4c/0x18e [ 65.736208] [<ffffffe000206bb0>]
ext4_dirty_inode+0x46/0x66 [ 65.736387] [<ffffffe000192914>]
__mark_inode_dirty+0x12c/0x3da [ 65.736576] [<ffffffe000180dd2>]
touch_atime+0x146/0x150 [ 65.736748] [<ffffffe00010d762>]
filemap_read+0x234/0x246 [ 65.736920] [<ffffffe00010d834>]
generic_file_read_iter+0xc0/0x114 [ 65.737114] [<ffffffe0001f5d7a>]
ext4_file_read_iter+0x42/0xea [ 65.737310] [<ffffffe000163f2c>]
new_sync_read+0xe2/0x15a [ 65.737483] [<ffffffe000165814>]
vfs_read+0xca/0xf2 [ 65.737641] [<ffffffe000165bae>] ksys_read+0x5e/0xc8 [
65.737816] [<ffffffe000165c26>] sys_read+0xe/0x16 [ 65.737973]
[<ffffffe000003972>] ret_from_syscall+0x0/0x2 [ 65.738858] —[ end trace
fe93f985456c935d ]— A simple reproducer looks like: echo ‘p:myprobe
sys_read fd=%a0 buf=%a1 count=%a2’ >
/sys/kernel/debug/tracing/kprobe_events echo 1 >
/sys/kernel/debug/tracing/events/kprobes/myprobe/enable cat
/sys/kernel/debug/tracing/trace Here’s what happens to hit that BUG_ON():

1. After installing kprobe at entry of sys_read, the first instruction is
   replaced by ‘ebreak’ instruction on riscv64 platform. 2) Once kernel reach
   the ‘ebreak’ instruction at the entry of sys_read, it trap into the riscv
   breakpoint handler, where it do something to setup for coming single-step
   of origin instruction, including backup the ‘sstatus’ in pt_regs, followed
   by disable interrupt during single stepping via clear ‘SIE’ bit of
   ‘sstatus’ in pt_regs. 3) Then kernel restore to the instruction slot
   contains two instructions, one is original instruction at entry of
   sys_read, the other is ‘ebreak’. Here it trigger a ‘Instruction page fault’
   exception (value at ‘scause’ is ‘0xc’), if PF is not filled into PageTabe
   for that slot yet. 4) Again kernel trap into page fault exception handler,
   where it choose different policy according to the state of running kprobe.
   Because afte 2) the state is KPROBE_HIT_SS, so kernel reset the current kp
   —truncated—

Notes