Re: [RFC PATCH] drm/radeon: fixup locking inversion between mmap_sem and reservations

[Jerome Glisse <j.glisse@gmail.com>]

On Tue, Oct 08, 2013 at 06:29:35PM +0200, Thomas Hellstrom wrote:
> On 10/08/2013 04:55 PM, Jerome Glisse wrote:
> >On Tue, Oct 08, 2013 at 04:45:18PM +0200, Christian König wrote:
> >>Am 08.10.2013 16:33, schrieb Jerome Glisse:
> >>>On Tue, Oct 08, 2013 at 04:14:40PM +0200, Maarten Lankhorst wrote:
> >>>>Allocate and copy all kernel memory before doing reservations. This prevents a locking
> >>>>inversion between mmap_sem and reservation_class, and allows us to drop the trylocking
> >>>>in ttm_bo_vm_fault without upsetting lockdep.
> >>>>
> >>>>Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
> >>>I would say NAK. Current code only allocate temporary page in AGP case.
> >>>So AGP case is userspace -> temp page -> cs checker -> radeon ib.
> >>>
> >>>Non AGP is directly memcpy to radeon IB.
> >>>
> >>>Your patch allocate memory memcpy userspace to it and it will then be
> >>>memcpy to IB. Which means you introduce an extra memcpy in the process
> >>>not something we want.
> >>Totally agree. Additional to that there is no good reason to provide
> >>anything else than anonymous system memory to the CS ioctl, so the
> >>dependency between the mmap_sem and reservations are not really
> >>clear to me.
> >>
> >>Christian.
> >I think is that in other code path you take mmap_sem first then reserve
> >bo. But here we reserve bo and then we take mmap_sem because of copy
> >from user.
> >
> >Cheers,
> >Jerome
> >
> Actually the log message is a little confusing. I think the mmap_sem
> locking inversion problem is orthogonal to what's being fixed here.
> 
> This patch fixes the possible recursive bo::reserve caused by
> malicious user-space handing a pointer to ttm memory so that the ttm
> fault handler is called when bos are already reserved. That may
> cause a (possibly interruptible) livelock.
> 
> Once that is fixed, we are free to choose the mmap_sem ->
> bo::reserve locking order. Currently it's bo::reserve->mmap_sem(),
> but the hack required in the ttm fault handler is admittedly a bit
> ugly.  The plan is to change the locking order to
> mmap_sem->bo::reserve
> 
> I'm not sure if it applies to this particular case, but it should be
> possible to make sure that copy_from_user_inatomic() will always
> succeed, by making sure the pages are present using
> get_user_pages(), and release the pages after
> copy_from_user_inatomic() is done. That way there's no need for a
> double memcpy slowpath, but if the copied data is very fragmented I
> guess the resulting code may look ugly. The get_user_pages()
> function will return an error if it hits TTM pages.
> 
> /Thomas

get_user_pages + copy_from_user_inatomic is overkill. We should just
do get_user_pages which fails with ttm memory and then use copy_highpage
helper.

Cheers,
Jerome