CVE-2021-47566 proc/vmcore: fix clearing user buffer by properly using clear_user()

2024-05-2415:12:53

Linux

www.cve.org

linux kernel

vulnerability resolution

proc/vmcore

clear_user()

virtio-mem device

vmcore_cb

linux memory block

bug

page fault

supervisor write access

smap

x86-64 cpus

clac()+stac()

user buffer

6.3 Medium

AI Score

Confidence

Low

0.0004 Low

EPSS

Percentile

13.0%

JSON

In the Linux kernel, the following vulnerability has been resolved:

proc/vmcore: fix clearing user buffer by properly using clear_user()

To clear a user buffer we cannot simply use memset, we have to use
clear_user(). With a virtio-mem device that registers a vmcore_cb and
has some logically unplugged memory inside an added Linux memory block,
I can easily trigger a BUG by copying the vmcore via “cp”:

systemd[1]: Starting Kdump Vmcore Save Service…
kdump[420]: Kdump is using the default log level(3).
kdump[453]: saving to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
kdump[458]: saving vmcore-dmesg.txt to /sysroot/var/crash/127.0.0.1-2021-11-11-14:59:22/
kdump[465]: saving vmcore-dmesg.txt complete
kdump[467]: saving vmcore
BUG: unable to handle page fault for address: 00007f2374e01000
#PF: supervisor write access in kernel mode
#PF: error_code(0x0003) - permissions violation
PGD 7a523067 P4D 7a523067 PUD 7a528067 PMD 7a525067 PTE 800000007048f867
Oops: 0003 [#1] PREEMPT SMP NOPTI
CPU: 0 PID: 468 Comm: cp Not tainted 5.15.0+ #6
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.14.0-27-g64f37cc530f1-prebuilt.qemu.org 04/01/2014
RIP: 0010:read_from_oldmem.part.0.cold+0x1d/0x86
Code: ff ff ff e8 05 ff fe ff e9 b9 e9 7f ff 48 89 de 48 c7 c7 38 3b 60 82 e8 f1 fe fe ff 83 fd 08 72 3c 49 8d 7d 08 4c 89 e9 89 e8 <49> c7 45 00 00 00 00 00 49 c7 44 05 f8 00 00 00 00 48 83 e7 f81
RSP: 0018:ffffc9000073be08 EFLAGS: 00010212
RAX: 0000000000001000 RBX: 00000000002fd000 RCX: 00007f2374e01000
RDX: 0000000000000001 RSI: 00000000ffffdfff RDI: 00007f2374e01008
RBP: 0000000000001000 R08: 0000000000000000 R09: ffffc9000073bc50
R10: ffffc9000073bc48 R11: ffffffff829461a8 R12: 000000000000f000
R13: 00007f2374e01000 R14: 0000000000000000 R15: ffff88807bd421e8
FS: 00007f2374e12140(0000) GS:ffff88807f000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f2374e01000 CR3: 000000007a4aa000 CR4: 0000000000350eb0
Call Trace:
read_vmcore+0x236/0x2c0
proc_reg_read+0x55/0xa0
vfs_read+0x95/0x190
ksys_read+0x4f/0xc0
do_syscall_64+0x3b/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xae

Some x86-64 CPUs have a CPU feature called “Supervisor Mode Access
Prevention (SMAP)”, which is used to detect wrong access from the kernel
to user buffers like this: SMAP triggers a permissions violation on
wrong access. In the x86-64 variant of clear_user(), SMAP is properly
handled via clac()+stac().

To fix, properly use clear_user() when we’re dealing with a user buffer.

CNA Affected

[
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "unaffected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "fs/proc/vmcore.c"
    ],
    "versions": [
      {
        "version": "997c136f518c",
        "lessThan": "a9e164bd160b",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "997c136f518c",
        "lessThan": "33a7d698f30f",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "997c136f518c",
        "lessThan": "99d348b82bcb",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "997c136f518c",
        "lessThan": "9ef384ed300d",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "997c136f518c",
        "lessThan": "fd7974c547ab",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "997c136f518c",
        "lessThan": "a8a917058faf",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "997c136f518c",
        "lessThan": "7b3a34f08d11",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "997c136f518c",
        "lessThan": "c1e631177119",
        "status": "affected",
        "versionType": "git"
      }
    ]
  },
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "affected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "fs/proc/vmcore.c"
    ],
    "versions": [
      {
        "version": "3.0",
        "status": "affected"
      },
      {
        "version": "0",
        "lessThan": "3.0",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "4.4.294",
        "lessThanOrEqual": "4.4.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "4.9.292",
        "lessThanOrEqual": "4.9.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "4.14.257",
        "lessThanOrEqual": "4.14.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "4.19.219",
        "lessThanOrEqual": "4.19.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.4.163",
        "lessThanOrEqual": "5.4.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.10.83",
        "lessThanOrEqual": "5.10.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.15.6",
        "lessThanOrEqual": "5.15.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.16",
        "lessThanOrEqual": "*",
        "status": "unaffected",
        "versionType": "original_commit_for_fix"
      }
    ]
  }
]