linux-kernel - Re: [PATCH] fix array-index-out-of-bounds in bpf_prog_select

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Date: Sun, 5 May 2024 01:21:12 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Camila Alvarez <cam.alvarez.i@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	syzbot+d2a2c639d03ac200a4f1@...kaller.appspotmail.com
Subject: Re: [PATCH] fix array-index-out-of-bounds in bpf_prog_select_runtime

On Sat, May 4, 2024 at 6:49 PM Camila Alvarez <cam.alvarez.i@...il.com> wrote:
>
> The error indicates that the verifier is letting through a program with
> a stack depth bigger than 512.
>
> This is due to the verifier not checking the stack depth after
> instruction rewrites are perfomed. For example, the MAY_GOTO instruction
> adds 8 bytes to the stack, which means that if the stack at the moment
> was already 512 bytes it would overflow after rewriting the instruction.

This is by design. may_goto and other constructs like bpf_loop
inlining can consume a few words above 512 limit.

> The fix involves adding a stack depth check after all instruction
> rewrites are performed.
>
> Reported-by: syzbot+d2a2c639d03ac200a4f1@...kaller.appspotmail.com

This syzbot report is likely unrelated.
It says that it bisected it to may_goto, but it has this report
before may_goto was introduced, so bisection is incorrect.

pw-bot: cr

> Signed-off-by: Camila Alvarez <cam.alvarez.i@...il.com>
> ---
>  kernel/bpf/verifier.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 63749ad5ac6b..a9e23b6b8e8f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -21285,6 +21285,10 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr, bpfptr_t uattr, __u3
>         if (ret == 0)
>                 ret = do_misc_fixups(env);
>
> +        /* max stack depth verification must be done after rewrites as well */
> +        if (ret == 0)
> +                ret = check_max_stack_depth(env);
> +
>         /* do 32-bit optimization after insn patching has done so those patched
>          * insns could be handled correctly.
>          */
> --
> 2.34.1
>