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

[Catalin Marinas]

On Tue, Feb 16, 2016 at 01:31:36PM -0800, Arjan van de Ven wrote:
> On 2/16/2016 10:50 AM, Linus Torvalds wrote:
> >On Tue, Feb 16, 2016 at 9:04 AM, Will Deacon <will.deacon@xxxxxxx> wrote:
> >>[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.
> 
> how does this work if the adjacent page is not accessible?

do_strncpy_from_user() assumes by default that it can read a word at a
time using get_user() but checks its return value in case it failed and
falls back to byte at a time. What happens on arm64 is that for
alignment faults we don't have a handler that would search the exception
table and run the get_user() fixup.

> isn't the general rule for such basic functions "don't touch memory
> unless you KNOW it is there"

Well, user access routines are in general safe with this via the
exception handling + fixup mechanism (which usually returns -EFAULT).
That's what do_strncpy_from_user() tries to do by optimising away the
boundary checks.

-- 
Catalin