linux-kernel - Re: [PATCH v2] arm64: add alignment fault hanling

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Date:	Tue, 16 Feb 2016 16:00:55 +0000
From:	Will Deacon <will.deacon@....com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Robin Murphy <robin.murphy@....com>,
	EunTaik Lee <eun.taik.lee@...sung.com>,
	"vladimir.murzin@....com" <vladimir.murzin@....com>,
	"suzuki.poulose@....com" <suzuki.poulose@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"salyzyn@...roid.com" <salyzyn@...roid.com>,
	"riandrews@...roid.com" <riandrews@...roid.com>,
	"james.morse@....com" <james.morse@....com>,
	"Dave.Martin@....com" <Dave.Martin@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] arm64: add alignment fault hanling

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.

Will