linux-cve-announce - CVE-2024-56702: bpf: Mark raw_tp arguments with PTR_MAYBE

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Message-ID: <2024122837-CVE-2024-56702-172d@gregkh>
Date: Sat, 28 Dec 2024 10:45:54 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-56702: bpf: Mark raw_tp arguments with PTR_MAYBE_NULL

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

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

bpf: Mark raw_tp arguments with PTR_MAYBE_NULL

Arguments to a raw tracepoint are tagged as trusted, which carries the
semantics that the pointer will be non-NULL.  However, in certain cases,
a raw tracepoint argument may end up being NULL. More context about this
issue is available in [0].

Thus, there is a discrepancy between the reality, that raw_tp arguments
can actually be NULL, and the verifier's knowledge, that they are never
NULL, causing explicit NULL checks to be deleted, and accesses to such
pointers potentially crashing the kernel.

To fix this, mark raw_tp arguments as PTR_MAYBE_NULL, and then special
case the dereference and pointer arithmetic to permit it, and allow
passing them into helpers/kfuncs; these exceptions are made for raw_tp
programs only. Ensure that we don't do this when ref_obj_id > 0, as in
that case this is an acquired object and doesn't need such adjustment.

The reason we do mask_raw_tp_trusted_reg logic is because other will
recheck in places whether the register is a trusted_reg, and then
consider our register as untrusted when detecting the presence of the
PTR_MAYBE_NULL flag.

To allow safe dereference, we enable PROBE_MEM marking when we see loads
into trusted pointers with PTR_MAYBE_NULL.

While trusted raw_tp arguments can also be passed into helpers or kfuncs
where such broken assumption may cause issues, a future patch set will
tackle their case separately, as PTR_TO_BTF_ID (without PTR_TRUSTED) can
already be passed into helpers and causes similar problems. Thus, they
are left alone for now.

It is possible that these checks also permit passing non-raw_tp args
that are trusted PTR_TO_BTF_ID with null marking. In such a case,
allowing dereference when pointer is NULL expands allowed behavior, so
won't regress existing programs, and the case of passing these into
helpers is the same as above and will be dealt with later.

Also update the failure case in tp_btf_nullable selftest to capture the
new behavior, as the verifier will no longer cause an error when
directly dereference a raw tracepoint argument marked as __nullable.

  [0]: https://lore.kernel.org/bpf/ZrCZS6nisraEqehw@jlelli-thinkpadt14gen4.remote.csb

The Linux kernel CVE team has assigned CVE-2024-56702 to this issue.

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

	Issue introduced in 6.2 with commit 3f00c52393445ed49aadc1a567aa502c6333b1a1 and fixed in 6.11.11 with commit c9b91d2d54175f781ad2c361cb2ac2c0e29b14b6
	Issue introduced in 6.2 with commit 3f00c52393445ed49aadc1a567aa502c6333b1a1 and fixed in 6.12.2 with commit 3634d4a310820567fc634bf8f1ee2b91378773e8
	Issue introduced in 6.2 with commit 3f00c52393445ed49aadc1a567aa502c6333b1a1 and fixed in 6.13-rc1 with commit cb4158ce8ec8a5bb528cc1693356a5eb8058094d

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-2024-56702
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:
	include/linux/bpf.h
	kernel/bpf/btf.c
	kernel/bpf/verifier.c
	tools/testing/selftests/bpf/progs/test_tp_btf_nullable.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/c9b91d2d54175f781ad2c361cb2ac2c0e29b14b6
	https://git.kernel.org/stable/c/3634d4a310820567fc634bf8f1ee2b91378773e8
	https://git.kernel.org/stable/c/cb4158ce8ec8a5bb528cc1693356a5eb8058094d