CVE-2026-97944: x86/cfi: Fix FineIBT hash offset in cfi_get_func_hash()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

x86/cfi: Fix FineIBT hash offset in cfi_get_func_hash()

The switch of the FineIBT preamble from "subl $hash, %r10d" to the
shorter "subl $hash, %eax" moved the hash immediate from offset 7 to
offset 5 of the preamble. fineibt_preamble_hash was updated to match,
but the open-coded offset in cfi_get_func_hash() was missed and it
still reads the hash at offset 7.

cfi_get_func_hash() is used by the BPF JIT to give a struct_ops
trampoline the CFI hash of the stub function it stands in for. With
FineIBT the trampoline now gets the upper half of the real hash
followed by the first two bytes of the next instruction, so the first
indirect call from the kernel into a struct_ops program,
tcp_init_congestion_control() calling ->init() of a BPF congestion
control for example, fails the FineIBT check and the kernel dies with
a CFI failure.

Move the FineIBT preamble template and its offset defines above
cfi_get_func_hash() and use fineibt_preamble_hash there, so every
reader of the preamble shares one definition of its layout. The
CFI_FINEIBT arm is only built with CONFIG_FINEIBT, the only
configuration in which cfi_mode can take that value.
cfi_get_func_arity() does not need the same treatment: the __bhi_args
call whose displacement it reads still ends at the function address.

The Linux kernel CVE team has assigned CVE-2026-97944 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.18 with commit 85a2d4a890dce3cfc9c14aa91afc3dd7af8e3bf5 and fixed in 6.18.53 with commit cd9b4595d6283f9b8a35d7ae607508de45a351ce
	Issue introduced in 6.18 with commit 85a2d4a890dce3cfc9c14aa91afc3dd7af8e3bf5 and fixed in 7.2.7 with commit 67cf94c30a8b5ebc839f282f5c5a522b35903524
	Issue introduced in 6.18 with commit 85a2d4a890dce3cfc9c14aa91afc3dd7af8e3bf5 and fixed in 7.3-rc4 with commit 5a5d26f2cfe13467166219f6bf58099326912ddb

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-97944
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/x86/kernel/alternative.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/cd9b4595d6283f9b8a35d7ae607508de45a351ce
	https://git.kernel.org/stable/c/67cf94c30a8b5ebc839f282f5c5a522b35903524
	https://git.kernel.org/stable/c/5a5d26f2cfe13467166219f6bf58099326912ddb