CVE-2021-46921 locking/qrwlock: Fix ordering in queued_write_lock_slowpath()

2024-02-2709:36:26

Linux

github.com

linux kernel

vulnerability

fix

locking

queued_write_lock_slowpath

a-b-a problem

atomic_cond_read

cmpxchg

acquire semantics

relaxed semantics

AI Score

6.8

Confidence

Low

SSVC

Exploitation

none

Automatable

Technical Impact

partial

JSON

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

locking/qrwlock: Fix ordering in queued_write_lock_slowpath()

While this code is executed with the wait_lock held, a reader can
acquire the lock without holding wait_lock. The writer side loops
checking the value with the atomic_cond_read_acquire(), but only truly
acquires the lock when the compare-and-exchange is completed
successfully which isn’t ordered. This exposes the window between the
acquire and the cmpxchg to an A-B-A problem which allows reads
following the lock acquisition to observe values speculatively before
the write lock is truly acquired.

We’ve seen a problem in epoll where the reader does a xchg while
holding the read lock, but the writer can see a value change out from
under it.

Writer | Reader

A core can order the read of the ovflist ahead of the
atomic_cmpxchg_relaxed(). Switching the cmpxchg to use acquire
semantics addresses this issue at which point the atomic_cond_read can
be switched to use relaxed semantics.

[peterz: use try_cmpxchg()]

CNA Affected

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    "vendor": "Linux",
    "product": "Linux",
    "versions": [
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        "status": "affected",
        "version": "b519b56e378e",
        "lessThan": "5902f9453a31",
        "versionType": "git"
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    ],
    "programFiles": [
      "kernel/locking/qrwlock.c"
    ],
    "defaultStatus": "unaffected"
  },
  {
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "vendor": "Linux",
    "product": "Linux",
    "versions": [
      {
        "status": "affected",
        "version": "4.15"
      },
      {
        "status": "unaffected",
        "version": "0",
        "lessThan": "4.15",
        "versionType": "custom"
      },
      {
        "status": "unaffected",
        "version": "4.19.189",
        "versionType": "custom",
        "lessThanOrEqual": "4.19.*"
      },
      {
        "status": "unaffected",
        "version": "5.4.115",
        "versionType": "custom",
        "lessThanOrEqual": "5.4.*"
      },
      {
        "status": "unaffected",
        "version": "5.10.33",
        "versionType": "custom",
        "lessThanOrEqual": "5.10.*"
      },
      {
        "status": "unaffected",
        "version": "5.11.17",
        "versionType": "custom",
        "lessThanOrEqual": "5.11.*"
      },
      {
        "status": "unaffected",
        "version": "5.12",
        "versionType": "original_commit_for_fix",
        "lessThanOrEqual": "*"
      }
    ],
    "programFiles": [
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]