Re: [PATCH] drm/i915: Convert WARNs during userptr revoke to SIGBUS

Chris Wilson
2015-09-28T13:52:38Z

On Mon, Sep 28, 2015 at 03:42:22PM +0200, Daniel Vetter wrote:
> On Wed, Sep 23, 2015 at 09:07:24PM +0100, Chris Wilson wrote:
> > If the client revokes the virtual address it asked to be mapped into GPU
> > space via userptr (by using anything along the lines of mmap, mprotect,
> > madvise, munmap, ftruncate etc) the mmu notifier sends a range
> > invalidate command to userptr. Upon receiving the invalidation signal
> > for the revoked range, we try to release the struct pages we pinned into
> > the GTT. However, this can fail if any of the GPU's VMA are pinned for
> > use by the hardware (i.e. despite the user's intention we cannot
> > relinquish the client's address range and keep uptodate with whatever is
> > placed in there). Currently we emit a few WARN so that we would notice
> > if this every occurred in the wild; it has. Sadly this means we need to
> > replace those WARNs with the proper SIGBUS to the offending clients
> > instead.
> >
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > Cc: Michał Winiarski <michal.winiarski@intel.com>
> > ---
> > drivers/gpu/drm/i915/i915_gem_userptr.c | 41 +++++++++++++++++++++++++++++----
> > 1 file changed, 37 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
> > index f75d90118888..efb404b9fe69 100644
> > --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> > +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> > @@ -81,11 +81,44 @@ static void __cancel_userptr__worker(struct work_struct *work)
> > was_interruptible = dev_priv->mm.interruptible;
> > dev_priv->mm.interruptible = false;
> >
> > - list_for_each_entry_safe(vma, tmp, &obj->vma_list, obj_link) {
> > - int ret = i915_vma_unbind(vma);
> > - WARN_ON(ret && ret != -EIO);
> > + list_for_each_entry_safe(vma, tmp, &obj->vma_list, obj_link)
> > + i915_vma_unbind(vma);
> > + if (i915_gem_object_put_pages(obj)) {
> > + struct task_struct *p;
> > +
> > + DRM_ERROR("Unable to revoke ownership by userptr of"
> > + " invalidated address range, sending SIGBUS"
> > + " to attached clients.\n");
> > +
> > + rcu_read_lock();
> > + for_each_process(p) {
> > + siginfo_t info;
> > +
> > + /* This doesn't capture everyone who has
> > + * the pages pinned behind a VMA as we
> > + * do not have that tracking information
> > + * available, it does however kill the
> > + * original process (and siblings) who
> > + * created the userptr and presumably tried
> > + * to reuse the address space despite having
> > + * pinned it (possibly indirectly) to the hw.
> > + * Arguably, we don't even want to kill the
> > + * other processes as they are not at fault,
> > + * likely to be a display server, and hopefully
> > + * will release the pages in due course once
> > + * the client is dead.
> > + */
> > + if (p->mm != obj->userptr.mm->mm)
> > + continue;
> > +
> > + info.si_signo = SIGBUS;
> > + info.si_errno = 0;
> > + info.si_code = BUS_ADRERR;
> > + info.si_addr = (void __user *)obj->userptr.ptr;
> > + force_sig_info(SIGBUS, &info, p);
> > + }
> > + rcu_read_unlock();
>
> Why do we need to send a SIGBUS? It won't tear down the offending gem bo,
> any new users will hopefully get it, and abusing SIGBUS without the thread
> actually doing a memory access is a bit surprising. DRM_DEBUG seems to be
> the most we can do here I think - I think userspace being able to do this
> is just a fundamental property of userptr.

It is not the bo that is at fault but the *client's* *address* *space*
that is being changed. It is equivalent to mmap on a truncated file i.e.
if the client tries to use its mmapping after it has truncated the file
it is scolded via SIGBUS.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre