ALSA: rawmidi: Fix racy buffer resize under concurrent accesses

commit c1f6e3c818dd734c30f6a7eeebf232ba2cf3181d upstream.

The rawmidi core allows user to resize the runtime buffer via ioctl,
and this may lead to UAF when performed during concurrent reads or
writes: the read/write functions unlock the runtime lock temporarily
during copying form/to user-space, and that's the race window.

This patch fixes the hole by introducing a reference counter for the
runtime buffer read/write access and returns -EBUSY error when the
resize is performed concurrently against read/write.

Note that the ref count field is a simple integer instead of
refcount_t here, since the all contexts accessing the buffer is
basically protected with a spinlock, hence we need no expensive atomic
ops.  Also, note that this busy check is needed only against read /
write functions, and not in receive/transmit callbacks; the race can
happen only at the spinlock hole mentioned in the above, while the
whole function is protected for receive / transmit callbacks.

Reported-by: butt3rflyh4ck <butterflyhuangxx@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/CAFcO6XMWpUVK_yzzCpp8_XP7+=oUpQvuBeCbMffEDkpe8jWrfg@mail.gmail.com
Link: https://lore.kernel.org/r/s5heerw3r5z.wl-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 27 insertions, 4 deletions

diff --git a/sound/core/rawmidi.c b/sound/core/rawmidi.c
index c8b2309352d7..c7faee264415 100644
--- a/sound/core/rawmidi.c
+++ b/sound/core/rawmidi.c
@@ -108,6 +108,17 @@ static void snd_rawmidi_input_event_work(struct work_struct *work)
 		runtime->event(runtime->substream);
 }
 
+/* buffer refcount management: call with runtime->lock held */
+static inline void snd_rawmidi_buffer_ref(struct snd_rawmidi_runtime *runtime)
+{
+	runtime->buffer_ref++;
+}
+
+static inline void snd_rawmidi_buffer_unref(struct snd_rawmidi_runtime *runtime)
+{
+	runtime->buffer_ref--;
+}
+
 static int snd_rawmidi_runtime_create(struct snd_rawmidi_substream *substream)
 {
 	struct snd_rawmidi_runtime *runtime;
@@ -654,6 +665,11 @@ int snd_rawmidi_output_params(struct snd_rawmidi_substream *substream,
 		if (!newbuf)
 			return -ENOMEM;
 		spin_lock_irq(&runtime->lock);
+		if (runtime->buffer_ref) {
+			spin_unlock_irq(&runtime->lock);
+			kfree(newbuf);
+			return -EBUSY;
+		}
 		oldbuf = runtime->buffer;
 		runtime->buffer = newbuf;
 		runtime->buffer_size = params->buffer_size;
@@ -962,8 +978,10 @@ static long snd_rawmidi_kernel_read1(struct snd_rawmidi_substream *substream,
 	long result = 0, count1;
 	struct snd_rawmidi_runtime *runtime = substream->runtime;
 	unsigned long appl_ptr;
+	int err = 0;
 
 	spin_lock_irqsave(&runtime->lock, flags);
+	snd_rawmidi_buffer_ref(runtime);
 	while (count > 0 && runtime->avail) {
 		count1 = runtime->buffer_size - runtime->appl_ptr;
 		if (count1 > count)
@@ -982,16 +1000,19 @@ static long snd_rawmidi_kernel_read1(struct snd_rawmidi_substream *substream,
 		if (userbuf) {
 			spin_unlock_irqrestore(&runtime->lock, flags);
 			if (copy_to_user(userbuf + result,
-					 runtime->buffer + appl_ptr, count1)) {
-				return result > 0 ? result : -EFAULT;
-			}
+					 runtime->buffer + appl_ptr, count1))
+				err = -EFAULT;
 			spin_lock_irqsave(&runtime->lock, flags);
+			if (err)
+				goto out;
 		}
 		result += count1;
 		count -= count1;
 	}
+ out:
+	snd_rawmidi_buffer_unref(runtime);
 	spin_unlock_irqrestore(&runtime->lock, flags);
-	return result;
+	return result > 0 ? result : err;
 }
 
 long snd_rawmidi_kernel_read(struct snd_rawmidi_substream *substream,
@@ -1262,6 +1283,7 @@ static long snd_rawmidi_kernel_write1(struct snd_rawmidi_substream *substream,
 			return -EAGAIN;
 		}
 	}
+	snd_rawmidi_buffer_ref(runtime);
 	while (count > 0 && runtime->avail > 0) {
 		count1 = runtime->buffer_size - runtime->appl_ptr;
 		if (count1 > count)
@@ -1293,6 +1315,7 @@ static long snd_rawmidi_kernel_write1(struct snd_rawmidi_substream *substream,
 	}
       __end:
 	count1 = runtime->avail < runtime->buffer_size;
+	snd_rawmidi_buffer_unref(runtime);
 	spin_unlock_irqrestore(&runtime->lock, flags);
 	if (count1)
 		snd_rawmidi_output_trigger(substream, 1);