The following commit has been merged into the perf/urgent branch of tip:

Commit-ID: 58f6217e5d0132a9f14e401e62796916aa055c1b Gitweb: https://git.kernel.org/tip/58f6217e5d0132a9f14e401e62796916aa055c1b Author: Sean Christopherson seanjc@google.com AuthorDate: Fri, 25 Apr 2025 17:13:55 -07:00 Committer: Peter Zijlstra peterz@infradead.org CommitterDate: Wed, 30 Apr 2025 13:58:29 +02:00

perf/x86/intel: KVM: Mask PEBS_ENABLE loaded for guest with vCPU's value.

When generating the MSR_IA32_PEBS_ENABLE value that will be loaded on VM-Entry to a KVM guest, mask the value with the vCPU's desired PEBS_ENABLE value. Consulting only the host kernel's host vs. guest masks results in running the guest with PEBS enabled even when the guest doesn't want to use PEBS. Because KVM uses perf events to proxy the guest virtual PMU, simply looking at exclude_host can't differentiate between events created by host userspace, and events created by KVM on behalf of the guest.

Running the guest with PEBS unexpectedly enabled typically manifests as crashes due to a near-infinite stream of #PFs. E.g. if the guest hasn't written MSR_IA32_DS_AREA, the CPU will hit page faults on address '0' when trying to record PEBS events.

The issue is most easily reproduced by running `perf kvm top` from before commit 7b100989b4f6 ("perf evlist: Remove __evlist__add_default") (after which, `perf kvm top` effectively stopped using PEBS). The userspace side of perf creates a guest-only PEBS event, which intel_guest_get_msrs() misconstrues a guest-*owned* PEBS event.

Arguably, this is a userspace bug, as enabling PEBS on guest-only events simply cannot work, and userspace can kill VMs in many other ways (there is no danger to the host). However, even if this is considered to be bad userspace behavior, there's zero downside to perf/KVM restricting PEBS to guest-owned events.

Note, commit 854250329c02 ("KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations") fixed the case where host userspace is profiling KVM *and* userspace, but missed the case where userspace is profiling only KVM.

Fixes: c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS") Closes: https://lore.kernel.org/all/Z_VUswFkWiTYI0eD@do-x1carbon Reported-by: Seth Forshee sforshee@kernel.org Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Dapeng Mi dapeng1.mi@linux.intel.com Tested-by: "Seth Forshee (DigitalOcean)" sforshee@kernel.org Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20250426001355.1026530-1-seanjc@google.com --- arch/x86/events/intel/core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c index 00dfe48..c5f3854 100644 --- a/arch/x86/events/intel/core.c +++ b/arch/x86/events/intel/core.c @@ -4395,7 +4395,7 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data) arr[pebs_enable] = (struct perf_guest_switch_msr){ .msr = MSR_IA32_PEBS_ENABLE, .host = cpuc->pebs_enabled & ~cpuc->intel_ctrl_guest_mask, - .guest = pebs_mask & ~cpuc->intel_ctrl_host_mask, + .guest = pebs_mask & ~cpuc->intel_ctrl_host_mask & kvm_pmu->pebs_enable, };

if (arr[pebs_enable].host) {