[syzbot] [net?] [usb?] UBSAN: shift-out-of-bounds in __vcc_connect

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 26260251022f Merge tag 'livepatching-for-7.3' of git://git..
git tree: upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=14fa5179580000
kernel config: https://syzkaller.appspot.com/x/.config?x=a70bec3546180b6
dashboard link: https://syzkaller.appspot.com/bug?extid=f6ac161ee9699270b8dd
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12698625580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/858149e586b7/disk-26260251.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/8c4a5ad34e66/vmlinux-26260251.xz
kernel image: https://storage.googleapis.com/syzbot-assets/fa4efe98315e/bzImage-26260251.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+f6ac16...@syzkaller.appspotmail.com

------------[ cut here ]------------
UBSAN: shift-out-of-bounds in net/atm/common.c:382:32
shift exponent -1 is negative
CPU: 0 UID: 0 PID: 5987 Comm: syz.0.18 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/05/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
ubsan_epilogue+0xa/0x30 lib/ubsan.c:233
__ubsan_handle_shift_out_of_bounds+0x36d/0x400 lib/ubsan.c:494
__vcc_connect+0x14b4/0x19c0 net/atm/common.c:382
vcc_connect+0x328/0x8f0 net/atm/common.c:498
pvc_bind+0x272/0x380 net/atm/pvc.c:52
__sys_bind_socket net/socket.c:1945 [inline]
__sys_bind+0x2e3/0x410 net/socket.c:1976
__do_sys_bind net/socket.c:1981 [inline]
__se_sys_bind net/socket.c:1979 [inline]
__x64_sys_bind+0x7a/0x90 net/socket.c:1979
do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc16739e0d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff0c105ee8 EFLAGS: 00000246 ORIG_RAX: 0000000000000031
RAX: ffffffffffffffda RBX: 00007fc167625fa0 RCX: 00007fc16739e0d9
RDX: 000000000000000c RSI: 0000200000000300 RDI: 0000000000000004
RBP: 00007fc167435024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fc167625fac R14: 00007fc167625fa0 R15: 00007fc167625fa0
</TASK>
---[ end trace ]---


---
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***

Subject: [PATCH v2] usb: atm: usbatm: fix invalid ci_range initialization
Author: karti...@gmail.com

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


syzbot reported a shift-out-of-bounds in __vcc_connect():

UBSAN: shift-out-of-bounds in net/atm/common.c:382:32
shift exponent -1 is negative
CPU: 0 UID: 0 PID: 5987 Comm: syz.0.18 Not tainted syzkaller #0 PREEMPT(full)

Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 08/05/2026

Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
ubsan_epilogue+0xa/0x30 lib/ubsan.c:233
__ubsan_handle_shift_out_of_bounds+0x36d/0x400 lib/ubsan.c:494
__vcc_connect+0x14b4/0x19c0 net/atm/common.c:382
vcc_connect+0x328/0x8f0 net/atm/common.c:498
pvc_bind+0x272/0x380 net/atm/pvc.c:52

__sys_bind+0x2e3/0x410 net/socket.c:1976
__x64_sys_bind+0x7a/0x90 net/socket.c:1979
...

ATM device ci_range fields (vpi_bits and vci_bits) represent the
number of bits supported for VPI and VCI addressing on the device.
net/atm/common.c directly uses these fields as bit shift counts:
vpi >> dev->ci_range.vpi_bits
vci >> dev->ci_range.vci_bits
1 << vcc->dev->ci_range.vpi_bits
1 << vcc->dev->ci_range.vci_bits

usbatm_atm_init() sets ci_range.vpi_bits and ci_range.vci_bits to
ATM_CI_MAX (-1), which is defined in <uapi/linux/atmdev.h> as a
sentinel value for userspace ATM_SETCIRANGE requests, not a valid bit
count. Shifting by -1 is undefined behavior and triggers UBSAN
warnings.

ATM UNI cell headers allow up to 8 bits for VPI (0..255) and 16 bits
for VCI (0..65535). Initialize vpi_bits to 8 and vci_bits to 16, as
done by solos-pci.

Fixes: c59bba75fa50 ("[PATCH] USB ATM: new usbatm core")
Reported-by: syzbot+f6ac16...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f6ac161ee9699270b8dd
Signed-off-by: Deepanshu Kartikey <karti...@gmail.com>
---
drivers/usb/atm/usbatm.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/atm/usbatm.c b/drivers/usb/atm/usbatm.c
index 9600e1ec0993..7b0c791399ea 100644
--- a/drivers/usb/atm/usbatm.c
+++ b/drivers/usb/atm/usbatm.c
@@ -917,8 +917,8 @@ static int usbatm_atm_init(struct usbatm_data *instance)

instance->atm_dev = atm_dev;

- atm_dev->ci_range.vpi_bits = ATM_CI_MAX;
- atm_dev->ci_range.vci_bits = ATM_CI_MAX;
+ atm_dev->ci_range.vpi_bits = 8;
+ atm_dev->ci_range.vci_bits = 16;
atm_dev->signal = ATM_PHY_SIG_UNKNOWN;

/* temp init ATM device, set to 128kbit */
--
2.43.0

[syzbot]

Hello,

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

Reported-by: syzbot+f6ac16...@syzkaller.appspotmail.com
Tested-by: syzbot+f6ac16...@syzkaller.appspotmail.com

Tested on:

commit: 45c13f3f Merge tag 'hwlock-v7.3' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14ff6979580000
kernel config: https://syzkaller.appspot.com/x/.config?x=2210e2522d526007

dashboard link: https://syzkaller.appspot.com/bug?extid=f6ac161ee9699270b8dd
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=16276979580000

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