[PATCH] drm/amdgpu: fix an underflow on non-4KB-page systems

[Xi Ruoyao]

On 2021-03-30 03:40 +0800, Xi Ruoyao wrote:
> On 2021-03-29 21:36 +0200, Christian König wrote:
> > Am 29.03.21 um 21:27 schrieb Xi Ruoyao:
> > > Hi Christian,
> > > 
> > > I don't think there is any constraint implemented to ensure `num_entries %
> > > AMDGPU_GPU_PAGES_IN_CPU_PAGE == 0`.  For example, in `amdgpu_vm_bo_map()`:
> > > 
> > >          /* validate the parameters */
> > >          if (saddr & AMDGPU_GPU_PAGE_MASK || offset & AMDGPU_GPU_PAGE_MASK
> > > > > 
> > >              size == 0 || size & AMDGPU_GPU_PAGE_MASK)
> > >                  return -EINVAL;
> > > 
> > > /* snip */
> > > 
> > >          saddr /= AMDGPU_GPU_PAGE_SIZE;
> > >          eaddr /= AMDGPU_GPU_PAGE_SIZE;
> > > 
> > > /* snip */
> > > 
> > >          mapping->start = saddr;
> > >          mapping->last = eaddr;
> > > 
> > > 
> > > If we really want to ensure (mapping->last - mapping->start + 1) %
> > > AMDGPU_GPU_PAGES_IN_CPU_PAGE == 0, then we should replace
> > > "AMDGPU_GPU_PAGE_MASK"
> > > in "validate the parameters" with "PAGE_MASK".

It should be "~PAGE_MASK", "PAGE_MASK" has an opposite convention of
"AMDGPU_GPU_PAGE_MASK" :(.

> > Yeah, good point.
> > 
> > > I tried it and it broke userspace: Xorg startup fails with EINVAL with
> > > this
> > > change.
> > 
> > Well in theory it is possible that we always fill the GPUVM on a 4k 
> > basis while the native page size of the CPU is larger. Let me double 
> > check the code.

On my platform, there are two issues both causing the VM corruption.  One is
fixed by agd5f/linux at fe001e7.  Another is in Mesa from userspace:  it uses
`dev_info->gart_page_size` as the alignment, but the kernel AMDGPU driver
expects it to use `dev_info->virtual_address_alignment`.

If we can make the change to fill the GPUVM on a 4k basis, we can fix this issue
and make virtual_address_alignment = 4K.  Otherwise, we should fortify the
parameter validation, changing "AMDGPU_GPU_PAGE_MASK" to "~PAGE_MASK".  Then the
userspace will just get an EINVAL, instead of a slient VM corruption.  And
someone should tell Mesa developers to fix the code in this case.
--
Xi Ruoyao <xry111 at mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University