Em Tue, 08 Jan 2019 18:05:57 +0200
Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxx> escreveu:
> > > > > Em Tue, 8 Jan 2019 10:58:34 +0200
> > > > >
> > > > > Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > > > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > > > Reviewed-by: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>
> > > > > > ---
> > > > > >
> > > > > > 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
