[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 09:08:49 PDT 2014

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.


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