Re: [PATCH bpf-next] bpf: dup xlated insns with kvmalloc+memcpy

[Yonghong Song]

On 12/15/22 9:54 PM, Hao Sun wrote:
> Currently, kmemdup() is used for allocating and copying xlated insns
> in bpf_insn_prepare_dump(). The following warning can be triggered
> when dup large amount of insns (roughly BPF_COMPLEXITY_LIMIT_INSNS/2)
> because kmemdup() uses kmalloc() which would fail when allocing size
> is too big, leading to failure in dump xlated insns:
>
> WARNING: CPU: 2 PID: 7060 at mm/page_alloc.c:5534
> Call Trace:
>   <TASK>
>   __alloc_pages_node include/linux/gfp.h:237 [inline]
>   alloc_pages_node include/linux/gfp.h:260 [inline]
>   __kmalloc_large_node+0x81/0x160 mm/slab_common.c:1096
>   __do_kmalloc_node mm/slab_common.c:943 [inline]
>   __kmalloc_node_track_caller.cold+0x5/0x5d mm/slab_common.c:975
>   kmemdup+0x29/0x60 mm/util.c:129
>   kmemdup include/linux/fortify-string.h:585 [inline]
>   bpf_insn_prepare_dump kernel/bpf/syscall.c:3820 [inline]
>   bpf_prog_get_info_by_fd+0x9a3/0x2cb0 kernel/bpf/syscall.c:3975
>   bpf_obj_get_info_by_fd kernel/bpf/syscall.c:4297 [inline]
>   __sys_bpf+0x3928/0x56f0 kernel/bpf/syscall.c:5004
>   __do_sys_bpf kernel/bpf/syscall.c:5069 [inline]
>   __se_sys_bpf kernel/bpf/syscall.c:5067 [inline]
>   ...
>
> So use kvmalloc()+memcpy() to fix this, for small size of insns,
> this is same as kmemdup(), but this also support dup large amount
> of xlated insns.
>
> Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
> ---
>   kernel/bpf/syscall.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 35972afb6850..06229fddac0d 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3831,10 +3831,10 @@ static struct bpf_insn *bpf_insn_prepare_dump(const struct bpf_prog *prog,
>   	u8 code;
>   	int i;
>   
> -	insns = kmemdup(prog->insnsi, bpf_prog_insn_size(prog),
> -			GFP_USER);

Does kmemdup(prog->insnsi, bpf_prog_insn_size(prog), GFP_USER | 
__GFP_NOWARN) work?

> -	if (!insns)
> +	insns = kvmalloc(bpf_prog_insn_size(prog), GFP_USER | __GFP_NOWARN);
> +	if (unlikely(!insns))
>   		return insns;
> +	memcpy(insns, prog->insnsi, bpf_prog_insn_size(prog));
>   
>   	for (i = 0; i < prog->len; i++) {
>   		code = insns[i].code;
> @@ -3992,7 +3992,7 @@ static int bpf_prog_get_info_by_fd(struct file *file,
>   		uinsns = u64_to_user_ptr(info.xlated_prog_insns);
>   		ulen = min_t(u32, info.xlated_prog_len, ulen);
>   		fault = copy_to_user(uinsns, insns_sanitized, ulen);
> -		kfree(insns_sanitized);
> +		kvfree(insns_sanitized);
>   		if (fault)
>   			return -EFAULT;
>   	}
>
> base-commit: 0e43662e61f2569500ab83b8188c065603530785