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 PERF_SAMPLE_BRANCH_USER is requested. On AMD processors supporting X86_FEATURE_BRS (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, amd_brs_match_plm() 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 PERF_SAMPLE_BRANCH_USER is requested.
References
- https://git.kernel.org/stable/c/046f6244da9b68e463a849b21446b9424e531491
- https://git.kernel.org/stable/c/2e706be56f418718bb3ae66c0aa94f9b61150e6d
- https://git.kernel.org/stable/c/47915e855fb38b42133e31ba917d99565f862154
- https://git.kernel.org/stable/c/90843d00dbc61220b66408ea0d8775cae9e51f70
- https://git.kernel.org/stable/c/ac44b4a3d6137489f8fa2e794b12e849c6b22eaa