linux-cve-announce - CVE-2024-49888: bpf: Fix a sdiv overflow issue

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Message-ID: <2024102117-CVE-2024-49888-027c@gregkh>
Date: Mon, 21 Oct 2024 20:01:36 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-49888: bpf: Fix a sdiv overflow issue

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

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

bpf: Fix a sdiv overflow issue

Zac Ecob reported a problem where a bpf program may cause kernel crash due
to the following error:
  Oops: divide error: 0000 [#1] PREEMPT SMP KASAN PTI

The failure is due to the below signed divide:
  LLONG_MIN/-1 where LLONG_MIN equals to -9,223,372,036,854,775,808.
LLONG_MIN/-1 is supposed to give a positive number 9,223,372,036,854,775,808,
but it is impossible since for 64-bit system, the maximum positive
number is 9,223,372,036,854,775,807. On x86_64, LLONG_MIN/-1 will
cause a kernel exception. On arm64, the result for LLONG_MIN/-1 is
LLONG_MIN.

Further investigation found all the following sdiv/smod cases may trigger
an exception when bpf program is running on x86_64 platform:
  - LLONG_MIN/-1 for 64bit operation
  - INT_MIN/-1 for 32bit operation
  - LLONG_MIN%-1 for 64bit operation
  - INT_MIN%-1 for 32bit operation
where -1 can be an immediate or in a register.

On arm64, there are no exceptions:
  - LLONG_MIN/-1 = LLONG_MIN
  - INT_MIN/-1 = INT_MIN
  - LLONG_MIN%-1 = 0
  - INT_MIN%-1 = 0
where -1 can be an immediate or in a register.

Insn patching is needed to handle the above cases and the patched codes
produced results aligned with above arm64 result. The below are pseudo
codes to handle sdiv/smod exceptions including both divisor -1 and divisor 0
and the divisor is stored in a register.

sdiv:
      tmp = rX
      tmp += 1 /* [-1, 0] -> [0, 1]
      if tmp >(unsigned) 1 goto L2
      if tmp == 0 goto L1
      rY = 0
  L1:
      rY = -rY;
      goto L3
  L2:
      rY /= rX
  L3:

smod:
      tmp = rX
      tmp += 1 /* [-1, 0] -> [0, 1]
      if tmp >(unsigned) 1 goto L1
      if tmp == 1 (is64 ? goto L2 : goto L3)
      rY = 0;
      goto L2
  L1:
      rY %= rX
  L2:
      goto L4  // only when !is64
  L3:
      wY = wY  // only when !is64
  L4:

  [1] https://lore.kernel.org/bpf/tPJLTEh7S_DxFEqAI2Ji5MBSoZVg7_G-Py2iaZpAaWtM961fFTWtsnlzwvTbzBzaUzwQAoNATXKUlt0LZOFgnDcIyKCswAnAGdUF3LBrhGQ=@protonmail.com/

The Linux kernel CVE team has assigned CVE-2024-49888 to this issue.


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

	Fixed in 6.10.14 with commit 4902a6a0dc59
	Fixed in 6.11.3 with commit d22e45a369af
	Fixed in 6.12-rc1 with commit 7dd34d7b7dcf

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-2024-49888
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:
	kernel/bpf/verifier.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/4902a6a0dc593c82055fc8c9ada371bafe26c9cc
	https://git.kernel.org/stable/c/d22e45a369afc7c28f11acfa5b5e8e478227ca5d
	https://git.kernel.org/stable/c/7dd34d7b7dcf9309fc6224caf4dd5b35bedddcb7