linux-kernel - Re: [PATCH] bpf: selftest for late caller stack size increase

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Date:   Fri, 22 Dec 2017 11:11:53 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: selftest for late caller stack size increase

On Fri, Dec 22, 2017 at 07:12:35PM +0100, Jann Horn wrote:
> This checks that it is not possible to bypass the total stack size check in
> update_stack_depth() by calling a function that uses a large amount of
> stack memory *before* using a large amount of stack memory in the caller.
> 
> Currently, the first added testcase causes a rejection as expected, but
> the second testcase is (AFAICS incorrectly) accepted:
> 
> [...]
> #483/p calls: stack overflow using two frames (post-call access) FAIL
> Unexpected success to load!
> 0: (85) call pc+2
> caller:
>  R10=fp0,call_-1
> callee:
>  frame1: R1=ctx(id=0,off=0,imm=0) R10=fp0,call_0
> 3: (72) *(u8 *)(r10 -300) = 0
> 4: (b7) r0 = 0
> 5: (95) exit
> returning from callee:
>  frame1: R0_w=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0,call_0
> to caller at 1:
>  R0_w=inv0 R10=fp0,call_-1
> 
> from 5 to 1: R0=inv0 R10=fp0,call_-1
> 1: (72) *(u8 *)(r10 -300) = 0
> 2: (95) exit
> processed 6 insns, stack depth 300+300

got it. thanks for the test!
working on a fix.