[PATCH v2] usb: atm: usbatm: fix invalid ci_range initialization

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+6665d3db5fef15914802@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6665d3db5fef15914802
Suggested-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/all/20260824024620.23485-1-kartikey406@gmail.com/T/ (local) [v1]
Signed-off-by: Deepanshu Kartikey <redacted>
---
 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