UBUNTU-CVE-2021-46921

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    UBUNTU-CVE-2021-46921

    Published: 27 Feb 2024Last Modified: 9 Sept 2026
    Upstream:
    CVE-2021-46921

    Summary:

    Details: 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: https://ubuntu.com/security/CVE-2021-46921, https://git.kernel.org/linus/84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896, https://git.kernel.org/stable/c/5902f9453a313be8fe78cbd7e7ca9dba9319fc6e, https://git.kernel.org/stable/c/82808cc026811fbc3ecf0c0b267a12a339eead56, https://git.kernel.org/stable/c/82fa9ced35d88581cffa4a1c856fc41fca96d80a, https://git.kernel.org/stable/c/d558fcdb17139728347bccc60a16af3e639649d2, https://git.kernel.org/stable/c/84a24bf8c52e66b7ac89ada5e3cfbe72d65c1896, https://www.cve.org/CVERecord?id=CVE-2021-46921

    Affected packages

    Package

    Name: linux-azure

    Purl: pkg:deb/ubuntu/linux-azure?arch=source&distro=trusty%2Fesm

    Affected ranges

    Type: ECOSYSTEM

    Events:

    Introduced- 0
    Fixed -4.15.0-1115.128~14.04.1

    Affected versions

    4.15.0-1023.24~14.04.1
    4.15.0-1030.31~14.04.1
    4.15.0-1031.32~14.04.1
    4.15.0-1032.33~14.04.2
    4.15.0-1035.36~14.04.2
    4.15.0-1036.38~14.04.2
    4.15.0-1037.39~14.04.2
    4.15.0-1039.41~14.04.2
    4.15.0-1040.44~14.04.1
    4.15.0-1041.45~14.04.1
    4.15.0-1042.46~14.04.1
    4.15.0-1045.49~14.04.1
    4.15.0-1046.50~14.04.1
    4.15.0-1047.51~14.04.1
    4.15.0-1049.54~14.04.1
    4.15.0-1050.55~14.04.1
    4.15.0-1051.56~14.04.1
    4.15.0-1052.57~14.04.1
    4.15.0-1055.60~14.04.1
    4.15.0-1056.61~14.04.1
    4.15.0-1057.62~14.04.1
    4.15.0-1059.64~14.04.1
    4.15.0-1060.65~14.04.1
    4.15.0-1061.66~14.04.1
    4.15.0-1063.68~14.04.1
    4.15.0-1064.69~14.04.1
    4.15.0-1066.71~14.04.1
    4.15.0-1067.72~14.04.1
    4.15.0-1069.74~14.04.1
    4.15.0-1071.76~14.04.1
    4.15.0-1074.79~14.04.1
    4.15.0-1077.82~14.04.1
    4.15.0-1082.92~14.04.1
    4.15.0-1083.93~14.04.1
    4.15.0-1089.99~14.04.1
    4.15.0-1091.101~14.04.1
    4.15.0-1092.102~14.04.1
    4.15.0-1093.103~14.04.1
    4.15.0-1095.105~14.04.1
    4.15.0-1096.106~14.04.1
    4.15.0-1098.109~14.04.1
    4.15.0-1100.111~14.04.1
    4.15.0-1102.113~14.04.1
    4.15.0-1103.114~14.04.1
    4.15.0-1106.118~14.04.1
    4.15.0-1108.120~14.04.1
    4.15.0-1109.121~14.04.1
    4.15.0-1110.122~14.04.1
    4.15.0-1111.123~14.04.1
    4.15.0-1112.124~14.04.1
    4.15.0-1113.126~14.04.1
    4.15.0-1114.127~14.04.1

    Common Vulnerability Scoring System

    Attack Vector
    Network
    Adjacent
    Local
    Physical
    Privileges Required
    None
    Low
    High
    User Interaction
    None
    Required
    Scope
    Unchanged
    Changed
    Confidentiality
    None
    Low
    High
    Integrity
    None
    Low
    High
    Availability
    None
    Low
    High
    UBUNTU-CVE-2021-46921 | CVE-DB