[syzbot] [atm?] KMSAN: uninit-value in atmtcp_c_send

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 2c4a1f3fe03e Merge tag 'bpf-fixes' of git://git.kernel.org..
git tree: upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=1787d9d4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=42d51b7b9f9e61d
dashboard link: https://syzkaller.appspot.com/bug?extid=1d3c235276f62963e93a
compiler: Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1195ed70580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16187682580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/1313b3ad2bf4/disk-2c4a1f3f.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/15f719cfdf88/vmlinux-2c4a1f3f.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e7f531b0bef6/bzImage-2c4a1f3f.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1d3c23...@syzkaller.appspotmail.com

=====================================================
BUG: KMSAN: uninit-value in atmtcp_c_send+0x255/0xed0 drivers/atm/atmtcp.c:294
atmtcp_c_send+0x255/0xed0 drivers/atm/atmtcp.c:294
vcc_sendmsg+0xd7c/0xff0 net/atm/common.c:644
sock_sendmsg_nosec net/socket.c:712 [inline]
__sock_sendmsg+0x330/0x3d0 net/socket.c:727
____sys_sendmsg+0x7e0/0xd80 net/socket.c:2566
___sys_sendmsg+0x271/0x3b0 net/socket.c:2620
__sys_sendmsg net/socket.c:2652 [inline]
__do_sys_sendmsg net/socket.c:2657 [inline]
__se_sys_sendmsg net/socket.c:2655 [inline]
__x64_sys_sendmsg+0x211/0x3e0 net/socket.c:2655
x64_sys_call+0x32fb/0x3db0 arch/x86/include/generated/asm/syscalls_64.h:47
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd9/0x210 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:4154 [inline]
slab_alloc_node mm/slub.c:4197 [inline]
kmem_cache_alloc_node_noprof+0x818/0xf00 mm/slub.c:4249
kmalloc_reserve+0x13c/0x4b0 net/core/skbuff.c:579
__alloc_skb+0x347/0x7d0 net/core/skbuff.c:670
alloc_skb include/linux/skbuff.h:1336 [inline]
vcc_sendmsg+0xb40/0xff0 net/atm/common.c:628
sock_sendmsg_nosec net/socket.c:712 [inline]
__sock_sendmsg+0x330/0x3d0 net/socket.c:727
____sys_sendmsg+0x7e0/0xd80 net/socket.c:2566
___sys_sendmsg+0x271/0x3b0 net/socket.c:2620
__sys_sendmsg net/socket.c:2652 [inline]
__do_sys_sendmsg net/socket.c:2657 [inline]
__se_sys_sendmsg net/socket.c:2655 [inline]
__x64_sys_sendmsg+0x211/0x3e0 net/socket.c:2655
x64_sys_call+0x32fb/0x3db0 arch/x86/include/generated/asm/syscalls_64.h:47
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd9/0x210 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 1 UID: 0 PID: 5798 Comm: syz-executor192 Not tainted 6.16.0-rc1-syzkaller-00010-g2c4a1f3fe03e #0 PREEMPT(undef)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
=====================================================


---
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If the report is already addressed, let syzbot know by replying with:
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If you want syzbot to run the reproducer, reply with:
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If you want to undo deduplication, reply with:
#syz undup

[Kuniyuki Iwashima]

From: syzbot <syzbot+1d3c23...@syzkaller.appspotmail.com>
Date: Thu, 12 Jun 2025 16:41:26 -0700

#syz test

diff --git a/drivers/atm/atmtcp.c b/drivers/atm/atmtcp.c
index d4aa0f353b6c..c74dec9a2759 100644
--- a/drivers/atm/atmtcp.c
+++ b/drivers/atm/atmtcp.c
@@ -288,7 +288,11 @@ static int atmtcp_c_send(struct atm_vcc *vcc,struct sk_buff *skb)
struct sk_buff *new_skb;
int result = 0;

- if (!skb->len) return 0;
+ if (!skb->len)
+ return 0;
+ if (skb->len < sizeof(atmtcp_hdr))
+ return -EINVAL;
+
dev = vcc->dev_data;
hdr = (struct atmtcp_hdr *) skb->data;
if (hdr->length == ATMTCP_HDR_MAGIC) {

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

drivers/atm/atmtcp.c:293:24: error: use of undeclared identifier 'atmtcp_hdr'; did you mean 'atm_tcp_ops'?


Tested on:

commit: 27605c8c Merge tag 'net-6.16-rc2' of git://git.kernel...
git tree: upstream

kernel config: https://syzkaller.appspot.com/x/.config?x=42d51b7b9f9e61d
dashboard link: https://syzkaller.appspot.com/bug?extid=1d3c235276f62963e93a
compiler: Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6

patch: https://syzkaller.appspot.com/x/patch.diff?x=1183ed70580000

[Kuniyuki Iwashima]

From: syzbot <syzbot+1d3c23...@syzkaller.appspotmail.com>
Date: Thu, 12 Jun 2025 19:40:03 -0700

> Hello,
>
> syzbot tried to test the proposed patch but the build/boot failed:
>
> drivers/atm/atmtcp.c:293:24: error: use of undeclared identifier 'atmtcp_hdr'; did you mean 'atm_tcp_ops'?

Oops, I forgot struct :/

#syz test

diff --git a/drivers/atm/atmtcp.c b/drivers/atm/atmtcp.c

index d4aa0f353b6c..96c0969ce584 100644

--- a/drivers/atm/atmtcp.c
+++ b/drivers/atm/atmtcp.c
@@ -288,7 +288,11 @@ static int atmtcp_c_send(struct atm_vcc *vcc,struct sk_buff *skb)
struct sk_buff *new_skb;
int result = 0;

- if (!skb->len) return 0;
+ if (!skb->len)
+ return 0;

+ if (skb->len < sizeof(struct atmtcp_hdr))

[syzbot]

Hello,

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

Reported-by: syzbot+1d3c23...@syzkaller.appspotmail.com
Tested-by: syzbot+1d3c23...@syzkaller.appspotmail.com

Tested on:

commit: 27605c8c Merge tag 'net-6.16-rc2' of git://git.kernel...
git tree: upstream

console output: https://syzkaller.appspot.com/x/log.txt?x=16d17682580000

kernel config: https://syzkaller.appspot.com/x/.config?x=42d51b7b9f9e61d
dashboard link: https://syzkaller.appspot.com/bug?extid=1d3c235276f62963e93a
compiler: Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6

patch: https://syzkaller.appspot.com/x/patch.diff?x=129e7682580000

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

[Kuniyuki Iwashima]

From: syzbot <syzbot+1d3c23...@syzkaller.appspotmail.com>
Date: Thu, 12 Jun 2025 20:48:02 -0700

I noticed the original code didn't call dev_kfree_skb() and leaked
sk_buff_head.

Also, most of the ATM drivers don't call atm_return() to revert
the memory accounting by atm_account_tx() in vcc_sendmsg(), but
that's another bug...

#syz test

diff --git a/drivers/atm/atmtcp.c b/drivers/atm/atmtcp.c

index d4aa0f353b6c..5ce3c6c066e1 100644

--- a/drivers/atm/atmtcp.c
+++ b/drivers/atm/atmtcp.c
@@ -288,7 +288,11 @@ static int atmtcp_c_send(struct atm_vcc *vcc,struct sk_buff *skb)
struct sk_buff *new_skb;
int result = 0;

- if (!skb->len) return 0;

+ if (skb->len < sizeof(struct atmtcp_hdr)) {

+ dev_kfree_skb(skb);
+ return -EINVAL;
+ }

[syzbot]

Hello,

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

Reported-by: syzbot+1d3c23...@syzkaller.appspotmail.com
Tested-by: syzbot+1d3c23...@syzkaller.appspotmail.com

Tested on:

commit: 27605c8c Merge tag 'net-6.16-rc2' of git://git.kernel...
git tree: upstream

console output: https://syzkaller.appspot.com/x/log.txt?x=15c21e0c580000

kernel config: https://syzkaller.appspot.com/x/.config?x=42d51b7b9f9e61d
dashboard link: https://syzkaller.appspot.com/bug?extid=1d3c235276f62963e93a
compiler: Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6

patch: https://syzkaller.appspot.com/x/patch.diff?x=10c0d10c580000