Re: [PATCH] net: atm: fix shift-out-of-bounds in __vcc_connect()

[Eric Dumazet]

On Wed, Aug 26, 2026 at 1:59 PM Simon Horman <horms@xxxxxxxxxx> wrote:
>
> This is an AI-generated review of your patch. The human sending this
> email has considered the AI review valid, or at least plausible.
> Full review at: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260824024620.23485-1-kartikey406@xxxxxxxxx?part=1
> ---
> net: atm: fix shift-out-of-bounds in __vcc_connect()
>
> __vcc_connect() used dev->ci_range.vpi_bits and vci_bits directly as shift
> amounts, which is a negative shift when those fields still hold the
> ATM_CI_MAX (-1) sentinel.  The patch skips the VPI/VCI range check
> entirely when either field equals ATM_CI_MAX.
>
> Closes: https://syzkaller.appspot.com/bug?extid=6665d3db5fef15914802

A simpler fix would be something like

   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 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 was defined in <uapi/linux/atmdev.h> as a sentinel
    value for user-space ATM_SETCIRANGE requests, not as 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.

 ...


diff --git a/drivers/usb/atm/usbatm.c b/drivers/usb/atm/usbatm.c
index 9600e1ec099304e465dddb00c36817339f538b3b..7b0c791399eaac9ebe00541a2c41d715f465a24f
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 */