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SubjectRe: [PATCH] bpf: selftest for late caller stack size increase
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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!

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