Re: [PATCH bpf-next 1/2] bpf: Fix sleepable check for tracing prog

[Viktor Malik]

On 7/24/26 16:14, Leon Hwang wrote:
> When CONFIG_FUNCTION_ERROR_INJECTION is disabled, a sleepable tracing prog
> is allowed to attach to '__x64_'-alike prefix symbols.
> 
> It is because the verifier does not verify whether the symbol is a kernel
> function or a bpf prog. That said, a sleepable tracing prog is allowed to
> attach to a bpf prog target whose name has '__x64_'-alike prefix.
> 
> For example, a sleepable fentry prog attaches to a '__x64_sys_nop' XDP
> prog, and copies buffer from a user pointer with bpf_copy_from_user()
> helper. After attaching the XDP prog to lo interface, the kernel BUG
> could be triggered by 'ping -c 1 -W 1 127.0.0.1':
> 
> [    3.460756] BUG: sleeping function called from invalid context at kernel/bpf/trampoline.c:1324
> 
> Fix it by disallowing sleepable tracing prog always when its target is
> bpf prog.
> 
> Fixes: 16d9c5660692 ("bpf: Always allow sleepable programs on syscalls")
> Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
> ---
>  kernel/bpf/verifier.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 52be0a118cce..40d567b90d24 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -18935,13 +18935,16 @@ static bool is_tracing_multi_id(const struct bpf_prog *prog, u32 btf_id)
>  }
>  
>  static int btf_id_allow_sleepable(u32 btf_id, unsigned long addr, const struct bpf_prog *prog,
> -				  const struct btf *btf)
> +				  const struct btf *btf, const struct bpf_prog *tgt_prog)
>  {
>  	const struct btf_type *t;
>  	const char *tname;
>  
>  	switch (prog->type) {
>  	case BPF_PROG_TYPE_TRACING:
> +		if (tgt_prog)
> +			return prog->sleepable ? -EINVAL : 0;

The prog->sleepable check is redundant since btf_id_allow_sleepable() is
only called if prog->sleepable is true.

Other than that:

Acked-by: Viktor Malik <vmalik@xxxxxxxxxx>

> +
>  		t = btf_type_by_id(btf, btf_id);
>  		if (!t)
>  			return -EINVAL;
> @@ -19324,7 +19327,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
>  		}
>  
>  		if (prog->sleepable) {
> -			ret = btf_id_allow_sleepable(btf_id, addr, prog, btf);
> +			ret = btf_id_allow_sleepable(btf_id, addr, prog, btf, tgt_prog);
>  			if (ret) {
>  				module_put(mod);
>  				bpf_log(log, "%s is not sleepable\n", tname);
> @@ -19575,7 +19578,7 @@ int bpf_check_attach_btf_id_multi(struct btf *btf, struct bpf_prog *prog, u32 bt
>  
>  	/* Check sleepable program attachment. */
>  	if (prog->sleepable) {
> -		err = btf_id_allow_sleepable(btf_id, addr, prog, btf);
> +		err = btf_id_allow_sleepable(btf_id, addr, prog, btf, NULL);
>  		if (err)
>  			return err;
>  	}