[PATCH 5.6 140/194] ALSA: rawmidi: Fix racy buffer resize under concurrent accesses

Greg Kroah-Hartman
2020-05-18T18:10:30Z

From: Takashi Iwai <tiwai@suse.de>

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>

---
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
@@ -61,6 +61,7 @@ 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
@@ -120,6 +120,17 @@ 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;
@@ -669,6 +680,11 @@ 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;
@@ -1019,8 +1035,10 @@ 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)
@@ -1039,16 +1057,19 @@ 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,
@@ -1342,6 +1363,7 @@ 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)
@@ -1373,6 +1395,7 @@ 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);