CVE-2026-63809

In the Linux kernel, the following vulnerability has been resolved:

bpf: use kvfree() for replaced sysctl write buffer

proc_sys_call_handler() allocates its temporary sysctl buffer with

kvzalloc() and passes it to __cgroup_bpf_run_filter_sysctl(). Since

kvzalloc() may fall back to vmalloc() for large allocations, freeing

that buffer with kfree() is wrong and can corrupt memory.

Use kvfree() to safely handle both kmalloc and kvzalloc()/vmalloc

allocations.

The bug was first flagged by an experimental analysis tool we are

developing for kernel memory-management bugs while analyzing

v6.13-rc1. The tool is still under development and is not yet publicly

available. Manual inspection confirms that the bug is still

present in v7.1-rc5.

Reproduced the bug based on v7.1-rc4 in a QEMU x86_64 guest booted with

KASAN and CONFIG_FAILSLAB enabled. To exercise the replacement path, the

test tree also included the accompanying fix for the stale ret == 1

check in __cgroup_bpf_run_filter_sysctl(). The reproducer confines

failslab injections to the proc_sys_call_handler() range, uses

stacktrace-depth=32, and injects fail-nth=1 while writing 8191 bytes to

/proc/sys/kernel/domainname from a task in the target cgroup. Under

that setup, fail-nth=1 triggered the fault:

BUG: unable to handle page fault for address: ffffeb0200024d48

#PF: supervisor read access in kernel mode

#PF: error_code(0x0000) - not-present page

PGD 0 P4D 0

Oops: Oops: 0000 SMP KASAN NOPTI

CPU: 2 UID: 0 PID: 209 Comm: repro_proc_sys_ Not tainted 7.1.0-rc4-00686-g97625979a5d4 PREEMPT(lazy)

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.15.0-1 04/01/2014

RIP: 0010:kfree+0x6e/0x510

...

Call Trace:

<TASK>

? __cgroup_bpf_run_filter_sysctl+0x626/0xc30

__cgroup_bpf_run_filter_sysctl+0x74d/0xc30

? __pfx___cgroup_bpf_run_filter_sysctl+0x10/0x10

? srso_return_thunk+0x5/0x5f

? __kvmalloc_node_noprof+0x345/0x870

? proc_sys_call_handler+0x250/0x480

? srso_return_thunk+0x5/0x5f

proc_sys_call_handler+0x3a2/0x480

? __pfx_proc_sys_call_handler+0x10/0x10

? srso_return_thunk+0x5/0x5f

? selinux_file_permission+0x39f/0x500

? srso_return_thunk+0x5/0x5f

? lock_is_held_type+0x9e/0x120

vfs_write+0x98e/0x1000

...

</TASK>

With this fix applied on top of the same test setup, rerunning the

reproducer with fail-nth=1 yields no corresponding Oops reports.