Re: [PATCH v2 1/3] videobuf2-core: Prevent size alignment wrapping buffer size to 0

[Mauro Carvalho Chehab <mchehab+samsung@kernel.org>]

Em Tue, 08 Jan 2019 18:05:57 +0200
Laurent Pinchart <laurent.pinchart@ideasonboard.com> escreveu:

> On Tuesday, 8 January 2019 16:30:22 EET Mauro Carvalho Chehab wrote:
> > Em Tue, 8 Jan 2019 15:40:47 +0200
> > 
> > Sakari Ailus <sakari.ailus@linux.intel.com> escreveu:  
> > > On Tue, Jan 08, 2019 at 10:59:55AM -0200, Mauro Carvalho Chehab wrote:  
> > > > Em Tue, 8 Jan 2019 10:52:12 -0200
> > > > 
> > > > Mauro Carvalho Chehab <mchehab@kernel.org> escreveu:  
> > > > > Em Tue,  8 Jan 2019 10:58:34 +0200
> > > > > 
> > > > > Sakari Ailus <sakari.ailus@linux.intel.com> escreveu:  
> > > > > > PAGE_ALIGN() may wrap the buffer size around to 0. Prevent this by
> > > > > > checking that the aligned value is not smaller than the unaligned
> > > > > > one.
> > > > > > 
> > > > > > Note on backporting to stable: the file used to be under
> > > > > > drivers/media/v4l2-core, it was moved to the current location after
> > > > > > 4.14.
> > > > > > 
> > > > > > Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> > > > > > Cc: stable@vger.kernel.org
> > > > > > Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> > > > > > ---
> > > > > > 
> > > > > >  drivers/media/common/videobuf2/videobuf2-core.c | 4 ++++
> > > > > >  1 file changed, 4 insertions(+)
> > > > > > 
> > > > > > diff --git a/drivers/media/common/videobuf2/videobuf2-core.c
> > > > > > b/drivers/media/common/videobuf2/videobuf2-core.c index
> > > > > > 0ca81d495bda..0234ddbfa4de 100644
> > > > > > --- a/drivers/media/common/videobuf2/videobuf2-core.c
> > > > > > +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> > > > > > @@ -207,6 +207,10 @@ static int __vb2_buf_mem_alloc(struct
> > > > > > vb2_buffer *vb)
> > > > > > 
> > > > > >  	for (plane = 0; plane < vb->num_planes; ++plane) {
> > > > > >  	
> > > > > >  		unsigned long size = PAGE_ALIGN(vb->planes[plane].length);
> > > > > > 
> > > > > > +		/* Did it wrap around? */
> > > > > > +		if (size < vb->planes[plane].length)
> > > > > > +			goto free;
> > > > > > +  
> > > > > 
> > > > > Sorry, but I can't see how this could ever happen (except for a very
> > > > > serious bug at the compiler or at the hardware).
> > > > > 
> > > > > See, the definition at PAGE_ALIGN is (from mm.h):
> > > > > 	#define PAGE_ALIGN(addr) ALIGN(addr, PAGE_SIZE)
> > > > > 
> > > > > and the macro it uses come from kernel.h:
> > > > > 	#define __ALIGN_KERNEL(x, a)		__ALIGN_KERNEL_MASK(x, (typeof(x))  
> (a) -
> > > > > 	1)
> > > > > 	#define __ALIGN_KERNEL_MASK(x, mask)	(((x) + (mask)) & ~(mask))
> > > > > 	..
> > > > > 	#define ALIGN(x, a)		__ALIGN_KERNEL((x), (a))
> > > > > 
> > > > > So, this:
> > > > > 	size = PAGE_ALIGN(length);
> > > > > 
> > > > > (assuming PAGE_SIZE= 0x1000)
> > > > > 
> > > > > becomes:
> > > > > 	size = (length + 0x0fff) & ~0xfff;
> > > > > 
> > > > > so, size will *always* be >= length.  
> > > > 
> > > > Hmm... after looking at patch 2, now I understand what's your concern...
> > > > 
> > > > If someone indeed uses length = INT_MAX, size will indeed be zero.
> > > > 
> > > > Please adjust the description accordingly, as it doesn't reflect
> > > > that.  
> > > 
> > > How about:
> > > 
> > > PAGE_ALIGN() may wrap the buffer length around to 0 if the value to be
> > > aligned is close to the top of the value range of the type. Prevent this
> > > by
> > > checking that the aligned value is not smaller than the unaligned one.  
> > 
> > I would be a way more clear, as this is there to prevent a single
> > special case: length == ULEN_MAX. Something like:
> > 
> > 	If one tried to allocate a buffer with sizeof(ULEN_MAX), this will cause
> > 	an overflow at PAGE_ALIGN(), making it return zero as the size of the
> > 	buffer, causing the code to fail.
> > 
> > I would even let it clearer at the code itself. So, instead of the
> > hunk you proposed, I would do:
> > 
> > 	unsigned long size = vb->planes[plane].length;
> > 
> > 	/* Prevent PAGE_ALIGN overflow */
> > 	if (WARN_ON(size == ULONG_MAX))
> > 		goto free;  
> 
> ULONG_MAX - PAGE_SIZE + 2 to ULONG_MAX would all cause the same issue.

True. The actual check should be:

	if (WARN_ON(size > ULONG_MAX - PAGE_SIZE + 1))

Not that the original proposal of checking after the overflow is wrong, but, 
IMHO, something linking the size to ULONG_MAX makes clearer about what kind
of issue the code is meant to solve. A good comment before that would work
fine.

Thanks,
Mauro