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

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Date: Sun, 5 May 2024 19:18:39 -0400 (-04)
From: Camila Alvarez Inostroza <cam.alvarez.i@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
cc: Camila Alvarez <cam.alvarez.i@...il.com>, 
    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 Sun, 5 May 2024, Alexei Starovoitov wrote:

> 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.
>

Is this the only case where the verifier should allow the stack to go over 
the 512 limit? If that's the case, maybe we could use the extra stack 
depth to store how much the rewrites affect the stack depth? This would 
only be used to obtain the correct interpreter when 
CONFIG_BPF_JIT_ALWAYS_ON is not set.
That would allow choosing the interpreter by considering the stack depth 
before the rewrites.

>> 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

I can see that may_goto was introduced on march 6th, and the first report 
was on march 13th. Is there any report I'm missing?

>
>> 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
>>
>