netdev - Re: [PATCH bpf-next v3 06/11] bpf: Fix compare error in function retval_range

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Message-ID: <m3pwq4fhoh4pecl5mahz7fhjiav4atebtbr22jfk4eqqq5hiya@g3vsc2zqlcy6>
Date: Fri, 12 Apr 2024 16:53:35 +0800
From: Shung-Hsi Yu <shung-hsi.yu@...e.com>
To: Xu Kuohai <xukuohai@...weicloud.com>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, Dave Thaler <dthaler1968@...glemail.com>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, 
	linux-security-module@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Matt Bobrowski <mattbobrowski@...gle.com>, 
	Brendan Jackman <jackmanb@...omium.org>, Paul Moore <paul@...l-moore.com>, 
	James Morris <jmorris@...ei.org>, "Serge E . Hallyn" <serge@...lyn.com>, 
	Khadija Kamran <kamrankhadijadj@...il.com>, Casey Schaufler <casey@...aufler-ca.com>, 
	Ondrej Mosnacek <omosnace@...hat.com>, Kees Cook <keescook@...omium.org>, 
	John Johansen <john.johansen@...onical.com>, Lukas Bulwahn <lukas.bulwahn@...il.com>, 
	Roberto Sassu <roberto.sassu@...wei.com>, Toke Høiland-Jørgensen <toke@...hat.com>, 
	bpf@...f.org, David Vernet <void@...ifault.com>
Subject: Re: [PATCH bpf-next v3 06/11] bpf: Fix compare error in function
 retval_range_within

On Thu, Apr 11, 2024 at 08:27:47PM +0800, Xu Kuohai wrote:
> [...]
> 24: (b4) w0 = -1                      ; R0_w=0xffffffff
> ; int BPF_PROG(test_int_hook, struct vm_area_struct *vma, @ lsm.c:89
> 25: (95) exit
> At program exit the register R0 has smin=4294967295 smax=4294967295 should have been in [-4095, 0]
> 
> It can be seen that instruction "w0 = -1" zero extended -1 to 64-bit
> register r0, setting both smin and smax values of r0 to 4294967295.
> This resulted in a false reject when r0 was checked with range [-4095, 0].
> 
> Given bpf_retval_range is a 32-bit range, this patch fixes it by
> changing the compare between r0 and return range from 64-bit
> operation to 32-bit operation.
> 
> Fixes: 8fa4ecd49b81 ("bpf: enforce exact retval range on subprog/callback exit")
> Signed-off-by: Xu Kuohai <xukuohai@...wei.com>
> ---
>  kernel/bpf/verifier.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 05c7c5f2bec0..5393d576c76f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9879,7 +9879,7 @@ static bool in_rbtree_lock_required_cb(struct bpf_verifier_env *env)
>  
>  static bool retval_range_within(struct bpf_retval_range range, const struct bpf_reg_state *reg)
>  {
> -	return range.minval <= reg->smin_value && reg->smax_value <= range.maxval;
> +	return range.minval <= reg->s32_min_value && reg->s32_max_value <= range.maxval;

Logic-wise LGTM

While the status-quo is that the return value is always truncated to
32-bit, looking back there was an attempt to use 64-bit return value for
bpf_prog_run[1] (not merged due to issue on 32-bit architectures). Also
from the reading of BPF standardization ABI it would be inferred that
return value is in 64-bit range:

  BPF has 10 general purpose registers and a read-only frame pointer register,
  all of which are 64-bits wide.
  
  The BPF calling convention is defined as:
  
  * R0: return value from function calls, and exit value for BPF programs
  ...

So add relevant people into the thread for opinions.

1: https://lore.kernel.org/bpf/20221115193911.u6prvskdzr5jevni@apollo/