linux-cve-announce - CVE-2025-21922: ppp: Fix KMSAN uninit-value warning with bpf

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Message-ID: <2025040132-CVE-2025-21922-8c36@gregkh>
Date: Tue,  1 Apr 2025 16:39:46 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2025-21922: ppp: Fix KMSAN uninit-value warning with bpf

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

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

ppp: Fix KMSAN uninit-value warning with bpf

Syzbot caught an "KMSAN: uninit-value" warning [1], which is caused by the
ppp driver not initializing a 2-byte header when using socket filter.

The following code can generate a PPP filter BPF program:
'''
struct bpf_program fp;
pcap_t *handle;
handle = pcap_open_dead(DLT_PPP_PPPD, 65535);
pcap_compile(handle, &fp, "ip and outbound", 0, 0);
bpf_dump(&fp, 1);
'''
Its output is:
'''
(000) ldh [2]
(001) jeq #0x21 jt 2 jf 5
(002) ldb [0]
(003) jeq #0x1 jt 4 jf 5
(004) ret #65535
(005) ret #0
'''
Wen can find similar code at the following link:
https://github.com/ppp-project/ppp/blob/master/pppd/options.c#L1680
The maintainer of this code repository is also the original maintainer
of the ppp driver.

As you can see the BPF program skips 2 bytes of data and then reads the
'Protocol' field to determine if it's an IP packet. Then it read the first
byte of the first 2 bytes to determine the direction.

The issue is that only the first byte indicating direction is initialized
in current ppp driver code while the second byte is not initialized.

For normal BPF programs generated by libpcap, uninitialized data won't be
used, so it's not a problem. However, for carefully crafted BPF programs,
such as those generated by syzkaller [2], which start reading from offset
0, the uninitialized data will be used and caught by KMSAN.

[1] https://syzkaller.appspot.com/bug?extid=853242d9c9917165d791
[2] https://syzkaller.appspot.com/text?tag=ReproC&x=11994913980000

The Linux kernel CVE team has assigned CVE-2025-21922 to this issue.


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

	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 5.4.291 with commit d685096c8129c9a92689975193e268945fd21dbf
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 5.10.235 with commit 2f591cb158807bdcf424f66f1fbfa6e4e50f3757
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 5.15.179 with commit 4e2191b0fd0c064d37b0db67396216f2d4787e0f
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.1.131 with commit 3de809a768464528762757e433cd50de35bcb3c1
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.6.83 with commit 1eacd47636a9de5bee25d9d5962dc538a82d9f0b
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.12.19 with commit 8aa8a40c766b3945b40565a70349d5581458ff63
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.13.7 with commit c036f5f2680cbdabdbbace86baee3c83721634d6
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.14 with commit 4c2d14c40a68678d885eab4008a0129646805bae

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-2025-21922
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:
	drivers/net/ppp/ppp_generic.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/d685096c8129c9a92689975193e268945fd21dbf
	https://git.kernel.org/stable/c/2f591cb158807bdcf424f66f1fbfa6e4e50f3757
	https://git.kernel.org/stable/c/4e2191b0fd0c064d37b0db67396216f2d4787e0f
	https://git.kernel.org/stable/c/3de809a768464528762757e433cd50de35bcb3c1
	https://git.kernel.org/stable/c/1eacd47636a9de5bee25d9d5962dc538a82d9f0b
	https://git.kernel.org/stable/c/8aa8a40c766b3945b40565a70349d5581458ff63
	https://git.kernel.org/stable/c/c036f5f2680cbdabdbbace86baee3c83721634d6
	https://git.kernel.org/stable/c/4c2d14c40a68678d885eab4008a0129646805bae