linux-kernel - [RFC bpf-next PATCH] bpf: Clamp trace length in __bpf_get

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Message-ID: <20251110211640.963-1-listout@listout.xyz>
Date: Tue, 11 Nov 2025 02:46:40 +0530
From: Brahmajit Das <listout@...tout.xyz>
To: syzbot+d1b7fa1092def3628bd7@...kaller.appspotmail.com
Cc: andrii@...nel.org,
	ast@...nel.org,
	bpf@...r.kernel.org,
	contact@...aud-lcm.com,
	daniel@...earbox.net,
	eddyz87@...il.com,
	haoluo@...gle.com,
	john.fastabend@...il.com,
	jolsa@...nel.org,
	kpsingh@...nel.org,
	linux-kernel@...r.kernel.org,
	martin.lau@...ux.dev,
	netdev@...r.kernel.org,
	sdf@...ichev.me,
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	syzkaller-bugs@...glegroups.com,
	yonghong.song@...ux.dev
Subject: [RFC bpf-next PATCH] bpf: Clamp trace length in __bpf_get_stack to fix OOB write

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@...kaller.appspotmail.com
Fixes: e17d62fedd10 ("bpf: Refactor stack map trace depth calculation into helper function")
Signed-off-by: Brahmajit Das <listout@...tout.xyz>
---
 kernel/bpf/stackmap.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 2365541c81dd..885130e4ab0d 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, size / elem_size);
 	copy_len = trace_nr * elem_size;
 
 	ips = trace->ip + skip;
-- 
2.51.2