Re: [PATCH] gpu: drm: drm_mm: Fix a sleep-in-atomic-context bug in show_leaks()

[Christian KÃnig]

Am 03.09.2018 um 09:52 schrieb Daniel Vetter:
> On Sat, Sep 01, 2018 at 01:32:54PM +0100, Chris Wilson wrote:
> > Quoting Jia-Ju Bai (2018-09-01 13:20:41)
> > > The driver may sleep with holding a spinlock.
> > >
> > > The function call paths (from bottom to top) in Linux-4.16 are:
> > >
> > > [FUNC] kmalloc(GFP_KERNEL)
> > > drivers/gpu/drm/drm_mm.c, 130:
> > >          kmalloc in show_leaks
> > > drivers/gpu/drm/drm_mm.c, 913:
> > >          show_leaks in drm_mm_takedown
> > > drivers/gpu/drm/drm_vma_manager.c, 107:
> > >          drm_mm_takedown in drm_vma_offset_manager_destroy
> > > drivers/gpu/drm/drm_vma_manager.c, 106:
> > >          _raw_write_lock in drm_vma_offset_manager_destroy
> > >
> > > [FUNC] kmalloc(GFP_KERNEL)
> > > drivers/gpu/drm/drm_mm.c, 130:
> > >          kmalloc in show_leaks
> > > drivers/gpu/drm/drm_mm.c, 913:
> > >          show_leaks in drm_mm_takedown
> > > drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c, 71:
> > >          drm_mm_takedown in amdgpu_vram_mgr_fini
> > > drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c, 70:
> > >          spin_lock in amdgpu_vram_mgr_fini
> > >
> > > [FUNC] kmalloc(GFP_KERNEL)
> > > drivers/gpu/drm/drm_mm.c, 130:
> > >          kmalloc in show_leaks
> > > drivers/gpu/drm/drm_mm.c, 913:
> > >          show_leaks in drm_mm_takedown
> > > drivers/gpu/drm/ttm/ttm_bo_manager.c, 128:
> > >          drm_mm_takedown in ttm_bo_man_takedown
> > > drivers/gpu/drm/ttm/ttm_bo_manager.c, 126:
> > >          spin_lock in ttm_bo_man_takedown
> > >
> > > To fix this bug, GFP_KERNEL is replaced with GFP_ATOMIC.
> > The bug are above, since those spinlocks do not protect the data and
> > imply use-after-free.
> Adding amdgpu, since that's where the bug seems to be.

When we have use after free we might have concurrent uses as well.

I think taking the lock here is probably a good idea if you don't want 
to accidentally access freed memory in show_leaks.

So Chris change sounds valid to me.

Christian.

> -Daniel