Subject: Re: [PATCH v2] powerpc/64: Fix memcmp reading past the end of src/dest

From: Segher Boessenkool <[email-masked]>
Date: Thu, 7 Feb 2019 06:52:56 -0600

On Thu, Feb 07, 2019 at 10:53:13PM +1100, Michael Ellerman wrote:
> Chandan reported that fstests' generic/026 test hit a crash:

> The instruction dump decodes as:
>   subfic  r6,r5,8
>   rlwinm  r6,r6,3,0,28
>   ldbrx   r9,0,r3
>   ldbrx   r10,0,r4	<-
> 
> Which shows us doing an 8 byte load from c00000062ac3fff9, which
> crosses the page boundary at c00000062ac40000 and faults.
> 
> It's not OK for memcmp to read past the end of the source or
> destination buffers.

It's not okay to access memory pages unsolicited.  Reading past the end
is fine per se.

Segher