CVE-2026-98641: bpf, arm64: set up the frame pointer for the exception callback

CVE-2026-98641: bpf, arm64: set up the frame pointer for the exception callback

[Greg Kroah-Hartman]

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

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

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

bpf, arm64: set up the frame pointer for the exception callback

A program acting as exception boundary saves all callee-saved registers,
so build_prologue() takes the exception_cb path and never calls
push_callee_regs(). That is the only place find_used_callee_regs() runs,
and with it the only place ctx->fp_used is set, so the callback prologue
does not emit the

  mov x25, sp

that points BPF_REG_FP at the frame the callback runs on. x25 keeps
whatever it held when bpf_throw() was called. If the throw came from a
subprogram that uses its own BPF stack, that is the subprogram's frame
pointer, and since the subprogram never returns it never restores x25
either.

Stack accesses through BPF_REG_FP are rewritten to be stack pointer
relative, so those still land in the callback's own frame. Materializing
the register does not: a callback that passes the address of a local
variable to a helper hands over an address in the dead subprogram's
frame. That address is below the callback's stack pointer by then, and
the helper's own call chain covers it, so the helper can write over its
own return address. 0x1234 below is the value the helper was asked to
store:

  pc : 0x1234
  lr : 0x1234
  Call trace:
   0x1234 (P)
   bpf_test_run+0x188/0x3e0
   bpf_prog_test_run_skb+0x47c/0x998
   __sys_bpf+0xbdc/0xdd8
  Kernel panic - not syncing: Oops: Fatal exception in interrupt

Set ctx->fp_used on the exception callback path so that the existing code
further down sets x25 from the stack pointer. The epilogue restores it
from the main program's save area along with the other callee-saved
registers, as it already does. x86 sets the frame pointer for the
callback from the argument it is passed, and powerpc computes it from
the stack pointer.

The Linux kernel CVE team has assigned CVE-2026-98641 to this issue.


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

	Issue introduced in 6.12 with commit 5d4fa9ec5643a5c75d3c1e6abf50fb9284caf1ff and fixed in 6.12.112 with commit ecdd770a9fc3eaf6c3127c5991312b84774a7b0f
	Issue introduced in 6.12 with commit 5d4fa9ec5643a5c75d3c1e6abf50fb9284caf1ff and fixed in 6.18.55 with commit b67b4c763f211e7a6391c356fef552e2ea63f08a
	Issue introduced in 6.12 with commit 5d4fa9ec5643a5c75d3c1e6abf50fb9284caf1ff and fixed in 7.2.9 with commit d7cd5ebe13989f265015910faa25e5d50faa4c22
	Issue introduced in 6.12 with commit 5d4fa9ec5643a5c75d3c1e6abf50fb9284caf1ff and fixed in 7.3-rc5 with commit ef1fb82f12186dd26153b14d9fbcf4ec98db81b3

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-2026-98641
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/arm64/net/bpf_jit_comp.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/ecdd770a9fc3eaf6c3127c5991312b84774a7b0f
	https://git.kernel.org/stable/c/b67b4c763f211e7a6391c356fef552e2ea63f08a
	https://git.kernel.org/stable/c/d7cd5ebe13989f265015910faa25e5d50faa4c22
	https://git.kernel.org/stable/c/ef1fb82f12186dd26153b14d9fbcf4ec98db81b3