Re: [PATCH bpf-next v8 3/3] bpf: fix stackmap overflow check in __bpf_get_stackid()

["Lecomte, Arnaud" <contact@arnaud-lcm.com>]

Hi,
Can you confirm you received this one Alexei as I got an undelivered 
email reply.
Cheers,
Arnaud

On 05/09/2025 14:48, Arnaud lecomte wrote:
> From: Arnaud Lecomte <contact@arnaud-lcm.com>
>
> 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.
>
> Reported-by: syzbot+c9b724fbb41cf2538b7b@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=c9b724fbb41cf2538b7b
> Fixes: ee2a098851bf ("bpf: Adjust BPF stack helper functions to accommodate skip > 0")
> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
> Acked-by: Yonghong Song <yonghong.song@linux.dev>
> ---
> Changes in v2:
>   - Fixed max_depth names across get stack id
>
> Changes in v4:
>   - Removed unnecessary empty line in __bpf_get_stackid
>
> Changes in v6:
>   - Added back trace_len computation in __bpf_get_stackid
>
> Changes in v7:
>   - Removed usefull trace->nr assignation in bpf_get_stackid_pe
>   - Added restoration of trace->nr for both kernel and user traces
>     in bpf_get_stackid_pe
>
> Link to v7: https://lore.kernel.org/all/20250903234325.30212-1-contact@arnaud-lcm.com/
> ---
>   kernel/bpf/stackmap.c | 17 +++++++++++++----
>   1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 9f3ae426ddc3..9b57b8307565 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -369,6 +369,7 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
>   {
>   	struct perf_event *event = ctx->event;
>   	struct perf_callchain_entry *trace;
> +	u32 elem_size, max_depth;
>   	bool kernel, user;
>   	__u64 nr_kernel;
>   	int ret;
> @@ -390,15 +391,16 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
>   		return -EFAULT;
>   
>   	nr_kernel = count_kernel_ip(trace);
> +	elem_size = stack_map_data_size(map);
> +	__u64 nr = trace->nr; /* save original */
>   
>   	if (kernel) {
> -		__u64 nr = trace->nr;
> -
>   		trace->nr = nr_kernel;
> +		max_depth =
> +			stack_map_calculate_max_depth(map->value_size, elem_size, flags);
> +		trace->nr = min_t(u32, nr_kernel, max_depth);
>   		ret = __bpf_get_stackid(map, trace, flags);
>   
> -		/* restore nr */
> -		trace->nr = nr;
>   	} else { /* user */
>   		u64 skip = flags & BPF_F_SKIP_FIELD_MASK;
>   
> @@ -407,8 +409,15 @@ BPF_CALL_3(bpf_get_stackid_pe, struct bpf_perf_event_data_kern *, ctx,
>   			return -EFAULT;
>   
>   		flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
> +		max_depth =
> +			stack_map_calculate_max_depth(map->value_size, elem_size, flags);
> +		trace->nr = min_t(u32, trace->nr, max_depth);
>   		ret = __bpf_get_stackid(map, trace, flags);
>   	}
> +
> +	/* restore nr */
> +	trace->nr = nr;
> +
>   	return ret;
>   }
>