linux-kernel - Re: [PATCH v2 2/2] panthor: use drm_gpuva_unlink

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Message-ID: <20250911131817.3e0ab0d0@fedora>
Date: Thu, 11 Sep 2025 13:18:17 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Danilo Krummrich <dakr@...nel.org>, Matthew Brost
 <matthew.brost@...el.com>, "Thomas Hellström"
 <thomas.hellstrom@...ux.intel.com>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
 Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Steven Price <steven.price@....com>,
 Daniel Almeida <daniel.almeida@...labora.com>, Liviu Dudau
 <liviu.dudau@....com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 2/2] panthor: use drm_gpuva_unlink_defer()

On Thu, 11 Sep 2025 11:08:43 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> On Thu, Sep 11, 2025 at 12:15:37PM +0200, Boris Brezillon wrote:
> > On Tue, 09 Sep 2025 13:36:23 +0000
> > Alice Ryhl <aliceryhl@...gle.com> wrote:
> >   
> > >  static void panthor_vma_init(struct panthor_vma *vma, u32 flags)
> > > @@ -2084,12 +2010,12 @@ static int panthor_gpuva_sm_step_map(struct drm_gpuva_op *op, void *priv)
> > >  	if (ret)
> > >  		return ret;
> > >  
> > > -	/* Ref owned by the mapping now, clear the obj field so we don't release the
> > > -	 * pinning/obj ref behind GPUVA's back.
> > > -	 */
> > >  	drm_gpuva_map(&vm->base, &vma->base, &op->map);
> > >  	panthor_vma_link(vm, vma, op_ctx->map.vm_bo);
> > > +
> > > +	drm_gpuvm_bo_put_deferred(op_ctx->map.vm_bo);
> > >  	op_ctx->map.vm_bo = NULL;  
> > 
> > Hm, I don't see why we need a drm_gpuvm_bo_put_deferred() here. The
> > original idea was to delegate the vm_bo ownership to the VA being added
> > to the VM tree, so if we put it here, we have a UAF situation, don't we?  
> 
> The vm_bo refcount goes like this:
> 
> incr vm_bo_obtain()
> incr vma_link()
> decr vm_bo_put()
> 
> There is no decrement in panthor_vm_cleanup_op_ctx() due to this line:
> 
> 	op_ctx->map.vm_bo = NULL
> 
> So when everything is done, it is linked once and the refcount is
> incremented by one, which is correct.

Ah, right, I overlooked the change to panthor_vma_link() where you drop
the _put().