CVE-2026-98551: perf/core: Fill branch entries with a single assignment

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

perf/core: Fill branch entries with a single assignment

perfclearbranchentrybitfields() clears the bitfields of struct perfbranchentry 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: newtype and priv were added to perfbranchentry and never added here.

Only BRBE writes those two, and neither for every record. brbesetperfentrytype() leaves newtype alone for a branch type it does not recognise, and priv is not set for source-only records. armpmuv3.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.

perfclearbranchentrybitfields() has no callers left, so remove it. perfentryfrombrberegset() assigns an empty literal instead, since it fills from/to conditionally. PERFBRSPECNA is 0, so dropping the explicit spec assignment changes nothing.