CVE-2021-46921 locking/qrwlock: Fix ordering in queued_write_lock_slowpath()
A race condition in the Linux kernel's qrwlock implementation allows readers to observe stale data, potentially causing system instability or information disclosure.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2021-46921?
CVE-2021-46921 is a high-severity vulnerability in Linux kernel, published on 27 Feb 2024. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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()]
Weakness: CWE-668
Which versions are affected by CVE-2021-46921?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2021-46921?
Apply the vendor's patch.
Is CVE-2021-46921 being exploited?
There is no known exploitation of CVE-2021-46921 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as DEBIAN-CVE-2021-46921, GHSA-8j25-5vwv-hm2f, UBUNTU-CVE-2021-46921.