Re: [PATCH 2/3] kexec: ensure user memory sizes do not wrap

[Baoquan He <bhe@redhat.com>]

On 04/18/16 at 09:37am, Russell King - ARM Linux wrote:
> On Mon, Apr 18, 2016 at 01:35:34PM +0800, Baoquan He wrote:
> > On 04/14/16 at 09:00pm, Russell King wrote:
> > > Ensure that user memory sizes do not wrap around when validating the
> > > user input, which can lead to the following input validation working
> > > incorrectly.
> > > 
> > > Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
> > > ---
> > >  kernel/kexec_core.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > > index 8d34308ea449..d719a4d0ef55 100644
> > > --- a/kernel/kexec_core.c
> > > +++ b/kernel/kexec_core.c
> > > @@ -169,6 +169,8 @@ int sanity_check_segment_list(struct kimage *image)
> > >  
> > >  		mstart = image->segment[i].mem;
> > >  		mend   = mstart + image->segment[i].memsz;
> > > +		if (mstart > mend)
> > > +			return result;
> > 
> > These segments are built in kexec utility, their availability should be
> > guaranteed there. If without some alignment handling, wrapping around
> > might not happen here. But I don't have strong objection to it.
> 
> In which case, what's the point of all the other validation which is done,
> like the check below:
> 
> > >  		if ((mstart & ~PAGE_MASK) || (mend & ~PAGE_MASK))
> > >  			return result;
> > >  		if (mend >= KEXEC_DESTINATION_MEMORY_LIMIT)
> 
> Your reply is contradictory to the whole suite of tests which kexec does
> to validate its input from userspace.

It's not contradictory. In kexec utility it will call
valid_memory_segment() to check each segment. And there it will check if
the start is bigger than end. What I meant is if start is 5000, end is
5100, an alignment of end will make start> end case happen. Anyway I am
fine with this check adding, the safer, the better.

> 
> -- 
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