From c1f6e3c818dd734c30f6a7eeebf232ba2cf3181d Mon Sep 17 00:00:00 2001
From: Takashi Iwai <tiwai@suse.de>
Date: Thu, 7 May 2020 13:44:56 +0200
Subject: [PATCH] ALSA: rawmidi: Fix racy buffer resize under concurrent accesses
Git-commit: c1f6e3c818dd734c30f6a7eeebf232ba2cf3181d
Patch-mainline: v5.7-rc6
References: CVE-2020-27786 bsc#1179601

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>

---
 include/sound/rawmidi.h |    1 +
 sound/core/rawmidi.c    |   31 +++++++++++++++++++++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

--- a/include/sound/rawmidi.h
+++ b/include/sound/rawmidi.h
@@ -76,6 +76,7 @@ struct snd_rawmidi_runtime {
 	size_t avail_min;	/* min avail for wakeup */
 	size_t avail;		/* max used buffer for wakeup */
 	size_t xruns;		/* over/underruns counter */
+	int buffer_ref;		/* buffer reference count */
 	/* misc */
 	spinlock_t lock;
 	wait_queue_head_t sleep;
--- a/sound/core/rawmidi.c
+++ b/sound/core/rawmidi.c
@@ -112,6 +112,17 @@ static void snd_rawmidi_input_event_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;
@@ -661,6 +672,11 @@ static int resize_runtime_buffer(struct
 		if (!newbuf)
 			return -ENOMEM;
 		spin_lock_irq(&runtime->lock);
+		if (runtime->buffer_ref) {
+			spin_unlock_irq(&runtime->lock);
+			kvfree(newbuf);
+			return -EBUSY;
+		}
 		oldbuf = runtime->buffer;
 		runtime->buffer = newbuf;
 		runtime->buffer_size = params->buffer_size;
@@ -960,8 +976,10 @@ static long snd_rawmidi_kernel_read1(str
 	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)
@@ -980,16 +998,19 @@ static long snd_rawmidi_kernel_read1(str
 		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,
@@ -1283,6 +1304,7 @@ static long snd_rawmidi_kernel_write1(st
 			return -EAGAIN;
 		}
 	}
+	snd_rawmidi_buffer_ref(runtime);
 	while (count > 0 && runtime->avail > 0) {
 		count1 = runtime->buffer_size - runtime->appl_ptr;
 		if (count1 > count)
@@ -1314,6 +1336,7 @@ static long snd_rawmidi_kernel_write1(st
 	}
       __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);