Subject: Re: [PATCH 2/5] powerpc: Allow 4096 bytes of stack expansion for the signal frame

From: Daniel Axtens <[email-masked]>
Date: Thu, 23 Jul 2020 23:35:36 +1000

Hi Michael,

Unfortunately, this patch doesn't completely solve the problem.

Trying the original reproducer, I'm still able to trigger the crash even
with this patch, although not 100% of the time. (If I turn ASLR off
outside of tmux it reliably crashes, if I turn ASLR off _inside_ of tmux
it reliably succeeds; all of this is on a serial console.)

./foo 1241000 & sleep 1; killall -USR1 foo; echo ok

If I add some debugging information, I see that I'm getting
address + 4096 = 7fffffed0fa0
gpr1 =           7fffffed1020

So address + 4096 is 0x80 bytes below the 4k window. I haven't been able
to figure out why, gdb gives me a NIP in __kernel_sigtramp_rt64 but I
don't know what to make of that.

Kind regards,
Daniel

P.S. I don't know what your policy on linking to kernel bugzilla is, but
if you want:

Link: https://bugzilla.kernel.org/show_bug.cgi?id=205183

> Reported-by: Tom Lane <[email-masked]>
> Signed-off-by: Michael Ellerman <[email-masked]>
> ---
>  arch/powerpc/mm/fault.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 641fc5f3d7dd..ed01329dd12b 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -274,7 +274,7 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
>  	/*
>  	 * N.B. The POWER/Open ABI allows programs to access up to
>  	 * 288 bytes below the stack pointer.
> -	 * The kernel signal delivery code writes up to about 1.5kB
> +	 * The kernel signal delivery code writes up to 4KB
>  	 * below the stack pointer (r1) before decrementing it.
>  	 * The exec code can write slightly over 640kB to the stack
>  	 * before setting the user r1.  Thus we allow the stack to
> @@ -299,7 +299,7 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
>  		 * between the last mapped region and the stack will
>  		 * expand the stack rather than segfaulting.
>  		 */
> -		if (address + 2048 >= uregs->gpr[1])
> +		if (address + 4096 >= uregs->gpr[1])
>  			return false;
>  
>  		if ((flags & FAULT_FLAG_WRITE) && (flags & FAULT_FLAG_USER) &&
> -- 
> 2.25.1