linux-kernel - Re: [PATCH] drm/vgem: fix use-after-free when drm_gem_handle

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Message-ID: <155121487959.17933.15702334870310780013@skylake-alporthouse-com>
Date:   Tue, 26 Feb 2019 21:01:29 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...ux.ie>,
        Eric Biggers <ebiggers@...nel.org>,
        dri-devel@...ts.freedesktop.org
Cc:     syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/vgem: fix use-after-free when drm_gem_handle_create() fails

Quoting Eric Biggers (2019-02-26 20:47:26)
> From: Eric Biggers <ebiggers@...gle.com>
> 
> If drm_gem_handle_create() fails in vgem_gem_create(), then the
> drm_vgem_gem_object is freed twice: once when the reference is dropped
> by drm_gem_object_put_unlocked(), and again by __vgem_gem_destroy().
> 
> This was hit by syzkaller using fault injection.
> 
> Fix it by skipping the second free.
> 
> Reported-by: syzbot+e73f2fb5ed5a5df36d33@...kaller.appspotmail.com
> Fixes: 5ba6c9ff961a ("drm/vgem: Fix mmaping")

That's the wrong fixes line, it's
Fixes: af33a9190d02 ("drm/vgem: Enable dmabuf import interfaces")
Cc: Chris Wilson <chris@...is-wilson.co.uk>
Cc: Laura Abbott <labbott@...hat.com>
Cc: Daniel Vetter <daniel.vetter@...ll.ch>

Sadly I reviewed it so I'm still culpable, but the fix is correct as the
put purposely frees it on error.

> Cc: stable@...r.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

> ---
>  drivers/gpu/drm/vgem/vgem_drv.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
> index 5930facd6d2d8..70646d9da1596 100644
> --- a/drivers/gpu/drm/vgem/vgem_drv.c
> +++ b/drivers/gpu/drm/vgem/vgem_drv.c
> @@ -189,15 +189,13 @@ static struct drm_gem_object *vgem_gem_create(struct drm_device *dev,
>                 return ERR_CAST(obj);
>  
>         ret = drm_gem_handle_create(file, &obj->base, handle);
> +
>         drm_gem_object_put_unlocked(&obj->base);
> +

The pattern in the other GEM drivers is not to have these extra
newlines.

Reviewed-by: Chris Wilson <chris@...is-wilson.co.uk>

>         if (ret)
> -               goto err;
> +               return ERR_PTR(ret);
>  
>         return &obj->base;
> -
> -err:
> -       __vgem_gem_destroy(obj);
> -       return ERR_PTR(ret);
>  }
>  
>  static int vgem_gem_dumb_create(struct drm_file *file, struct drm_device *dev,
> -- 
> 2.21.0.rc2.261.ga7da99ff1b-goog
>