Vulnerability CVE-2021-46990

Vulnerability CVE-2021-46990

Published: 2024-02-28

Description:

In the Linux kernel, the following vulnerability has been resolved:

powerpc/64s: Fix crashes when toggling entry flush barrier

The entry flush mitigation can be enabled/disabled at runtime via a
debugfs file (entry_flush), which causes the kernel to patch itself to
enable/disable the relevant mitigations.

However depending on which mitigation we're using, it may not be safe to
do that patching while other CPUs are active. For example the following
crash:

sleeper[15639]: segfault (11) at c000000000004c20 nip c000000000004c20 lr c000000000004c20

Shows that we returned to userspace with a corrupted LR that points into
the kernel, due to executing the partially patched call to the fallback
entry flush (ie. we missed the LR restore).

Fix it by doing the patching under stop machine. The CPUs that aren't
doing the patching will be spinning in the core of the stop machine
logic. That is currently sufficient for our purposes, because none of
the patching we do is to that code or anywhere in the vicinity.

References:

https://git.kernel.org/stable/c/8382b15864e5014261b4f36c2aa89723612ee058

https://git.kernel.org/stable/c/0c25a7bb697f2e6ee65b6d63782f675bf129511a

https://git.kernel.org/stable/c/ee4b7aab93c2631c3bb0753023c5dda592bb666b

https://git.kernel.org/stable/c/2db22ba4e0e103f00e0512e0ecce36ac78c644f8

https://git.kernel.org/stable/c/0b4eb172cc12dc102cd0ad013e53ee4463db9508

https://git.kernel.org/stable/c/d2e3590ca39ccfd8a5a46d8c7f095cb6c7b9ae92

https://git.kernel.org/stable/c/dd0d6117052faace5440db20fc37175efe921c7d

https://git.kernel.org/stable/c/5bc00fdda1e934c557351a9c751a205293e68cbf

https://git.kernel.org/stable/c/aec86b052df6541cc97c5fca44e5934cbea4963b

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