[PATCH bpf-next v2] bpf: Clamp trace length in __bpf_get_stack to fix OOB write

[Brahmajit Das]

syzbot reported a stack-out-of-bounds write in __bpf_get_stack()
triggered via bpf_get_stack() when capturing a kernel stack trace.

After the recent refactor that introduced stack_map_calculate_max_depth(),
the code in stack_map_get_build_id_offset() (and related helpers) stopped
clamping the number of trace entries (`trace_nr`) to the number of elements
that fit into the stack map value (`num_elem`).

As a result, if the captured stack contained more frames than the map value
can hold, the subsequent memcpy() would write past the end of the buffer,
triggering a KASAN report like:

    BUG: KASAN: stack-out-of-bounds in __bpf_get_stack+0x...
    Write of size N at addr ... by task syz-executor...

Restore the missing clamp by limiting `trace_nr` to `num_elem` before
computing the copy length. This mirrors the pre-refactor logic and ensures
we never copy more bytes than the destination buffer can hold.

No functional change intended beyond reintroducing the missing bound check.

Reported-by: syzbot+d1b7fa1092def3628bd7@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: e17d62fedd10 ("bpf: Refactor stack map trace depth calculation into helper function")
Signed-off-by: Brahmajit Das <listout@xxxxxxxxxxx>
---
Changes in v2:
- Use max_depth instead of num_elem logic, this logic is similar to what
we are already using __bpf_get_stackid

Changes in v1:
- RFC patch that restores the number of trace entries by setting
trace_nr to trace_nr or num_elem based on whichever is the smallest.
Link: https://lore.kernel.org/all/20251110211640.963-1-listout@xxxxxxxxxxx/
---
 kernel/bpf/stackmap.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 2365541c81dd..f9081de43689 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -480,6 +480,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 	}
 
 	trace_nr = trace->nr - skip;
+	trace_nr = min_t(u32, trace_nr, max_depth - skip);
 	copy_len = trace_nr * elem_size;
 
 	ips = trace->ip + skip;
-- 
2.51.2