In the Linux kernel, the following vulnerability has been resolved: bpf: use kvfree() for replaced sysctl write buffer proc_sys_call_handler()… (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.
AI Analysis
Technical Summary
The Linux kernel vulnerability CVE-2026-63809 involves incorrect memory deallocation in proc_sys_call_handler(), where a buffer allocated with kvzalloc() (which can fall back to vmalloc()) was freed using kfree(), leading to potential memory corruption. The correct function kvfree() was introduced to handle both kmalloc and kvzalloc/vmalloc allocations safely. The bug was identified through experimental memory management analysis and reproduced on kernel version 7.1-rc4 with KASAN and CONFIG_FAILSLAB enabled. Applying the fix prevents kernel oops and memory faults triggered by this improper free operation.
Potential Impact
Improper memory deallocation can cause kernel memory corruption, leading to kernel crashes (oops) and instability. While no exploits are known in the wild, this vulnerability could potentially be leveraged to cause denial of service or other unpredictable kernel behavior due to memory corruption.
Mitigation Recommendations
A fix has been implemented in the Linux kernel to replace kfree() with kvfree() for the affected sysctl write buffer deallocation. Users and administrators should apply the official kernel updates that include this fix. Patch status is not explicitly stated in the provided data; therefore, check the vendor's Linux kernel advisory or mainline kernel repositories for the fixed version and apply updates accordingly.
In the Linux kernel, the following vulnerability has been resolved: bpf: use kvfree() for replaced sysctl write buffer proc_sys_call_handler()… (CVE-2026-63809)
Description
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.
CVSS v3.1
Score 7.8high
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The Linux kernel vulnerability CVE-2026-63809 involves incorrect memory deallocation in proc_sys_call_handler(), where a buffer allocated with kvzalloc() (which can fall back to vmalloc()) was freed using kfree(), leading to potential memory corruption. The correct function kvfree() was introduced to handle both kmalloc and kvzalloc/vmalloc allocations safely. The bug was identified through experimental memory management analysis and reproduced on kernel version 7.1-rc4 with KASAN and CONFIG_FAILSLAB enabled. Applying the fix prevents kernel oops and memory faults triggered by this improper free operation.
Potential Impact
Improper memory deallocation can cause kernel memory corruption, leading to kernel crashes (oops) and instability. While no exploits are known in the wild, this vulnerability could potentially be leveraged to cause denial of service or other unpredictable kernel behavior due to memory corruption.
Mitigation Recommendations
A fix has been implemented in the Linux kernel to replace kfree() with kvfree() for the affected sysctl write buffer deallocation. Users and administrators should apply the official kernel updates that include this fix. Patch status is not explicitly stated in the provided data; therefore, check the vendor's Linux kernel advisory or mainline kernel repositories for the fixed version and apply updates accordingly.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- GHSA-vchv-wvp9-hcw9
- Osv Schema Version
- 1.4.0
- Aliases
- ["CVE-2026-63809"]
- Ecosystems
- []
- Database Specific Severity
- null
- Cvss Version
- 3.1
Threat ID: 6a5d27ad2a4a8d59891315ba
Added to database: 07/19/2026, 19:38:21 UTC
Last enriched: 07/19/2026, 20:19:40 UTC
Last updated: 08/26/2026, 22:52:12 UTC
Views: 54
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