[PATCH 6.6.y] bpf: Fix stackmap overflow check in __bpf_get_stackid()

[Artem Dinaburg <artem@trailofbits.com>]

From: Arnaud Lecomte <contact@arnaud-lcm.com>

[ Upstream commit 23f852daa4bab4d579110e034e4d513f7d490846 ]

Syzkaller reported a KASAN slab-out-of-bounds write in __bpf_get_stackid()
when copying stack trace data. The issue occurs when the perf trace
 contains more stack entries than the stack map bucket can hold,
 leading to an out-of-bounds write in the bucket's data array.

Fixes: ee2a098851bf ("bpf: Adjust BPF stack helper functions to accommodate skip > 0")
Reported-by: syzbot+c9b724fbb41cf2538b7b@syzkaller.appspotmail.com
Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20251025192941.1500-1-contact@arnaud-lcm.com

Closes: https://syzkaller.appspot.com/bug?extid=c9b724fbb41cf2538b7b
Assisted-by: LLM
[ Backport to 6.6.y: stack_map_calculate_max_depth() was introduced by
  e17d62fedd10 ("bpf: Refactor stack map trace depth calculation into
  helper function"), which is not in 6.6.y; open-code the same
  computation in __bpf_get_stackid(). ]
Signed-off-by: Artem Dinaburg <artem@trailofbits.com>
---
Hi,

Please consider this backport of the fix for CVE-2025-68378 to 6.6.y.

The only difference from upstream is that the
stack_map_calculate_max_depth() helper, which 6.6.y lacks, is
open-coded in __bpf_get_stackid().

The complete fix is already present in 6.12.y, 6.18.y and 7.2.y.

Build-tested only as part of a cumulative 6.6.y series on x86_64
(allmodconfig, CONFIG_WERROR=y); no runtime testing.

Thanks,
Artem Dinaburg

 kernel/bpf/stackmap.c |   20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -216,7 +216,8 @@
 	struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
 	struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
 	u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
-	u32 hash, id, trace_nr, trace_len;
+	u32 hash, id, trace_nr, trace_len, max_depth;
+	u32 curr_sysctl_max_stack;
 	bool user = flags & BPF_F_USER_STACK;
 	u64 *ips;
 	bool hash_matches;
@@ -225,7 +226,12 @@
 		/* skipping more than usable stack trace */
 		return -EFAULT;
 
-	trace_nr = trace->nr - skip;
+	curr_sysctl_max_stack = READ_ONCE(sysctl_perf_event_max_stack);
+	max_depth = map->value_size / stack_map_data_size(map);
+	max_depth += skip;
+	if (max_depth > curr_sysctl_max_stack)
+		max_depth = curr_sysctl_max_stack;
+	trace_nr = min_t(u32, trace->nr - skip, max_depth - skip);
 	trace_len = trace_nr * sizeof(u64);
 	ips = trace->ip + skip;
 	hash = jhash2((u32 *)ips, trace_len / sizeof(u32), 0);
@@ -354,15 +360,11 @@
 		return -EFAULT;
 
 	nr_kernel = count_kernel_ip(trace);
+	__u64 nr = trace->nr; /* save original */
 
 	if (kernel) {
-		__u64 nr = trace->nr;
-
 		trace->nr = nr_kernel;
 		ret = __bpf_get_stackid(map, trace, flags);
-
-		/* restore nr */
-		trace->nr = nr;
 	} else { /* user */
 		u64 skip = flags & BPF_F_SKIP_FIELD_MASK;
 
@@ -373,6 +375,10 @@
 		flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
 		ret = __bpf_get_stackid(map, trace, flags);
 	}
+
+	/* restore nr */
+	trace->nr = nr;
+
 	return ret;
 }