Vulnerability CVE-2024-26885
Published: 2024-04-17

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

bpf: Fix DEVMAP_HASH overflow check on 32-bit arches

The devmap code allocates a number hash buckets equal to the next power
of two of the max_entries value provided when creating the map. When
rounding up to the next power of two, the 32-bit variable storing the
number of buckets can overflow, and the code checks for overflow by
checking if the truncated 32-bit value is equal to 0. However, on 32-bit
arches the rounding up itself can overflow mid-way through, because it
ends up doing a left-shift of 32 bits on an unsigned long value. If the
size of an unsigned long is four bytes, this is undefined behaviour, so
there is no guarantee that we'll end up with a nice and tidy 0-value at
the end.

Syzbot managed to turn this into a crash on arm32 by creating a
DEVMAP_HASH with max_entries > 0x80000000 and then trying to update it.
Fix this by moving the overflow check to before the rounding up
operation.

 References:
https://git.kernel.org/stable/c/225da02acdc97af01b6bc6ce1a3e5362bf01d3fb
https://git.kernel.org/stable/c/c826502bed93970f2fd488918a7b8d5f1d30e2e3
https://git.kernel.org/stable/c/edf7990baa48de5097daa9ac02e06cb4c798a737
https://git.kernel.org/stable/c/250051acc21f9d4c5c595e4fcb55986ea08c4691
https://git.kernel.org/stable/c/22079b3a423382335f47d9ed32114e6c9fe88d7c
https://git.kernel.org/stable/c/e89386f62ce9a9ab9a94835a9890883c23d9d52c
https://git.kernel.org/stable/c/281d464a34f540de166cee74b723e97ac2515ec3
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