cve-2021-46921
Vulnerability from cvelistv5
Published
2024-02-27 09:36
Modified
2024-12-19 07:31
Severity ?
EPSS score ?
Summary
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
--------------------------------------------------------------------------------
ep_scan_ready_list() |
|- write_lock_irq() |
|- queued_write_lock_slowpath() |
|- atomic_cond_read_acquire() |
| read_lock_irqsave(&ep->lock, flags);
--> (observes value before unlock) | chain_epi_lockless()
| | epi->next = xchg(&ep->ovflist, epi);
| | read_unlock_irqrestore(&ep->lock, flags);
| |
| atomic_cmpxchg_relaxed() |
|-- READ_ONCE(ep->ovflist); |
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()]
References
Impacted products
| Vendor | Product | Version | |||||
|---|---|---|---|---|---|---|---|
| ▼ | Linux | Linux |
Version: b519b56e378ee82caf9b079b04f5db87dedc3251 Version: b519b56e378ee82caf9b079b04f5db87dedc3251 Version: b519b56e378ee82caf9b079b04f5db87dedc3251 Version: b519b56e378ee82caf9b079b04f5db87dedc3251 Version: b519b56e378ee82caf9b079b04f5db87dedc3251 |
||||
|
|||||||
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"datePublished": "2024-02-27T09:36:26.461Z",
"dateReserved": "2024-02-25T13:45:52.719Z",
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}
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Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.