CVE-2026-98551 perf/core: Fill branch entries with a single assignment
Local attackers can read uninitialized kernel memory via perf branch entries on arm64 systems due to missing field initialization.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-98551?
CVE-2026-98551 is a vulnerability in Linux, published on 18 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
perf/core: Fill branch entries with a single assignment
perf_clear_branch_entry_bitfields() clears the bitfields of struct perf_branch_entry one by one and leaves from/to alone, since callers overwrite those straight away. The list has to be kept in sync with the struct by hand and has already fallen behind: new_type and priv were added to perf_branch_entry and never added here.
Only BRBE writes those two, and neither for every record. brbe_set_perf_entry_type() leaves new_type alone for a branch type it does not recognise, and priv is not set for source-only records. arm_pmuv3.c allocates the per-CPU branch stack with kmalloc(), so such a record reaches userspace with whatever the slot held: uninitialised kmalloc() data on the first pass over the buffer, the previous record's values after that. Nothing under arch/x86/events/ writes either field, so only arm64 is affected.
Assign the whole entry at each site instead. Everything not named is then zero, and there is no list to keep in sync. The bitfields add up to exactly 64 bits, so the struct has no padding to leave undefined.
perf_clear_branch_entry_bitfields() has no callers left, so remove it. perf_entry_from_brbe_regset() assigns an empty literal instead, since it fills from/to conditionally. PERF_BR_SPEC_NA is 0, so dropping the explicit spec assignment changes nothing.
Which versions are affected by CVE-2026-98551?
How do I fix CVE-2026-98551?
Apply the vendor's patch.
Is CVE-2026-98551 being exploited?
There is no known exploitation of CVE-2026-98551 in the wild and no public exploit code.
Sources
Also known as AZL-109389, BELL-CVE-2026-98551, DEBIAN-CVE-2026-98551, GHSA-r5w5-qjwh-6pq3, UBUNTU-CVE-2026-98551.