[PATCH 2/5] drm/radeon: add userptr flag to limit it to anonymous memory v2

Jerome Glisse j.glisse at gmail.com
Wed Aug 6 11:34:16 PDT 2014

On Wed, Aug 06, 2014 at 07:17:25PM +0200, Christian König wrote:
> Am 06.08.2014 um 18:08 schrieb Jerome Glisse:
> >On Wed, Aug 06, 2014 at 08:55:28AM +0200, Christian König wrote:
> >>Am 06.08.2014 um 00:13 schrieb Jerome Glisse:
> >>>On Tue, Aug 05, 2014 at 07:45:21PM +0200, Christian König wrote:
> >>>>Am 05.08.2014 um 19:39 schrieb Jerome Glisse:
> >>>>>On Tue, Aug 05, 2014 at 06:05:29PM +0200, Christian König wrote:
> >>>>>>From: Christian König <christian.koenig at amd.com>
> >>>>>>
> >>>>>>Avoid problems with writeback by limiting userptr to anonymous memory.
> >>>>>>
> >>>>>>v2: add commit and code comments
> >>>>>I guess, i have not expressed myself clearly. This is bogus, you pretend
> >>>>>you want to avoid writeback issue but you still allow userspace to map
> >>>>>file backed pages (which by the way might be a regular bo object from
> >>>>>another device for instance and that would be fun).
> >>>>>
> >>>>>So this patch is a no go and i would rather see that this userptr to
> >>>>>be restricted to anon vma only no matter what. No flags here.
> >>>>Mapping of non anonymous memory (e.g. everything get_user_pages won't fail
> >>>>with) is restricted to read only access by the GPU.
> >>>>
> >>>>I'm fine with making it a hard requirement for all mappings if you say it's
> >>>>a must have.
> >>>>
> >>>Well for time being you should force read only. The way you implement write
> >>>is broken. Here is how it can abuse to allow write to a file backed mmap.
> >>>
> >>>mmap(fixaddress,fixedsize,NOFD)
> >>>userptr_ioctl(fixedaddress, RADEON_GEM_USERPTR_ANONONLY)
> >>>// bo is created successfully because fixedaddress is part of anonvma
> >>>munmap(fixedaddress,fixedsize)
> >>>// radeon get mmu_notifier_range_start callback and unbind page from the
> >>>// bo but radeon does not know there was an unmap.
> >>>mmap(fixaddress,fixedsize,fd_to_this_read_only_file_i_want_to_write_to)
> >>>radeon_ioctl_use_my_userptrbo
> >>>// bo is bind again by radeon and because all flag are set at creation
> >>>// it is map with write permission allowing someone to write to a file
> >>>// that might be read only for the user.
> >>>//
> >>>// Script kiddies it's time to learn about gpu ...
> >>>
> >>>Of course if you this patch (kind of selling my own junk here) :
> >>>
> >>>http://www.spinics.net/lists/linux-mm/msg75878.html
> >>>
> >>>then you could know inside the range_start that you should remove the
> >>>write permission and that it should be rechecked on next bind.
> >>>
> >>>Note that i have not read much of your code so maybe you handle this
> >>>case somehow.
> >>I've stumbled over this attack vector as well and it's the reason why I've
> >>moved checking the access rights to the bind callback instead of BO creation
> >>time with V5 of the patch.
> >>
> >>This way you get an -EFAULT if you try something like this on command
> >>submission time.
> >So you seem immune to that issue but you are still not checking if the anon
> >vma is writeable which you should again security concern here.
> We check the access rights of the pointer using:
> >        if (!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
> >(long)gtt->userptr,
> >                       ttm->num_pages * PAGE_SIZE))
> >                return -EFAULT;
> Shouldn't that be enough?

No, access_ok only check against special area on some architecture and i am
pretty sure on x86 the VERIFY_WRITE or VERIFY_READ is just flat out ignored.

What you need to test is the vma vm_flags somethings like

if (write && !(vma->vm_flags VM_WRITE))
   return -EPERM;

Which need to happen on all bind.


> Christian.
> >
> >Cheers,
> >Jérôme

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