bpf: Clamp trace length in __bpf_get_stack to fix OOB write #10250
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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: [email protected]
Fixes: e17d62f ("bpf: Refactor stack map trace depth calculation into helper function")
Signed-off-by: Brahmajit Das <[email protected]>
Acked-by: Yonghong Song <[email protected]>
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Pull request for series with
subject: bpf: Clamp trace length in __bpf_get_stack to fix OOB write
version: 3
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1021938