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: 9996508b3353 ("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>

On Sat, 2 Dec 2023 at 08:05, Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> 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: 9996508b3353 ("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>
>
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 420f155d251f..a3bbdd6e60fc 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/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;
> -                       }
> -                       data_avail = bytes_read;
> -               }
> -
> -               if (!data_avail) {
> -                       if (filp->f_flags & O_NONBLOCK) {
> +                       } else if (bytes_read == 0 &&
> +                                  (filp->f_flags & O_NONBLOCK)) {
>                                 err = -EAGAIN;
>                                 goto out_unlock_reading;
>                         }
> -               } else {
> -                       len = data_avail;
> +
> +                       data_avail = bytes_read;
> +               }
> +
> +               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:
> --
> Email: Herbert Xu <herbert@gondor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
>

Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>

Regards,
PrasannaKumar