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

dev->ci_range.vpi_bits and vci_bits can legitimately be ATM_CI_MAX (-1),
a sentinel meaning "no range configured, use maximum" (see
include/uapi/linux/atmdev.h). Some drivers, such as usbatm_atm_init()
in drivers/usb/atm/usbatm.c, set this sentinel and never resolve it to
an actual bit width.

__vcc_connect() uses these fields directly as a shift amount without
checking for the sentinel, so binding a PVC socket on such a device
triggers a negative shift:

  UBSAN: shift-out-of-bounds in net/atm/common.c:381:10
  shift exponent -1 is negative

Skip the range check when ci_range.vpi_bits/vci_bits is still
ATM_CI_MAX, since that value means "unrestricted".

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+6665d3db5fef15914802@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6665d3db5fef15914802
Tested-by: syzbot+6665d3db5fef15914802@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <redacted>
---
 net/atm/common.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/atm/common.c b/net/atm/common.c
index 81195727fa18..73057bb016f7 100644
--- a/net/atm/common.c
+++ b/net/atm/common.c

@@ -378,8 +378,11 @@ static int __vcc_connect(struct atm_vcc *vcc, struct atm_dev *dev, short vpi,
 	int error;
 
 	if ((vpi != ATM_VPI_UNSPEC && vpi != ATM_VPI_ANY &&
+	    dev->ci_range.vpi_bits != ATM_CI_MAX &&
 	    vpi >> dev->ci_range.vpi_bits) || (vci != ATM_VCI_UNSPEC &&
-	    vci != ATM_VCI_ANY && vci >> dev->ci_range.vci_bits))
+	    vci != ATM_VCI_ANY &&
+	    dev->ci_range.vci_bits != ATM_CI_MAX &&
+	    vci >> dev->ci_range.vci_bits))
 		return -EINVAL;
 	if (vci > 0 && vci < ATM_NOT_RSV_VCI && !capable(CAP_NET_BIND_SERVICE))
 		return -EPERM;

-- 
2.34.1