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

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Date: Wed, 27 Mar 2024 15:14:53 -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 Mon, Mar 25, 2024 at 5:41 PM Camila Alvarez <cam.alvarez.i@...ilcom> wrote:
>
> BPF documentation specifies that the maximum stack depth for a BPF
> program is 512 bytes. This is not enforced when selecting a bpf
> interpreter, thus casuing an index out of bounds error when trying to
> obtain an interpreter with a bigger stack size.
>
> This patch enforces the stack size to be not bigger than
> 512.
>
> Reported-by: syzbot+d2a2c639d03ac200a4f1@...kaller.appspotmail.com
> Signed-off-by: Camila Alvarez <cam.alvarez.i@...il.com>
> ---
>  kernel/bpf/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 696bc55de8e8..8167b3a721e9 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2196,7 +2196,7 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn)
>                 BUG_ON(1);
>                 return 0;
>  }
> -
> +#define BPF_MAX_STACK_SIZE 512
>  #define PROG_NAME(stack_size) __bpf_prog_run##stack_size
>  #define DEFINE_BPF_PROG_RUN(stack_size) \
>  static unsigned int PROG_NAME(stack_size)(const void *ctx, const struct bpf_insn *insn) \
> @@ -2345,7 +2345,7 @@ static void bpf_prog_select_func(struct bpf_prog *fp)
>  {
>  #ifndef CONFIG_BPF_JIT_ALWAYS_ON
>         u32 stack_depth = max_t(u32, fp->aux->stack_depth, 1);
> -
> +       stack_depth = min_t(u32, stack_depth, BPF_MAX_STACK_SIZE);
>         fp->bpf_func = interpreters[(round_up(stack_depth, 32) / 32) - 1];

That's not the root cause of the issue.
syzbot is saying: index 16 is out of range for type '<unknown> *[16]'

somehow stack depth got bigger than 512.
The bug is somewhere in the verifier.
Please debug it further.