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@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. -Daniel