Re: [PATCH v2] arm64: add alignment fault hanling

[Robin Murphy]

On 16/02/16 10:31, Will Deacon wrote:
> On Tue, Feb 16, 2016 at 04:44:35AM +0000, EunTaik Lee wrote:
> > Userspace memory is mapped as below:
> > F2A7F000--F2A7FFFF Normal Memory
> > F2A80000--F2A80FFF Device nGnRnE
> >
> > And that userspace application makes a system call
> > as below:
> >
> > -009 |do_strncpy_from_user(inline)
> > -009 |strncpy_from_user()
> > -010 |getname_flags()
> > -011 |user_path_at_empty()
> > -012 |user_path_at()
> > -013 |SYSC_faccessat(inline)
> > -013 |sys_faccessat()
> > -014 |__sys_trace(asm)
> >   --> |exception
> >
> > The string spans from 0xF2A7FFC1 to 0xF2A7FFFB.
> >
> > When do_strncpy_from_user() reads the last (unsigned long)
> > value, the alignement fault is triggered. The 8 byte
> > from 0xF2A7FFC1 spans to the next page that is mapped as
> > Device nGnRnE, which does not allow an unaligned access,
> > causes the abort.
> >
> > The instruction which caused the alignment fault is registered
> > in the fixup table but the exception handler does not reach there.
> >
> > This patch registers a alignment fault handler and fixes up the
> > pc if appropriate.
>
> As discussed with Catalin previously, we should solve this by adding a
> guard page rather than handling the fault.

...especially since we may not even get a fault. See "Crossing a page 
boundary with different memory types or Shareability attributes" in the 
UNPREDICTABLE spec (K1.2.10 in the latest ARMv8 ARM).

Robin.

> Will
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel