2024-03-26 01:40:36

by Camila Alvarez

Subject: [PATCH] fix array-index-out-of-bounds in bpf_prog_select_runtime

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: [email protected]
Signed-off-by: Camila Alvarez <[email protected]>
---
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];
#else
fp->bpf_func = __bpf_prog_ret0_warn;
--
2.34.1

2024-03-27 22:15:19

by Alexei Starovoitov

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@gmailcom> 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: [email protected]
> Signed-off-by: Camila Alvarez <[email protected]>
> ---
> 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.