[syzbot] [net?] [can?] KMSAN: uninit-value in em_canid_match

[syzbot] [net?] [can?] KMSAN: uninit-value in em_canid_match

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    ac3fd01e4c1e Linux 6.18-rc7
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1703e612580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=61a9bf3cc5d17a01
dashboard link: https://syzkaller.appspot.com/bug?extid=5d8269a1e099279152bc
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12040e58580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=117d797c580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/227434a45737/disk-ac3fd01e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d8117003dbb5/vmlinux-ac3fd01e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a13125fb7a7d/bzImage-ac3fd01e.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5d8269a1e099279152bc@syzkaller.appspotmail.com

syzkaller0: entered promiscuous mode
syzkaller0: entered allmulticast mode
=====================================================
BUG: KMSAN: uninit-value in em_canid_match+0x2f0/0x360 net/sched/em_canid.c:104
 em_canid_match+0x2f0/0x360 net/sched/em_canid.c:104
 tcf_em_match net/sched/ematch.c:494 [inline]
 __tcf_em_tree_match+0x215/0xc70 net/sched/ematch.c:520
 tcf_em_tree_match include/net/pkt_cls.h:512 [inline]
 basic_classify+0x154/0x480 net/sched/cls_basic.c:50
 tc_classify include/net/tc_wrapper.h:197 [inline]
 __tcf_classify net/sched/cls_api.c:1764 [inline]
 tcf_classify+0x855/0x1cb0 net/sched/cls_api.c:1860
 multiq_classify net/sched/sch_multiq.c:39 [inline]
 multiq_enqueue+0x82/0x590 net/sched/sch_multiq.c:66
 dev_qdisc_enqueue net/core/dev.c:4118 [inline]
 __dev_xmit_skb net/core/dev.c:4214 [inline]
 __dev_queue_xmit+0x1d91/0x5e60 net/core/dev.c:4729
 dev_queue_xmit include/linux/netdevice.h:3365 [inline]
 packet_xmit+0x8f/0x710 net/packet/af_packet.c:275
 packet_snd net/packet/af_packet.c:3076 [inline]
 packet_sendmsg+0x9173/0xa2a0 net/packet/af_packet.c:3108
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg+0x333/0x3d0 net/socket.c:742
 ____sys_sendmsg+0x7e0/0xd80 net/socket.c:2630
 ___sys_sendmsg+0x271/0x3b0 net/socket.c:2684
 __sys_sendmsg net/socket.c:2716 [inline]
 __do_sys_sendmsg net/socket.c:2721 [inline]
 __se_sys_sendmsg net/socket.c:2719 [inline]
 __x64_sys_sendmsg+0x211/0x3e0 net/socket.c:2719
 x64_sys_call+0x1dfd/0x3e30 arch/x86/include/generated/asm/syscalls_64.h:47
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd9/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
 slab_post_alloc_hook mm/slub.c:4985 [inline]
 slab_alloc_node mm/slub.c:5288 [inline]
 kmem_cache_alloc_node_noprof+0x989/0x16b0 mm/slub.c:5340
 kmalloc_reserve+0x13c/0x4b0 net/core/skbuff.c:579
 __alloc_skb+0x347/0x7d0 net/core/skbuff.c:670
 alloc_skb include/linux/skbuff.h:1383 [inline]
 alloc_skb_with_frags+0xc5/0xa60 net/core/skbuff.c:6671
 sock_alloc_send_pskb+0xacc/0xc60 net/core/sock.c:2965
 packet_alloc_skb net/packet/af_packet.c:2926 [inline]
 packet_snd net/packet/af_packet.c:3019 [inline]
 packet_sendmsg+0x743d/0xa2a0 net/packet/af_packet.c:3108
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg+0x333/0x3d0 net/socket.c:742
 ____sys_sendmsg+0x7e0/0xd80 net/socket.c:2630
 ___sys_sendmsg+0x271/0x3b0 net/socket.c:2684
 __sys_sendmsg net/socket.c:2716 [inline]
 __do_sys_sendmsg net/socket.c:2721 [inline]
 __se_sys_sendmsg net/socket.c:2719 [inline]
 __x64_sys_sendmsg+0x211/0x3e0 net/socket.c:2719
 x64_sys_call+0x1dfd/0x3e30 arch/x86/include/generated/asm/syscalls_64.h:47
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd9/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 0 UID: 0 PID: 6067 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(none) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
=====================================================


---
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Re: [syzbot] [net?] [can?] KMSAN: uninit-value in em_canid_match

[shaurya]

[-- Attachment #1: Type: text/plain, Size: 83 bytes --]

#syz test:
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

[-- Attachment #2: 0001-net-sched-em_canid-add-length-check-before-reading-C.patch --]
[-- Type: text/x-patch, Size: 1229 bytes --]

From 137086f25c8aa52e4222355345e4218e410188c5 Mon Sep 17 00:00:00 2001
From: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
Date: Wed, 26 Nov 2025 02:06:51 +0530
Subject: [PATCH] net/sched: em_canid: add length check before reading CAN ID

Add a check to verify that the skb has at least sizeof(canid_t) bytes
before reading the CAN ID from skb->data. This prevents reading
uninitialized memory when processing malformed packets that don't
contain a valid CAN frame.

Reported-by: syzbot+5d8269a1e099279152bc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5d8269a1e099279152bc
Fixes: f057bbb6f9ed ("net: em_canid: Ematch rule to match CAN frames according to their identifiers")
Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
---
 net/sched/em_canid.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sched/em_canid.c b/net/sched/em_canid.c
index 5337bc462755..a9b6cab70ff1 100644
--- a/net/sched/em_canid.c
+++ b/net/sched/em_canid.c
@@ -99,6 +99,9 @@ static int em_canid_match(struct sk_buff *skb, struct tcf_ematch *m,
 	int i;
 	const struct can_filter *lp;
 
+	if (skb->len < sizeof(canid_t))
+		return 0;
+
 	can_id = em_canid_get_id(skb);
 
 	if (can_id & CAN_EFF_FLAG) {
-- 
2.34.1

Re: [syzbot] [net?] [can?] KMSAN: uninit-value in em_canid_match

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+5d8269a1e099279152bc@syzkaller.appspotmail.com
Tested-by: syzbot+5d8269a1e099279152bc@syzkaller.appspotmail.com

Tested on:

commit:         30f09200 Merge tag 'arm64-fixes' of git://git.kernel.o..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13236e92580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=61a9bf3cc5d17a01
dashboard link: https://syzkaller.appspot.com/bug?extid=5d8269a1e099279152bc
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1230257c580000

Note: testing is done by a robot and is best-effort only.