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From:   Eric Biggers <ebiggers@kernel.org>
To:     dri-devel@lists.freedesktop.org
Cc:     syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        linux-kernel@vger.kernel.org,
        Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>,
        Haneen Mohammed <hamohammed.sa@gmail.com>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Chris Wilson <chris@chris-wilson.co.uk>, stable@vger.kernel.org
Subject: [PATCH] drm/vkms: fix use-after-free when drm_gem_handle_create() fails
Date:   Tue, 26 Feb 2019 14:08:58 -0800
Message-Id: <20190226220858.214438-1-ebiggers@kernel.org>
In-Reply-To: <20190226213053.GC218103@gmail.com>
References: <20190226213053.GC218103@gmail.com>
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From: Eric Biggers <ebiggers@google.com>

If drm_gem_handle_create() fails in vkms_gem_create(), then the
vkms_gem_object is freed twice: once when the reference is dropped by
drm_gem_object_put_unlocked(), and again by the extra calls to
drm_gem_object_release() and kfree().

Fix it by skipping the second release and free.

This bug was originally found in the vgem driver by syzkaller using
fault injection, but I noticed it's also present in the vkms driver.

Fixes: 559e50fd34d1 ("drm/vkms: Add dumb operations")
Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 drivers/gpu/drm/vkms/vkms_gem.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/vkms/vkms_gem.c b/drivers/gpu/drm/vkms/vkms_gem.c
index 138b0bb325cf9..69048e73377dc 100644
--- a/drivers/gpu/drm/vkms/vkms_gem.c
+++ b/drivers/gpu/drm/vkms/vkms_gem.c
@@ -111,11 +111,8 @@ struct drm_gem_object *vkms_gem_create(struct drm_device *dev,
 
 	ret = drm_gem_handle_create(file, &obj->gem, handle);
 	drm_gem_object_put_unlocked(&obj->gem);
-	if (ret) {
-		drm_gem_object_release(&obj->gem);
-		kfree(obj);
+	if (ret)
 		return ERR_PTR(ret);
-	}
 
 	return &obj->gem;
 }
-- 
2.21.0.rc2.261.ga7da99ff1b-goog