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Subject: Re: [PATCH] bpf: selftest for late caller stack size increase
To: Jann Horn <jannh@google.com>, Alexei Starovoitov <ast@kernel.org>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20171222181235.158636-1-jannh@google.com>
From: Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <43d7dc08-1c02-cb8c-40bd-14b715e31bba@iogearbox.net>
Date: Thu, 28 Dec 2017 00:07:09 +0100
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On 12/22/2017 07:12 PM, 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
> [...]
> Summary: 704 PASSED, 1 FAILED
> 
> AFAICS the JIT-generated code for the second testcase shows that this
> really causes the stack pointer to be decremented by 300+300:
> 
> first function:
> 00000000  55                push rbp
> 00000001  4889E5            mov rbp,rsp
> 00000004  4881EC58010000    sub rsp,0x158
> 0000000B  4883ED28          sub rbp,byte +0x28
> [...]
> 00000025  E89AB3AFE5        call 0xffffffffe5afb3c4
> 0000002A  C685D4FEFFFF00    mov byte [rbp-0x12c],0x0
> [...]
> 00000041  4883C528          add rbp,byte +0x28
> 00000045  C9                leave
> 00000046  C3                ret
> 
> second function:
> 00000000  55                push rbp
> 00000001  4889E5            mov rbp,rsp
> 00000004  4881EC58010000    sub rsp,0x158
> 0000000B  4883ED28          sub rbp,byte +0x28
> [...]
> 00000025  C685D4FEFFFF00    mov byte [rbp-0x12c],0x0
> [...]
> 0000003E  4883C528          add rbp,byte +0x28
> 00000042  C9                leave
> 00000043  C3                ret
> 
> Signed-off-by: Jann Horn <jannh@google.com>

Applied to bpf-next, thanks a lot Jann!