[PATCH v2] arm64: add alignment fault hanling

[will.deacon@arm.com (Will Deacon)]

[replying to self and adding some x86 people]

Background: Euntaik reports a problem where userspace has ended up with
a memory page mapped adjacent to an MMIO page (e.g. from /dev/mem or a
PCI memory bar from someplace in /sys). strncpy_from_user happens with
the word-at-a-time implementation, and we end up reading into the MMIO
page.

Question: Does x86 guarantee that this faults? (Arjan reckoned no, but
wasn't 100%).

On Tue, Feb 16, 2016 at 04:00:55PM +0000, Will Deacon wrote:
> On Tue, Feb 16, 2016 at 12:21:53PM +0000, Catalin Marinas wrote:
> > On Tue, Feb 16, 2016 at 10:57:49AM +0000, Robin Murphy wrote:
> > > 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.
> > 
> > I don't think we can trivially add this without implementing an arm64
> > specific arch_get_unmapped_area().
> 
> Even overriding arch_get_unmapped_area doesn't help as much as you might
> like since, in the case of something like /dev/mem, the memory is remapped
> using remap_pfn_range later on, so you can't necessarily tell what the
> final attributes are likely to be when you initially allocate the virtual
> space.

Thinking more about this, we could spit out a guard page between every
VMA, but it's likely to hamper any VMA merging.

Will