FAILED: patch "[PATCH] perf/x86/amd/brs: Fix kernel address leakage" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 47915e855fb38b42133e31ba917d99565f862154
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072053-paralysis-glancing-61b5@gregkh' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 47915e855fb38b42133e31ba917d99565f862154 Mon Sep 17 00:00:00 2001
From: Sandipan Das <[email protected]>
Date: Fri, 10 Jul 2026 22:04:49 +0530
Subject: [PATCH] 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.

index 06f35a6b58a5..dc564688f3d7 100644
@@ -259,13 +259,13 @@ void amd_brs_disable_all(void)
 		amd_brs_disable();
 }
 
-static bool amd_brs_match_plm(struct perf_event *event, u64 to)
+static bool amd_brs_match_plm(struct perf_event *event, u64 from, u64 to)
 {
 	int type = event->attr.branch_sample_type;
 	int plm_k = PERF_SAMPLE_BRANCH_KERNEL | PERF_SAMPLE_BRANCH_HV;
 	int plm_u = PERF_SAMPLE_BRANCH_USER;
 
-	if (!(type & plm_k) && kernel_ip(to))
+	if (!(type & plm_k) && (kernel_ip(to) || kernel_ip(from)))
 		return 0;
 
 	if (!(type & plm_u) && !kernel_ip(to))

@@ -338,11 +338,11 @@ void amd_brs_drain(void)
 		 */
 		to = (u64)(((s64)to << shift) >> shift);
 
-		if (!amd_brs_match_plm(event, to))
-			continue;
-
 		rdmsrq(brs_from(brs_idx), from);
 
+		if (!amd_brs_match_plm(event, from, to))
+			continue;
+
 		perf_clear_branch_entry_bitfields(br+nr);
 
 		br[nr].from = from;

commit 47915e855fb38b42133e31ba917d99565f862154 upstream.

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.

---
 arch/x86/events/amd/brs.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

index ed308719236c..01fcd43522bc 100644
@@ -259,13 +259,13 @@ void amd_brs_disable_all(void)
 		amd_brs_disable();
 }
 
-static bool amd_brs_match_plm(struct perf_event *event, u64 to)
+static bool amd_brs_match_plm(struct perf_event *event, u64 from, u64 to)
 {
 	int type = event->attr.branch_sample_type;
 	int plm_k = PERF_SAMPLE_BRANCH_KERNEL | PERF_SAMPLE_BRANCH_HV;
 	int plm_u = PERF_SAMPLE_BRANCH_USER;
 
-	if (!(type & plm_k) && kernel_ip(to))
+	if (!(type & plm_k) && (kernel_ip(to) || kernel_ip(from)))
 		return 0;
 
 	if (!(type & plm_u) && !kernel_ip(to))

@@ -338,11 +338,11 @@ void amd_brs_drain(void)
 		 */
 		to = (u64)(((s64)to << shift) >> shift);
 
-		if (!amd_brs_match_plm(event, to))
-			continue;
-
 		rdmsrl(brs_from(brs_idx), from);
 
+		if (!amd_brs_match_plm(event, from, to))
+			continue;
+
 		perf_clear_branch_entry_bitfields(br+nr);
 
 		br[nr].from = from;

2.53.0