fbmem: don't call copy_from/to_user() with mutex held

Avoid calling copy_from/to_user() with fb_info->lock mutex held in fbmem
ioctl().

fb_mmap() is called under mm->mmap_sem (A) held, that also acquires
fb_info->lock (B); fb_ioctl() takes fb_info->lock (B) and does
copy_from/to_user() that might acquire mm->mmap_sem (A), causing a
deadlock.

NOTE: it doesn't push down the fb_info->lock in each own driver's
fb_ioctl(), so there are still potential deadlocks elsewhere.

Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
Cc: Dave Jones <davej@redhat.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Johannes Weiner <hannes@saeurebad.de>
Cc: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/include/linux/fb.h b/include/linux/fb.h
index 818fe21257e8..31527e17076b 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -960,6 +960,21 @@ extern struct fb_info *registered_fb[FB_MAX];
 extern int num_registered_fb;
 extern struct class *fb_class;
 
+static inline int lock_fb_info(struct fb_info *info)
+{
+        mutex_lock(&info->lock);
+        if (!info->fbops) {
+                mutex_unlock(&info->lock);
+                return 0;
+        }
+        return 1;
+}
+
+static inline void unlock_fb_info(struct fb_info *info)
+{
+        mutex_unlock(&info->lock);
+}
+
 static inline void __fb_pad_aligned_buffer(u8 *dst, u32 d_pitch,
                                            u8 *src, u32 s_pitch, u32 height)
 {