Attacker Value
Unknown
(0 users assessed)
Exploitability
Unknown
(0 users assessed)
User Interaction
None
Privileges Required
Low
Attack Vector
Local
0

CVE-2021-46921

Disclosure Date: February 27, 2024 •
CVE-2021-46921 CVSS v3 Base Score: 5.5
Add MITRE ATT&CK tactics and techniques that apply to this CVE.

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()]

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CVSS V3 Severity and Metrics
Base Score:
5.5 Medium
Impact Score:
3.6
Exploitability Score:
1.8
Vector:
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
Attack Vector (AV):
Local
Attack Complexity (AC):
Low
Privileges Required (PR):
Low
User Interaction (UI):
None
Scope (S):
Unchanged
Confidentiality (C):
High
Integrity (I):
None
Availability (A):
None

General Information

Offensive Application
Unknown
Utility Class
Unknown
Ports
Unknown
OS
Unknown
Vulnerable Versions
Linux 5902f9453a31
Linux not down converted
Prerequisites
Unknown
Discovered By
Unknown
PoC Author
Unknown
Metasploit Module
Unknown
Reporter
Unknown

Vendors

  • linux

Products

  • linux kernel
Technical Analysis