[syzbot] [bridge?] KMSAN: uninit-value in br_dev_xmit (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: dccb07f2914c Merge tag 'for-6.9-rc7-tag' of git://git.kern..
git tree: upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=10b995a8980000
kernel config: https://syzkaller.appspot.com/x/.config?x=617171361dd3cd47
dashboard link: https://syzkaller.appspot.com/bug?extid=a63a1f6a062033cf0f40
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=122a37c0980000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=111a53c0980000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/fdbc7be30633/disk-dccb07f2.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a9e4c11aa835/vmlinux-dccb07f2.xz
kernel image: https://storage.googleapis.com/syzbot-assets/43c3a343ea93/bzImage-dccb07f2.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a63a1f...@syzkaller.appspotmail.com

=====================================================
BUG: KMSAN: uninit-value in br_dev_xmit+0x61d/0x1cb0 net/bridge/br_device.c:65
br_dev_xmit+0x61d/0x1cb0 net/bridge/br_device.c:65
__netdev_start_xmit include/linux/netdevice.h:4903 [inline]
netdev_start_xmit include/linux/netdevice.h:4917 [inline]
xmit_one net/core/dev.c:3531 [inline]
dev_hard_start_xmit+0x247/0xa20 net/core/dev.c:3547
__dev_queue_xmit+0x34db/0x5350 net/core/dev.c:4341
dev_queue_xmit include/linux/netdevice.h:3091 [inline]
__bpf_tx_skb net/core/filter.c:2136 [inline]
__bpf_redirect_common net/core/filter.c:2180 [inline]
__bpf_redirect+0x14a6/0x1620 net/core/filter.c:2187
____bpf_clone_redirect net/core/filter.c:2460 [inline]
bpf_clone_redirect+0x328/0x470 net/core/filter.c:2432
___bpf_prog_run+0x13fe/0xe0f0 kernel/bpf/core.c:1997
__bpf_prog_run512+0xb5/0xe0 kernel/bpf/core.c:2238
bpf_dispatcher_nop_func include/linux/bpf.h:1234 [inline]
__bpf_prog_run include/linux/filter.h:657 [inline]
bpf_prog_run include/linux/filter.h:664 [inline]
bpf_test_run+0x499/0xc30 net/bpf/test_run.c:425
bpf_prog_test_run_skb+0x14ea/0x1f20 net/bpf/test_run.c:1058
bpf_prog_test_run+0x6b7/0xad0 kernel/bpf/syscall.c:4269
__sys_bpf+0x6aa/0xd90 kernel/bpf/syscall.c:5678
__do_sys_bpf kernel/bpf/syscall.c:5767 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5765 [inline]
__x64_sys_bpf+0xa0/0xe0 kernel/bpf/syscall.c:5765
x64_sys_call+0x96b/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:322
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
slab_post_alloc_hook mm/slub.c:3810 [inline]
slab_alloc_node mm/slub.c:3851 [inline]
kmem_cache_alloc_node+0x622/0xc90 mm/slub.c:3894
kmalloc_reserve+0x13d/0x4a0 net/core/skbuff.c:577
pskb_expand_head+0x222/0x19d0 net/core/skbuff.c:2251
skb_ensure_writable+0x412/0x4a0 net/core/skbuff.c:6117
__bpf_try_make_writable net/core/filter.c:1665 [inline]
bpf_try_make_writable net/core/filter.c:1671 [inline]
bpf_try_make_head_writable net/core/filter.c:1679 [inline]
____bpf_clone_redirect net/core/filter.c:2454 [inline]
bpf_clone_redirect+0x17f/0x470 net/core/filter.c:2432
___bpf_prog_run+0x13fe/0xe0f0 kernel/bpf/core.c:1997
__bpf_prog_run512+0xb5/0xe0 kernel/bpf/core.c:2238
bpf_dispatcher_nop_func include/linux/bpf.h:1234 [inline]
__bpf_prog_run include/linux/filter.h:657 [inline]
bpf_prog_run include/linux/filter.h:664 [inline]
bpf_test_run+0x499/0xc30 net/bpf/test_run.c:425
bpf_prog_test_run_skb+0x14ea/0x1f20 net/bpf/test_run.c:1058
bpf_prog_test_run+0x6b7/0xad0 kernel/bpf/syscall.c:4269
__sys_bpf+0x6aa/0xd90 kernel/bpf/syscall.c:5678
__do_sys_bpf kernel/bpf/syscall.c:5767 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5765 [inline]
__x64_sys_bpf+0xa0/0xe0 kernel/bpf/syscall.c:5765
x64_sys_call+0x96b/0x3b50 arch/x86/include/generated/asm/syscalls_64.h:322
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcf/0x1e0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 0 PID: 5070 Comm: syz-executor183 Not tainted 6.9.0-rc7-syzkaller-00012-gdccb07f2914c #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
=====================================================


---
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[Edward Adam Davis]

please test uiv in br_dev_xmit

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git dccb07f2914c

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index b99127712e67..578590c83580 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2247,6 +2247,7 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
goto nodata;
size = SKB_WITH_OVERHEAD(size);

+ memset(data, 0, size);
/* Copy only real data... and, alas, header. This should be
* optimized for the cases when header is void.
*/

[syzbot]

Hello,

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

Reported-and-tested-by: syzbot+a63a1f...@syzkaller.appspotmail.com

Tested on:

commit: dccb07f2 Merge tag 'for-6.9-rc7-tag' of git://git.kern..
git tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=1617da5c980000

kernel config: https://syzkaller.appspot.com/x/.config?x=617171361dd3cd47
dashboard link: https://syzkaller.appspot.com/bug?extid=a63a1f6a062033cf0f40
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40

patch: https://syzkaller.appspot.com/x/patch.diff?x=1205acd0980000

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

[Nikolay Aleksandrov]

syzbot triggered an uninit value[1] error in bridge device's xmit path
by sending a short (less than ETH_HLEN bytes) skb. To fix it check if
we can actually pull that amount instead of assuming.

Tested with dropwatch:
drop at: br_dev_xmit+0xb93/0x12d0 [bridge] (0xffffffffc06739b3)
origin: software
timestamp: Mon May 13 11:31:53 2024 778214037 nsec
protocol: 0x88a8
length: 2
original length: 2
drop reason: PKT_TOO_SMALL

[1]

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+a63a1f...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=a63a1f6a062033cf0f40
Signed-off-by: Nikolay Aleksandrov <ra...@blackwall.org>
---
net/bridge/br_device.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
index c366ccc8b3db..ecac7886988b 100644
--- a/net/bridge/br_device.c
+++ b/net/bridge/br_device.c
@@ -27,6 +27,7 @@ EXPORT_SYMBOL_GPL(nf_br_ops);
/* net device transmit always called with BH disabled */
netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
{
+ enum skb_drop_reason reason = pskb_may_pull_reason(skb, ETH_HLEN);
struct net_bridge_mcast_port *pmctx_null = NULL;
struct net_bridge *br = netdev_priv(dev);
struct net_bridge_mcast *brmctx = &br->multicast_ctx;
@@ -38,6 +39,11 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
const unsigned char *dest;
u16 vid = 0;

+ if (unlikely(reason != SKB_NOT_DROPPED_YET)) {
+ kfree_skb_reason(skb, reason);
+ return NETDEV_TX_OK;
+ }
+
memset(skb->cb, 0, sizeof(struct br_input_skb_cb));
br_tc_skb_miss_set(skb, false);

--
2.44.0

[patchwork-b...@kernel.org]

Hello:

This patch was applied to netdev/net.git (main)
by David S. Miller <da...@davemloft.net>:

On Mon, 13 May 2024 13:34:19 +0300 you wrote:
> syzbot triggered an uninit value[1] error in bridge device's xmit path
> by sending a short (less than ETH_HLEN bytes) skb. To fix it check if
> we can actually pull that amount instead of assuming.
>
> Tested with dropwatch:
> drop at: br_dev_xmit+0xb93/0x12d0 [bridge] (0xffffffffc06739b3)
> origin: software
> timestamp: Mon May 13 11:31:53 2024 778214037 nsec
> protocol: 0x88a8
> length: 2
> original length: 2
> drop reason: PKT_TOO_SMALL
>

> [...]

Here is the summary with links:
- [net] net: bridge: xmit: make sure we have at least eth header len bytes
https://git.kernel.org/netdev/net/c/8bd67ebb50c0

You are awesome, thank you!
--
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