linux-cve-announce - CVE-2024-35917: s390/bpf: Fix bpf

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Message-ID: <2024051958-CVE-2024-35917-df0b@gregkh>
Date: Sun, 19 May 2024 10:35:31 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-35917: s390/bpf: Fix bpf_plt pointer arithmetic

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

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

s390/bpf: Fix bpf_plt pointer arithmetic

Kui-Feng Lee reported a crash on s390x triggered by the
dummy_st_ops/dummy_init_ptr_arg test [1]:

  [<0000000000000002>] 0x2
  [<00000000009d5cde>] bpf_struct_ops_test_run+0x156/0x250
  [<000000000033145a>] __sys_bpf+0xa1a/0xd00
  [<00000000003319dc>] __s390x_sys_bpf+0x44/0x50
  [<0000000000c4382c>] __do_syscall+0x244/0x300
  [<0000000000c59a40>] system_call+0x70/0x98

This is caused by GCC moving memcpy() after assignments in
bpf_jit_plt(), resulting in NULL pointers being written instead of
the return and the target addresses.

Looking at the GCC internals, the reordering is allowed because the
alias analysis thinks that the memcpy() destination and the assignments'
left-hand-sides are based on different objects: new_plt and
bpf_plt_ret/bpf_plt_target respectively, and therefore they cannot
alias.

This is in turn due to a violation of the C standard:

  When two pointers are subtracted, both shall point to elements of the
  same array object, or one past the last element of the array object
  ...

>>From the C's perspective, bpf_plt_ret and bpf_plt are distinct objects
and cannot be subtracted. In the practical terms, doing so confuses the
GCC's alias analysis.

The code was written this way in order to let the C side know a few
offsets defined in the assembly. While nice, this is by no means
necessary. Fix the noncompliance by hardcoding these offsets.

[1] https://lore.kernel.org/bpf/c9923c1d-971d-4022-8dc8-1364e929d34c@gmail.com/

The Linux kernel CVE team has assigned CVE-2024-35917 to this issue.

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

	Issue introduced in 6.3 with commit f1d5df84cd8c and fixed in 6.6.26 with commit c3062bdb859b
	Issue introduced in 6.3 with commit f1d5df84cd8c and fixed in 6.8.5 with commit d3d74e45a060
	Issue introduced in 6.3 with commit f1d5df84cd8c and fixed in 6.9 with commit 7ded842b356d

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-2024-35917
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/s390/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/c3062bdb859b6e2567e7f5c8cde20c0250bb130f
	https://git.kernel.org/stable/c/d3d74e45a060d218fe4b0c9174f0a77517509d8e
	https://git.kernel.org/stable/c/7ded842b356d151ece8ac4985940438e6d3998bb