CVE-2026-72237: perf/x86/amd/brs: Fix kernel address leakage

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

perf/x86/amd/brs: Fix kernel address leakage

A user-only branch stack can contain branches that originate from the kernel. As a result, kernel addresses are exposed to user space even when PERFSAMPLEBRANCHUSER is requested. On AMD processors supporting X86FEATUREBRS (Zen 3 only), perf can still report entries such as SYSRET/interrupt returns for which the branch-from addresses are in the kernel.

E.g.

$ perf record -j any,u -c 4000 -e branch-brs -o - -- \ perf bench syscall basic --loop 1000 | \ perf script -i - -F brstack|tr ' ' '\n'| \ grep -E '0x[89a-f][0-9a-f]{15}'

... 0xffffffff810001c4/0x72e2e32955eb/-/-/-/0//- 0xffffffff810001c4/0x72e2d94a9821/-/-/-/0//- 0xffffffff810001c4/0x72e2d94ffa1b/-/-/-/0//- ...

BRS provides no hardware branch filtering, so privilege level filtering is performed entirely in software. However, amdbrsmatchplm() only validates the branch-to address against the requested privilege levels. For branches from the kernel to user space, the branch-from address is left unchecked and is leaked. Extend the software filter to also validate the branch-from address, so that any branch record whose branch-from address is in the kernel is dropped when PERFSAMPLEBRANCHUSER is requested.