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

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

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.

Btw, in this particular case, I would use a WARN_ON(), as this is
something that indicates not only a driver bug (as the driver is
letting someone to request a buffer a way too big), but probably
also an attempt from a hacker to try to crack the system.

Thanks,
Mauro