kernel virtual memory access (from app) does not generate segfault

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Tue, Apr 20, 2010 at 06:09:44PM +0100, Ben Dooks wrote:
> On Tue, Apr 20, 2010 at 10:20:47PM +0800, anfei wrote:
> > On Tue, Apr 20, 2010 at 11:27:40AM +0100, Dave P. Martin wrote:
> > >  
> > > 
> > > > -----Original Message-----
> > > > From: linux-arm-kernel-bounces at lists.infradead.org 
> > > > [mailto:linux-arm-kernel-bounces at lists.infradead.org] On 
> > > > Behalf Of Ben Dooks
> > > > Sent: 20 April 2010 10:35
> > > > To: Sasha Sirotkin
> > > > Cc: linux-arm-kernel at lists.infradead.org
> > > > Subject: Re: kernel virtual memory access (from app) does not 
> > > 
> > > [..]
> > > 
> > > > > For instance, this code generates a segfault allright
> > > > >
> > > > > int * aa;
> > > > > aa = 0xc0000000;
> > > > > *aa=42;
> > > > >
> > > > > However this code does not, instead the process simply 
> > > > hangs (and can 
> > > > > be
> > > > > killed)
> > > > >
> > > > > void (*func)(void);
> > > > > func = 0xc0000000;
> > > > > func();
> > > > 
> > > > Your first example writes to an area, your second is 
> > > > execution. IIRC, this version of the ARM architecture equates 
> > > > read and execute permission and so you may actually have 
> > > > permission to read this area and thus execute code in it.

User programs do not have permission to read kernel addresses.  Trying to
do so _should_ generate a permission fault.

> > > I tried reading that address (albeit on an old 2.6.28 kernel), and I get a
> > > segfault.

... which is correct behaviour.

> > > Trying to execute in kernel space is the only thing that appears to hang.
> > > Attaching to the process in gdb, I observed that pc is always 0xc0000000
> > > when the process is stopped.
> > > 
> > > top accounts most of the CPU time as being consumed in the kernel.
> > > 
> > > I think what is going on here is that the kernel is catching the expected
> > > prefetch abort, but the handler fails to send SIGSEGV to the user process
> > > --- the process is resumed with the same pc and we end up in an endless
> > > spin.

Yes, that'd make sense.

> > diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> > index 9d40c34..cd4d15c 100644
> > --- a/arch/arm/mm/fault.c
> > +++ b/arch/arm/mm/fault.c
> > @@ -393,6 +393,9 @@ do_translation_fault(unsigned long addr, unsigned int fsr,
> >  	if (addr < TASK_SIZE)
> >  		return do_page_fault(addr, fsr, regs);
> >  
> > +	if (user_mode(regs) && addr >= TASK_SIZE)
> > +		goto bad_area;
> > +
> 
> technically, addr >= TASK_SIZE was guaranteed by the previous test
> on addr. The user_mode(regs) may well be a good idea, although I'm
> not sure if we get entered here if the kernel is attempting to access
> user-mode memory by forcing unpriveldged accesses.
> 
> probably best to get Russell's opinion.

	if (user_mode(regs))
		goto bad_area;

should be sufficient, since userspace should not be accessing anything
above TASK_SIZE, except for the exception page, which will always be
mapped.