Re: [PATCH bpf] bpf: fix BPF_F_CPU validation for sparse CPU IDs

[Leon Hwang]

Hi Hui,

On 2026/8/13 18:12, Hui Su wrote:
> BPF_F_CPU stores the target CPU ID in the upper 32 bits of the map
> operation flags. bpf_map_check_op_flags() currently compares that ID
> with num_possible_cpus(), which is the number of possible CPUs rather
> than a bound on CPU IDs.
> 
> On an arm64 QEMU guest with a CPU device-tree hole, the possible CPU

TIL about the possible CPU hole.

> mask was 0,2-3. A userspace program using raw bpf() syscalls creates
> a BPF_MAP_TYPE_PERCPU_ARRAY and performs update and lookup operations
> for each CPU by setting BPF_F_CPU and the CPU ID in the flags.
> 
> With the old check, CPU 1 is incorrectly accepted while valid CPU 3 is
> rejected with -ERANGE. The CPU 1 update then reaches the per-CPU map
> access path and triggers:
> 
>   Unable to handle kernel paging request at virtual address ...
>   pc : __pi_memcpy_generic+0x5c/0x22c
>   lr : bpf_percpu_array_update+0x2dc/0x2e8
>   Call trace:
>     __pi_memcpy_generic
>     bpf_map_update_value
>     map_update_elem
>     __sys_bpf
> 
> Check the CPU ID against nr_cpu_ids and cpu_possible() instead. This
> rejects CPU IDs outside the valid range and CPUs absent from the
> possible mask, while allowing valid sparse CPU IDs.
> 
> Fixes: 2b421662c788 ("bpf: Introduce BPF_F_CPU and BPF_F_ALL_CPUS flags")
> Signed-off-by: Hui Su <sh_def@xxxxxxx>
> ---
>  include/linux/bpf.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 7719f6528445..282f7022d984 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -4202,7 +4202,8 @@ static inline int bpf_map_check_op_flags(struct bpf_map *map, u64 flags, u64 all
>  			return -EINVAL;
>  
>  		cpu = flags >> 32;
> -		if ((flags & BPF_F_CPU) && cpu >= num_possible_cpus())
> +		if ((flags & BPF_F_CPU) &&
> +		    (cpu >= nr_cpu_ids || !cpu_possible(cpu)))

NIT: no need to put the cpu check on a new line, as 100 chars per line
are allowed.

Other than the NIT,

Acked-by: Leon Hwang <leon.hwang@xxxxxxxxx>

>  			return -ERANGE;
>  	}
>