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

[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".

I tried it and it broke userspace: Xorg startup fails with EINVAL with this
change.

On 2021-03-30 02:30 +0800, Xi Ruoyao wrote:
> On 2021-03-30 02:21 +0800, Xi Ruoyao wrote:
> > On 2021-03-29 20:10 +0200, Christian König wrote:
> > > You need to identify the root cause of this, most likely start or last 
> > > are not a multiple of AMDGPU_GPU_PAGES_IN_CPU_PAGE.
> > 
> > I printk'ed the value of start & last, they are all a multiple of 4
> > (AMDGPU_GPU_PAGES_IN_CPU_PAGE).
> > 
> > However... `num_entries = last - start + 1` so it became some irrational
> > thing...  Either this line is wrong, or someone called
> > amdgpu_vm_bo_update_mapping with [start, last) instead of [start, last], which
> > is unexpected.
> 
> I added BUG_ON(num_entries % AMDGPU_GPU_PAGES_IN_CPU_PAGE != 0), get:
> 
> > Mar 30 02:28:27 xry111-A1901 kernel: [<ffffffff83c90750>]
> > amdgpu_vm_bo_update_mapping.constprop.0+0x218/0xae8
> > Mar 30 02:28:27 xry111-A1901 kernel: [<ffffffff83c922b8>]
> > amdgpu_vm_bo_update+0x270/0x4c0
> > Mar 30 02:28:27 xry111-A1901 kernel: [<ffffffff83c823ec>]
> > amdgpu_gem_va_ioctl+0x40c/0x430
> > Mar 30 02:28:27 xry111-A1901 kernel: [<ffffffff83c1cae4>]
> > drm_ioctl_kernel+0xcc/0x120
> > Mar 30 02:28:27 xry111-A1901 kernel: [<ffffffff83c1cfd8>]
> > drm_ioctl+0x220/0x408
> > Mar 30 02:28:27 xry111-A1901 kernel: [<ffffffff83c5ea48>]
> > amdgpu_drm_ioctl+0x58/0x98
> > Mar 30 02:28:27 xry111-A1901 kernel: [<ffffffff838feac4>] sys_ioctl+0xcc/0xe8
> > Mar 30 02:28:27 xry111-A1901 kernel: [<ffffffff836e74f0>]
> > syscall_common+0x34/0x58
> > 
> 
> > > > > > BugLink: https://gitlab.freedesktop.org/drm/amd/-/issues/1549
> > > > > > Fixes: a39f2a8d7066 ("drm/amdgpu: nuke amdgpu_vm_bo_split_mapping v2")
> > > > > > Reported-by: Xi Ruoyao <xry111 at mengyan1223.wang>
> > > > > > Reported-by: Dan Horák <dan at danny.cz>
> > > > > > Cc: stable at vger.kernel.org
> > > > > > Signed-off-by: Xi Ruoyao <xry111 at mengyan1223.wang>
> > > > > > ---
> > > > > >    drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 2 +-
> > > > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > 
> > > > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > > > > > b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > > > > > index ad91c0c3c423..cee0cc9c8085 100644
> > > > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > > > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> > > > > > @@ -1707,7 +1707,7 @@ static int amdgpu_vm_bo_update_mapping(struct
> > > > > > amdgpu_device *adev,
> > > > > >                  }
> > > > > >                  start = tmp;
> > > > > >    
> > > > > > -       } while (unlikely(start != last + 1));
> > > > > > +       } while (unlikely(start < last + 1));
> > > > > >    
> > > > > >          r = vm->update_funcs->commit(&params, fence);
> > > > > >    
> > > > > > 
> > > > > > base-commit: a5e13c6df0e41702d2b2c77c8ad41677ebb065b3
> > > 
> > 
> 

-- 
Xi Ruoyao <xry111 at mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University