Re: [PATCH bpf] bpf: Fix out-of-bounds read in bpf_obj_memcpy

[Jiayuan Chen <mrpre@163.com>]

On 4/8/26 6:04 PM, Jiayuan Chen wrote:
> When copying map value between two maps in BPF program, an out-of-bounds
> read can occur in bpf_obj_memcpy(). Consider the following scenario:
>      // map1: BPF_MAP_TYPE_CGROUP_STORAGE, value_size = 4
>      // map2: BPF_MAP_TYPE_LRU_PERCPU_HASH, value_size = 4
>      void *src = bpf_get_local_storage(&map1, 0);  // 4-byte buffer
>      bpf_map_update_elem(&map2, &key, src, 0);     // copy src to map2
>
> The verifier validates that source buffer size >= destination map's
> value_size through check_helper_mem_access(). Since both maps have
> value_size=4, verification passes.
>
> However, at runtime bpf_obj_memcpy() rounds up the copy size to 8 bytes
> for long-aligned atomic copy:
>      bpf_long_memcpy(dst, src, round_up(size, 8));  // reads 8 bytes
>
> This causes a 4-byte over-read from the source buffer. Fix this by using
> round_down() to only copy complete 8-byte chunks with bpf_long_memcpy(),
> then copy any remaining bytes with regular memcpy().
>
> This ensures we never read beyond the validated buffer size while still
> maintaining atomic operations where possible.
>
> Fixes: d3bec0138bfbe ("bpf: Zero-fill re-used per-cpu map element")
> Closes: https://lore.kernel.org/bpf/14e6c70c.6c121.19c0399d948.Coremail.kaiyanm@hust.edu.cn/
> Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
> Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
> Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>   include/linux/bpf.h | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 05b34a6355b03..1b789f9f8a095 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -535,10 +535,15 @@ static inline void bpf_obj_memcpy(struct btf_record *rec,
>   	int i;
>   
>   	if (IS_ERR_OR_NULL(rec)) {
> -		if (long_memcpy)
> -			bpf_long_memcpy(dst, src, round_up(size, 8));
> -		else
> +		u32 aligned = round_down(size, 8);
> +
> +		if (long_memcpy && aligned) {
> +			bpf_long_memcpy(dst, src, aligned);
> +			if (size > aligned)
> +				memcpy(dst + aligned, src + aligned, size - aligned);
> +		} else {
>   			memcpy(dst, src, size);
> +		}
>   		return;
>   	}
>   

This seems to already exist....

https://lore.kernel.org/bpf/7653EEEC2BAB17DF+20260402073948.2185396-1-xulang@uniontech.com/

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