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: ? __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 …

With this fix applied on top of the same test setup, rerunning the reproducer with fail-nth=1 yields no corresponding Oops reports.

Affected Software

References

• https://git.kernel.org/stable/c/4c21b5927d4364bfe7365f2700da5fea0ed0d004
• https://git.kernel.org/stable/c/65bd0c0afb0e1bf3287458e342429b069624f7d4
• https://git.kernel.org/stable/c/70df4de46577fab5e25418f014583155a147c902
• https://git.kernel.org/stable/c/77355ef7a9f6b0d2bdf65be3b37f2c1f365e20d2
• https://git.kernel.org/stable/c/81fc9a13acae99966232f0e055eb2e445263b89a
• https://git.kernel.org/stable/c/838fe9c28121777c59a9406710a68fcf77bb8017
• https://git.kernel.org/stable/c/d0a81ed5ff5d0f9c3f63a4f9e5a4642c363ecd3e
• https://git.kernel.org/stable/c/e1d1e203a6000804c5d3b8a4aa4e52303c0c7ab2