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_l…

Overview

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()]

Affected

• linux_kernel >= 4.15.0, < 4.19.189
• linux_kernel >= 4.20.0, < 5.4.115
• linux_kernel >= 5.5.0, < 5.10.33
• linux_kernel >= 5.11.0, < 5.11.17

Remediation

Upgrade past the affected range:

• linux_kernel 5.11.17