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

[Jerome Glisse]

On Wed, Aug 06, 2014 at 10:24:31PM +0200, Daniel Vetter wrote:
> On Wed, Aug 06, 2014 at 02:34:16PM -0400, Jerome Glisse wrote:
> > 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.
> 
> access_ok is _only_ valid in combination with copy_from/to_user and
> friends and is an optimization of the access checks depending upon
> architecture. You always need them both, one alone is useless.

ENOPARSE, access_ok will always return the same value for a given address at least
on x86 so if address supplied at ioctl time is a valid userspace address then it
will still be a valid userspace address at buffer object bind time (note that the
user address is immutable here). So access_ok can be done once and only once inside
the ioctl and then for the write permission you need to recheck the vma each time
you bind the object (or rather each time the previous bind was invalidated by some
mmu_notifier call).

That being said access_ok is kind of useless given that get_user_page will fail on
kernel address and i assume for any special address any architecture might have. So
strictly speaking the access_ok is just a way to fail early and flatout instead of
delaying the failure to bind time.


Or do i need to go read x86 code again ?

Cheers,
Jérôme

> -Daniel
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch