Bug 2536730 (CVE-2026-90055)

Summary: CVE-2026-90055 kernel: usb: atm: usbatm: fix invalid ci_range initialization
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: low Docs Contact:
Priority: low    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's USB ATM (Asynchronous Transfer Mode) driver. An invalid initialization of the ci_range fields, specifically vpi_bits and vci_bits, to a negative sentinel value can lead to a shift-out-of-bounds error. This undefined behavior can be triggered by a local user interacting with ATM device functionality, potentially causing system instability or a denial of service (DoS).
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Description OSIDB Bzimport 2026-09-17 19:22:39 UTC 
In the Linux kernel, the following vulnerability has been resolved:

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.
Comment 1 Keith Grant 2026-09-22 14:59:44 UTC 
Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-90055.mbox