[PATCH v5] soc: aspeed: lpc-snoop: Fix usercopy overflow in snoop_file_read

[Karthikeyan KS <karthiproffesional@gmail.com>]

put_fifo_with_discard() acts as both producer and consumer on the kfifo:
it calls kfifo_skip() (advances out) and kfifo_put() (advances in) from
the IRQ handler without synchronizing with snoop_file_read(), which also
consumes via kfifo_to_user(). On SMP systems this concurrent access can
leave (in - out) larger than the ring buffer, so __kfifo_to_user()'s clamp
to (in - out) is ineffective and kfifo_copy_to_user() can attempt a
copy_to_user() past the kmalloc-2k backing store:

  usercopy: Kernel memory exposure attempt detected from SLUB object
  'kmalloc-2k' (offset 0, size 2049)!
  kernel BUG at mm/usercopy.c!
  Call trace:
   usercopy_abort
   __check_heap_object
   __check_object_size
   kfifo_copy_to_user
   __kfifo_to_user
   snoop_file_read
   vfs_read

Serialize kfifo access with a per-channel spinlock. The reader drains
into a bounce buffer under the lock with kfifo_out_spinlocked() and then
copies to userspace after dropping it, since copy_to_user() may sleep on
a page fault.

Fixes: 3772e5da4454 ("drivers/misc: Aspeed LPC snoop output using misc chardev")
Cc: stable@vger.kernel.org
Signed-off-by: Karthikeyan KS <karthiproffesional@gmail.com>
---
Andrew,

Thanks for the review.

Changes since v4:
- Use __free(kfree) for automatic cleanup
- Allocate clamped count instead of full SNOOP_FIFO_SIZE
- Use kfifo_out_spinlocked() in snoop_file_read
- Use scoped_guard(spinlock) in put_fifo_with_discard

 drivers/soc/aspeed/aspeed-lpc-snoop.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/soc/aspeed/aspeed-lpc-snoop.c b/drivers/soc/aspeed/aspeed-lpc-snoop.c
index b03310c0830d..c9c87a794228 100644
--- a/drivers/soc/aspeed/aspeed-lpc-snoop.c
+++ b/drivers/soc/aspeed/aspeed-lpc-snoop.c
@@ -11,6 +11,7 @@
  */
 
 #include <linux/bitops.h>
+#include <linux/cleanup.h>
 #include <linux/clk.h>
 #include <linux/dev_printk.h>
 #include <linux/interrupt.h>
@@ -74,6 +75,7 @@ struct aspeed_lpc_snoop_channel_cfg {
 struct aspeed_lpc_snoop_channel {
 	const struct aspeed_lpc_snoop_channel_cfg *cfg;
 	bool enabled;
+	spinlock_t		lock;    /* serialises @fifo: irq producer vs reader */
 	struct kfifo		fifo;
 	wait_queue_head_t	wq;
 	struct miscdevice	miscdev;
@@ -114,6 +116,7 @@ static ssize_t snoop_file_read(struct file *file, char __user *buffer,
 				size_t count, loff_t *ppos)
 {
 	struct aspeed_lpc_snoop_channel *chan = snoop_file_to_chan(file);
+	u8 *buf __free(kfree) = NULL;
 	unsigned int copied;
 	int ret = 0;
 
@@ -125,9 +128,16 @@ static ssize_t snoop_file_read(struct file *file, char __user *buffer,
 		if (ret == -ERESTARTSYS)
 			return -EINTR;
 	}
-	ret = kfifo_to_user(&chan->fifo, buffer, count, &copied);
-	if (ret)
-		return ret;
+
+	count = min_t(size_t, count, SNOOP_FIFO_SIZE);
+
+	buf = kmalloc(count, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	copied = kfifo_out_spinlocked(&chan->fifo, buf, count, &chan->lock);
+	if (copied && copy_to_user(buffer, buf, copied))
+		return -EFAULT;
 
 	return copied;
 }
@@ -153,9 +163,11 @@ static void put_fifo_with_discard(struct aspeed_lpc_snoop_channel *chan, u8 val)
 {
 	if (!kfifo_initialized(&chan->fifo))
 		return;
-	if (kfifo_is_full(&chan->fifo))
-		kfifo_skip(&chan->fifo);
-	kfifo_put(&chan->fifo, val);
+	scoped_guard(spinlock, &chan->lock) {
+		if (kfifo_is_full(&chan->fifo))
+			kfifo_skip(&chan->fifo);
+		kfifo_put(&chan->fifo, val);
+	}
 	wake_up_interruptible(&chan->wq);
 }
 
@@ -228,6 +240,7 @@ static int aspeed_lpc_enable_snoop(struct device *dev,
 		return -EBUSY;
 
 	init_waitqueue_head(&channel->wq);
+	spin_lock_init(&channel->lock);
 
 	channel->cfg = cfg;
 	channel->miscdev.minor = MISC_DYNAMIC_MINOR;
-- 
2.43.0