Le vendredi 07 juin 2019 à 16:58 +0300, Laurent Pinchart a écrit :
> Hi Marek,
>
> On Fri, Jun 07, 2019 at 03:55:05PM +0200, Marek Szyprowski wrote:
> > On 2019-06-07 15:40, Hans Verkuil wrote:
> > > On 6/7/19 2:47 PM, Hans Verkuil wrote:
> > > > On 6/7/19 2:23 PM, Hans Verkuil wrote:
> > > > > On 6/7/19 2:14 PM, Marek Szyprowski wrote:
> > > > > > On 2019-06-07 14:01, Hans Verkuil wrote:
> > > > > > > On 6/7/19 1:16 PM, Laurent Pinchart wrote:
> > > > > > > > Thank you for the patch.
> > > > > > > >
> > > > > > > > On Fri, Jun 07, 2019 at 10:45:31AM +0200, Hans Verkuil wrote:
> > > > > > > > > The __prepare_userptr() function made the incorrect assumption that if the
> > > > > > > > > same user pointer was used as the last one for which memory was acquired, then
> > > > > > > > > there was no need to re-acquire the memory. This assumption was never properly
> > > > > > > > > tested, and after doing that it became clear that this was in fact wrong.
> > > > > > > > Could you explain in the commit message why the assumption is not
> > > > > > > > correct ?
> > > > > > > You can free the memory, then allocate it again and you can get the same pointer,
> > > > > > > even though it is not necessarily using the same physical pages for the memory
> > > > > > > that the kernel is still using for it.
> > > > > > >
> > > > > > > Worse, you can free the memory, then allocate only half the memory you need and
> > > > > > > get back the same pointer. vb2 wouldn't notice this. And it seems to work (since
> > > > > > > the original mapping still remains), but this can corrupt userspace memory
> > > > > > > causing the application to crash. It's not quite clear to me how the memory can
> > > > > > > get corrupted. I don't know enough of those low-level mm internals to understand
> > > > > > > the sequence of events.
> > > > > > >
> > > > > > > I have test code for v4l2-compliance available if someone wants to test this.
> > > > > > I'm interested, I would really like to know what happens in the mm
> > > > > > subsystem in such case.
> > > > > Here it is:
> > > > >
> > > > > diff --git a/utils/v4l2-compliance/v4l2-test-buffers.cpp b/utils/v4l2-compliance/v4l2-test-buffers.cpp
> > > > > index be606e48..9abf41da 100644
> > > > > --- a/utils/v4l2-compliance/v4l2-test-buffers.cpp
> > > > > +++ b/utils/v4l2-compliance/v4l2-test-buffers.cpp
> > > > > @@ -797,7 +797,7 @@ int testReadWrite(struct node *node)
> > > > > return 0;
> > > > > }
> > > > >
> > > > > -static int captureBufs(struct node *node, const cv4l_queue &q,
> > > > > +static int captureBufs(struct node *node, cv4l_queue &q,
> > > > > const cv4l_queue &m2m_q, unsigned frame_count, int pollmode,
> > > > > unsigned &capture_count)
> > > > > {
> > > > > @@ -962,6 +962,21 @@ static int captureBufs(struct node *node, const cv4l_queue &q,
> > > > > buf.s_flags(V4L2_BUF_FLAG_REQUEST_FD);
> > > > > buf.s_request_fd(buf_req_fds[req_idx]);
> > > > > }
> > > > > + if (v4l_type_is_capture(buf.g_type()) && q.g_memory() == V4L2_MEMORY_USERPTR) {
> > > > > + printf("\nidx: %d", buf.g_index());
> > > > > + for (unsigned p = 0; p < q.g_num_planes(); p++) {
> > > > > + printf(" old buf[%d]: %p ", p, buf.g_userptr(p));
> > > > > + fflush(stdout);
> > > > > + free(buf.g_userptr(p));
> > > > > + void *m = calloc(1, q.g_length(p)/2);
> > > > > +
> > > > > + fail_on_test(m == NULL);
> > > > > + q.s_userptr(buf.g_index(), p, m);
> > > > > + printf("new buf[%d]: %p", p, m);
> > > > > + buf.s_userptr(m, p);
> > > > > + }
> > > > > + printf("\n");
> > > > > + }
> > > > > fail_on_test(buf.qbuf(node, q));
> > > > > fail_on_test(buf.g_flags() & V4L2_BUF_FLAG_DONE);
> > > > > if (buf.g_flags() & V4L2_BUF_FLAG_REQUEST_FD) {
> > > > >
> > > > >
> > > > >
> > > > > Load the vivid driver and just run 'v4l2-compliance -s10' and you'll see:
> > > > >
> > > > > ...
> > > > > Streaming ioctls:
> > > > > test read/write: OK
> > > > > test blocking wait: OK
> > > > > test MMAP (no poll): OK
> > > > > test MMAP (select): OK
> > > > > test MMAP (epoll): OK
> > > > > Video Capture: Frame #000
> > > > > idx: 0 old buf[0]: 0x7f71c6e7c010 new buf[0]: 0x7f71c6eb4010
> > > > > Video Capture: Frame #001
> > > > > idx: 1 old buf[0]: 0x7f71c6e0b010 new buf[0]: 0x7f71c6e7b010
> > > > > Video Capture: Frame #002
> > > > > idx: 0 old buf[0]: 0x7f71c6eb4010 free(): invalid pointer
> > > > > Aborted
> > > > To clarify: two full size buffers are allocated and queued (that happens in setupUserPtr()),
> > > > then streaming starts and captureBufs is called which basically just calls dqbuf
> > > > and qbuf.
> > > >
> > > > Tomasz pointed out that all the pointers in this log are actually different. That's
> > > > correct, but here is a log where the old and new buf ptr are the same:
> > > >
> > > > Streaming ioctls:
> > > > test read/write: OK
> > > > test blocking wait: OK
> > > > test MMAP (no poll): OK
> > > > test MMAP (select): OK
> > > > test MMAP (epoll): OK
> > > > Video Capture: Frame #000
> > > > idx: 0 old buf[0]: 0x7f1094e16010 new buf[0]: 0x7f1094e4e010
> > > > Video Capture: Frame #001
> > > > idx: 1 old buf[0]: 0x7f1094da5010 new buf[0]: 0x7f1094e15010
> > > > Video Capture: Frame #002
> > > > idx: 0 old buf[0]: 0x7f1094e4e010 new buf[0]: 0x7f1094e4e010
> > > > Video Capture: Frame #003
> > > > idx: 1 old buf[0]: 0x7f1094e15010 free(): invalid pointer
> > > > Aborted
> > > >
> > > > It's weird that the first log fails that way: if the pointers are different,
> > > > then vb2 will call get_userptr and it should discover that the buffer isn't
> > > > large enough, causing qbuf to fail. That doesn't seem to happen.
> > > I think that the reason for this corruption is that the memory pool used
> > > by glibc is now large enough for vb2 to think it can map the full length
> > > of the user pointer into memory, even though only the first half is actually
> > > from the buffer that's allocated. When you capture a frame you just overwrite
> > > a random part of the application's memory pool, causing this invalid pointer.
> > >
> > > But that's a matter of garbage in, garbage out. So that's not the issue here.
> > >
> > > The real question is what happens when you free the old buffer, allocate a
> > > new buffer, end up with the same userptr, but it's using one or more different
> > > pages for its memory compared to the mapping that the kernel uses.
> > >
> > > I managed to reproduce this with v4l2-ctl:
> > >
> > > diff --git a/utils/v4l2-ctl/v4l2-ctl-streaming.cpp b/utils/v4l2-ctl/v4l2-ctl-streaming.cpp
> > > index 28b2b3b9..8f2ed9b5 100644
> > > --- a/utils/v4l2-ctl/v4l2-ctl-streaming.cpp
> > > +++ b/utils/v4l2-ctl/v4l2-ctl-streaming.cpp
> > > @@ -1422,6 +1422,24 @@ static int do_handle_cap(cv4l_fd &fd, cv4l_queue &q, FILE *fout, int *index,
> > > * has the size that fits the old resolution and might not
> > > * fit to the new one.
> > > */
> > > + if (q.g_memory() == V4L2_MEMORY_USERPTR) {
> > > + printf("\nidx: %d", buf.g_index());
> > > + for (unsigned p = 0; p < q.g_num_planes(); p++) {
> > > + unsigned *pb = (unsigned *)buf.g_userptr(p);
> > > + printf(" old buf[%d]: %p first pixel: 0x%x", p, buf.g_userptr(p), *pb);
> > > + fflush(stdout);
> > > + free(buf.g_userptr(p));
> > > + void *m = calloc(1, q.g_length(p));
> > > +
> > > + if (m == NULL)
> > > + return QUEUE_ERROR;
> > > + q.s_userptr(buf.g_index(), p, m);
> > > + if (m == buf.g_userptr(p))
> > > + printf(" identical new buf");
> > > + buf.s_userptr(m, p);
> > > + }
> > > + printf("\n");
> > > + }
> > > if (fd.qbuf(buf) && errno != EINVAL) {
> > > fprintf(stderr, "%s: qbuf error\n", __func__);
> > > return QUEUE_ERROR;
> > >
> > >
> > > Load vivid, setup a pure white test pattern:
> > >
> > > v4l2-ctl -c test_pattern=6
> > >
> > > Now run v4l2-ctl --stream-user and you'll see:
> > >
> > > idx: 0 old buf[0]: 0x7f91551cb010 first pixel: 0x80ea80ea identical new buf
> > > <
> > > idx: 1 old buf[0]: 0x7f915515a010 first pixel: 0x80ea80ea identical new buf
> > > <
> > > idx: 2 old buf[0]: 0x7f91550e9010 first pixel: 0x80ea80ea identical new buf
> > > <
> > > idx: 3 old buf[0]: 0x7f9155078010 first pixel: 0x80ea80ea identical new buf
> > > <
> > > idx: 0 old buf[0]: 0x7f91551cb010 first pixel: 0x0 identical new buf
> > > <
> > > idx: 1 old buf[0]: 0x7f915515a010 first pixel: 0x0 identical new buf
> > > < 5.00 fps
> > >
> > > idx: 2 old buf[0]: 0x7f91550e9010 first pixel: 0x0 identical new buf
> > > <
> > > idx: 3 old buf[0]: 0x7f9155078010 first pixel: 0x0 identical new buf
> > >
> > > The first four dequeued buffers are filled with data, after that the
> > > returned buffer is empty because vivid is actually writing to different
> > > memory pages.
> > >
> > > With this patch the first pixel is always non-zero.
> >
> > Good catch. The question is weather we treat that as undefined behavior
> > and keep the optimization for 'good applications' or assume that every
> > broken userspace code has to be properly handled.
>
> Given how long we've been saying that USERPTR should be replaced by
> DMABUF, I would consider that any userspace code using USERPTR is broken
> :-) One could however question whether we were effective at getting that
> message across...
Just a reminder that DMABuf is not a replacement for USERPTR. It only
cover a subset in absence of an allocater for it. There is no clean way
to allocate a DMAbuf. Notably, memfds (which could have filled the gap)
are not DMABuf, even though they are they are similar to the buffers
allocated by vivid or uvcvideo.
>
> > The good thing is that
> > there is still imho no security issue. The physical pages gathered by
> > vb2 in worst case belongs to noone else (vb2 is their last user, they
> > are not yet returned to free pages pool).
> >
> > > I wonder if it isn't possible to just check the physical address of
> > > the received user pointer with the physical address of the previous
> > > user pointer. Or something like that. I'll dig around a bit more.
> >
> > Such check won't be so simple. Pages contiguous in the virtual memory
> > won't map to pages contiguous in the physical memory, so you would need
> > to check every single memory page. Make no sense. It is better to
> > reacquire buffer on every queue operation. This indeed show how broken
> > the USERPTR related part of v4l2 API is.
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