hwrng: core - Fix page fault dead lock on mmap-ed hwrng

There is a dead-lock in the hwrng device read path.  This triggers
when the user reads from /dev/hwrng into memory also mmap-ed from
/dev/hwrng.  The resulting page fault triggers a recursive read
which then dead-locks.

Fix this by using a stack buffer when calling copy_to_user.
Reported-by: Edward Adam Davis <eadavis@qq.com>
Reported-by: syzbot+c52ab18308964d248092@syzkaller.appspotmail.com
Fixes: 9996508b ("hwrng: core - Replace u32 in driver API with byte array")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/core.c

@@ -23,10 +23,13 @@
 #include <linux/sched.h>
 #include <linux/sched/signal.h>
 #include <linux/slab.h>
+#include <linux/string.h>
 #include <linux/uaccess.h>
 
 #define RNG_MODULE_NAME		"hw_random"
 
+#define RNG_BUFFER_SIZE (SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES)
+
 static struct hwrng *current_rng;
 /* the current rng has been explicitly chosen by user via sysfs */
 static int cur_rng_set_by_user;
@@ -58,7 +61,7 @@ static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
 
 static size_t rng_buffer_size(void)
 {
-	return SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES;
+	return RNG_BUFFER_SIZE;
 }
 
 static void add_early_randomness(struct hwrng *rng)
@@ -209,6 +212,7 @@ static inline int rng_get_data(struct hwrng *rng, u8 *buffer, size_t size,
 static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 			    size_t size, loff_t *offp)
 {
+	u8 buffer[RNG_BUFFER_SIZE];
 	ssize_t ret = 0;
 	int err = 0;
 	int bytes_read, len;
@@ -236,34 +240,37 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 			if (bytes_read < 0) {
 				err = bytes_read;
 				goto out_unlock_reading;
+			} else if (bytes_read == 0 &&
+				   (filp->f_flags & O_NONBLOCK)) {
+				err = -EAGAIN;
+				goto out_unlock_reading;
 			}
+
 			data_avail = bytes_read;
 		}
 
-		if (!data_avail) {
-			if (filp->f_flags & O_NONBLOCK) {
-				err = -EAGAIN;
-				goto out_unlock_reading;
-			}
-		} else {
-			len = data_avail;
+		len = data_avail;
+		if (len) {
 			if (len > size)
 				len = size;
 
 			data_avail -= len;
 
-			if (copy_to_user(buf + ret, rng_buffer + data_avail,
-								len)) {
+			memcpy(buffer, rng_buffer + data_avail, len);
+		}
+		mutex_unlock(&reading_mutex);
+		put_rng(rng);
+
+		if (len) {
+			if (copy_to_user(buf + ret, buffer, len)) {
 				err = -EFAULT;
-				goto out_unlock_reading;
+				goto out;
 			}
 
 			size -= len;
 			ret += len;
 		}
 
-		mutex_unlock(&reading_mutex);
-		put_rng(rng);
 
 		if (need_resched())
 			schedule_timeout_interruptible(1);
@@ -274,6 +281,7 @@ static ssize_t rng_dev_read(struct file *filp, char __user *buf,
 		}
 	}
 out:
+	memzero_explicit(buffer, sizeof(buffer));
 	return ret ? : err;
 
 out_unlock_reading: