CVE-2026-63876

Medium · 5.5

CVSS

5.5

EPSS

0.2%

Published

Jul 19, 2026

CISA due date

—

Cloudanix analysis

Schedule a fix SSVC: Track

A startup bug in certain older Linux systems could make the machine crash before it finishes booting, requiring an update to keep those machines running reliably.

Exploitation: Other
Attacker needs: None
Complexity: Theoretical
Impact: Denial Of Service
Component: OS kernel
Class: Memory Corruption
Remediation: Patch
Effort: Medium
Patch risk: Low

What it actually does

Null pointer dereference in serial_base_ctrl_add during zs serial port initialization when the parent device pointer is absent under legacy probing, resulting in kernel paging fault and oops.

Why it matters to the business

Systems using the affected DECstation Z85C30 serial hardware may crash on boot, causing startup failures and requiring manual intervention or hardware-specific workarounds until updated kernels are deployed.

How an attack unfolds

An attacker with physical or console access triggers a normal system boot on vulnerable hardware; the kernel initialization path for the zs driver dereferences a null-derived pointer before any user-space code runs, producing an unhandled oops that halts the boot process.

How to fix it

Update to a kernel containing the platform device conversion for the zs driver; ensure platform_driver_probe is used and resource handling requests only the actual address span.

How to confirm the fix worked

Confirm the zs driver binds via platform device in dmesg, verify absence of the serial_base_ctrl_add oops on boot, and check that the console handover occurs after interrupts are enabled without crashing.

Typical attack path

Boot affected DECstation hardware with legacy zs serial driver enabled
Kernel executes zs_init which calls serial_core_register_port
serial_base_ctrl_add receives a port structure whose parent device pointer is null
Dereference of offset 0x2c from the null-derived pointer triggers kernel oops

How to detect it

Inspect boot logs for the exact oops signature containing serial_base_ctrl_add+0x78 and BadVA 0000002c; search dmesg or journalctl for "Unable to handle kernel paging request" combined with zs_init or serial_core_register_port call traces.

What your security team should do

Prioritize kernel updates on any DECstation or Z85C30 hardware still running vulnerable versions
Test boot behavior with the updated driver before production deployment
Monitor kernel logs for similar null-dereference patterns in serial initialization paths

The record

In the Linux kernel, the following vulnerability has been resolved: serial: zs: Convert to use a platform device Prevent a crash from happening as the first serial port is initialised: Console: switching to mono frame buffer device 160x64 fb0: PMAG-AA frame buffer device at tc0 DECstation Z85C30 serial driver version 0.10 CPU 0 Unable to handle kernel paging request at virtual address 0000002c, epc == 803ab00c, ra == 803aafe0 Oops[#1]: CPU: 0 PID: 1 Comm: swapper Not tainted 6.4.0-rc3-00031-g84a9582fd203-dirty #57 $ 0 : 00000000 10012c00 803aaeb0 00000000 $ 4 : 80e12f60 80e12f50 80e12f58 81000030 $ 8 : 00000000 805ff37c 00000000 33433538 $12 : 65732030 00000006 80c2915d 6c616972 $16 : 80e12f00 807b7630 00000000 00000000 $20 : 00000004 00000348 000001a0 807623b8 $24 : 00000018 00000000 $28 : 80c24000 80c25d60 8078b148 803aafe0 Hi : 00000000 Lo : 00000000 epc : 803ab00c serial_base_ctrl_add+0x78/0xf4 ra : 803aafe0 serial_base_ctrl_add+0x4c/0xf4 Status: 10012c03 KERNEL EXL IE Cause : 00000008 (ExcCode 02) BadVA : 0000002c PrId : 00000440 (R4400SC) Modules linked in: Process swapper (pid: 1, threadinfo=(ptrval), task=(ptrval), tls=00000000) Stack : 80760000 00000cc0 00400044 00400040 803aa02c 80d61ab8 00000000 807b7630 80760000 807623b8 807b7628 803aa644 80386998 00000000 80e17780 80220f68 80e17780 80d61ab8 80c17d80 80e17780 80e17780 8063c798 80e17780 80383fa0 00000010 80e17780 00000000 80386998 807a0000 00000000 00400040 8038f848 807623b8 80d61ab8 00000004 80e17780 00000000 803a68e4 80c25e2c 803bb884 ... Call Trace: [<803ab00c>] serial_base_ctrl_add+0x78/0xf4 [<803aa644>] serial_core_register_port+0x174/0x69c [<8077e9ac>] zs_init+0xc8/0xfc [<800404d4>] do_one_initcall+0x40/0x2ac [<8076cecc>] kernel_init_freeable+0x1e4/0x270 [<80605bec>] kernel_init+0x20/0x108 [<800431e8>] ret_from_kernel_thread+0x14/0x1c Code: 2442aeb0 ae120024 ae0200d0 <8c67002c> 50e00001 8c670000 3c06806e 3c05806e afb30010 ---[ end trace 0000000000000000 ]--- (report at the offending commit) -- where a pointer is dereferenced that has been derived from a null pointer to the port's parent device. Since no device is available with legacy probing and it's not anymore a preferable way to discover devices anyway, switch the driver to using a platform device and use it as the port's parent device. Update resource handling accordingly and only request the actual span of addresses used within the slot, which will have had its resource already requested by generic platform device code. Use platform_driver_probe() not just because SCC devices are fixed with solder on board and not straightforward to remove, but foremost because the associated TTY's major device number is the same as used by the dz driver and the first driver to claim it will prevent the other one from using it. Either one DZ device or some SCC devices will be present in a given system but never both at a time, and therefore we want the major device number to be claimed by the first driver to actually successfully bind to its device and platform_driver_probe() is a way to fulfil that. An unfortunate consequence of the switch to a platform device is we now hand the console over from the bootconsole much later in the bootstrap. The firmware console handler appears good enough though to work so late and in particular with interrupts enabled. Since there is one way only remaining to reach zs_reset() now, remove the port initialisation marker as no longer needed and go through the channel reset unconditionally.

Affected packages

Ecosystem	Package	Introduced	Fixed