Re: [PATCH 2/2] nouveau/dmem: Fix memory leak in `migrate_to_ram` upon copy error

[Danilo Krummrich <dakr@kernel.org>]

On Mon, Oct 07, 2024 at 03:28:22PM +0300, Yonatan Maman wrote:
> 
> 
> On 30/09/2024 14:20, Danilo Krummrich wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > On Mon, Sep 23, 2024 at 01:54:58PM +0000, Yonatan Maman wrote:
> > > A copy push command might fail, causing `migrate_to_ram` to return a
> > > dirty HIGH_USER page to the user.
> > > 
> > > This exposes a security vulnerability in the nouveau driver. To prevent
> > > memory leaks in `migrate_to_ram` upon a copy error, allocate a zero
> > > page for the destination page.
> > 
> > So, you refer to the case where this function fails in nouveau_dmem_copy_one()?
> > 
> > If so, can you please explain why adding __GFP_ZERO to alloc_page_vma() helps
> > with that?
> > 
> 
> The nouveau_dmem_copy_one function ensures that the copy push command is
> sent to the device firmware but does not track whether it was executed
> successfully.
> 
> In the case of a copy error (e.g., firmware or hardware error), the command
> will be sent in the firmware channel, and nouveau_dmem_copy_one might
> succeed, as well as the migrate_to_ram function. Thus, a dirty page could be
> returned to the user.
> 
> It’s important to note that we attempted to use nouveau_fence_wait status to
> handle migration errors, but it does not catch all error types.
> 
> To avoid this vulnerability, we allocate a zero page. So that, in case of an
> error, a non-dirty (zero) page will be returned to the user.

I see, I got confused by calling this a 'memory leak'.

Please add this description in the commit message and avoid the term 'memory
leak' in this context.

> 
> > > 
> > > Signed-off-by: Yonatan Maman <Ymaman@Nvidia.com>
> > > Signed-off-by: Gal Shalom <GalShalom@Nvidia.com>
> > 
> > Since this is a bug, please also add a 'Fixes' tag, CC stable and add a
> > 'Co-developed-by' tag if appropriate.
> 
> sure, thanks, I will add, and push it as V2 patch-series.
> > 
> > > ---
> > >   drivers/gpu/drm/nouveau/nouveau_dmem.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > > index 6fb65b01d778..097bd3af0719 100644
> > > --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > > +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > > @@ -193,7 +193,7 @@ static vm_fault_t nouveau_dmem_migrate_to_ram(struct vm_fault *vmf)
> > >        if (!spage || !(src & MIGRATE_PFN_MIGRATE))
> > >                goto done;
> > > 
> > > -     dpage = alloc_page_vma(GFP_HIGHUSER, vmf->vma, vmf->address);
> > > +     dpage = alloc_page_vma(GFP_HIGHUSER | __GFP_ZERO, vmf->vma, vmf->address);
> > >        if (!dpage)
> > >                goto done;
> > > 
> > > --
> > > 2.34.1
> > > 
>