[PATCH] staging: android: ashmem: Avoid range_alloc() allocation with ashmem_mutex held.

[Joel Fernandes]

On Fri, Feb 22, 2019 at 3:04 AM Tetsuo Handa
<penguin-kernel at i-love.sakura.ne.jp> wrote:
>
> ashmem_pin() is calling range_shrink() without checking whether
> range_alloc() succeeded. Also, doing memory allocation with ashmem_mutex
> held should be avoided because ashmem_shrink_scan() tries to hold it.
>
> Therefore, move memory allocation for range_alloc() to ashmem_pin_unpin()
> and make range_alloc() not to fail.
>
> This patch is mostly meant for backporting purpose for fuzz testing on
> stable/distributor kernels, for there is a plan to remove this code in
> near future.
>
> Signed-off-by: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
> Cc: stable at vger.kernel.org

Properly this time,
Reviewed-by: Joel Fernandes <joel at joelfernandes.org>

thanks,

 - Joel



> ---
>  drivers/staging/android/ashmem.c | 42 +++++++++++++++++++++++-----------------
>  1 file changed, 24 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
> index 5d5b091..74d497d 100644
> --- a/drivers/staging/android/ashmem.c
> +++ b/drivers/staging/android/ashmem.c
> @@ -172,19 +172,15 @@ static inline void lru_del(struct ashmem_range *range)
>   * @end:          The ending page (inclusive)
>   *
>   * This function is protected by ashmem_mutex.
> - *
> - * Return: 0 if successful, or -ENOMEM if there is an error
>   */
> -static int range_alloc(struct ashmem_area *asma,
> -                      struct ashmem_range *prev_range, unsigned int purged,
> -                      size_t start, size_t end)
> +static void range_alloc(struct ashmem_area *asma,
> +                       struct ashmem_range *prev_range, unsigned int purged,
> +                       size_t start, size_t end,
> +                       struct ashmem_range **new_range)
>  {
> -       struct ashmem_range *range;
> -
> -       range = kmem_cache_zalloc(ashmem_range_cachep, GFP_KERNEL);
> -       if (!range)
> -               return -ENOMEM;
> +       struct ashmem_range *range = *new_range;
>
> +       *new_range = NULL;
>         range->asma = asma;
>         range->pgstart = start;
>         range->pgend = end;
> @@ -194,8 +190,6 @@ static int range_alloc(struct ashmem_area *asma,
>
>         if (range_on_lru(range))
>                 lru_add(range);
> -
> -       return 0;
>  }
>
>  /**
> @@ -597,7 +591,8 @@ static int get_name(struct ashmem_area *asma, void __user *name)
>   *
>   * Caller must hold ashmem_mutex.
>   */
> -static int ashmem_pin(struct ashmem_area *asma, size_t pgstart, size_t pgend)
> +static int ashmem_pin(struct ashmem_area *asma, size_t pgstart, size_t pgend,
> +                     struct ashmem_range **new_range)
>  {
>         struct ashmem_range *range, *next;
>         int ret = ASHMEM_NOT_PURGED;
> @@ -650,7 +645,7 @@ static int ashmem_pin(struct ashmem_area *asma, size_t pgstart, size_t pgend)
>                          * second half and adjust the first chunk's endpoint.
>                          */
>                         range_alloc(asma, range, range->purged,
> -                                   pgend + 1, range->pgend);
> +                                   pgend + 1, range->pgend, new_range);
>                         range_shrink(range, range->pgstart, pgstart - 1);
>                         break;
>                 }
> @@ -664,7 +659,8 @@ static int ashmem_pin(struct ashmem_area *asma, size_t pgstart, size_t pgend)
>   *
>   * Caller must hold ashmem_mutex.
>   */
> -static int ashmem_unpin(struct ashmem_area *asma, size_t pgstart, size_t pgend)
> +static int ashmem_unpin(struct ashmem_area *asma, size_t pgstart, size_t pgend,
> +                       struct ashmem_range **new_range)
>  {
>         struct ashmem_range *range, *next;
>         unsigned int purged = ASHMEM_NOT_PURGED;
> @@ -690,7 +686,8 @@ static int ashmem_unpin(struct ashmem_area *asma, size_t pgstart, size_t pgend)
>                 }
>         }
>
> -       return range_alloc(asma, range, purged, pgstart, pgend);
> +       range_alloc(asma, range, purged, pgstart, pgend, new_range);
> +       return 0;
>  }
>
>  /*
> @@ -723,10 +720,17 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
>         struct ashmem_pin pin;
>         size_t pgstart, pgend;
>         int ret = -EINVAL;
> +       struct ashmem_range *range = NULL;
>
>         if (copy_from_user(&pin, p, sizeof(pin)))
>                 return -EFAULT;
>
> +       if (cmd == ASHMEM_PIN || cmd == ASHMEM_UNPIN) {
> +               range = kmem_cache_zalloc(ashmem_range_cachep, GFP_KERNEL);
> +               if (!range)
> +                       return -ENOMEM;
> +       }
> +
>         mutex_lock(&ashmem_mutex);
>         wait_event(ashmem_shrink_wait, !atomic_read(&ashmem_shrink_inflight));
>
> @@ -751,10 +755,10 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
>
>         switch (cmd) {
>         case ASHMEM_PIN:
> -               ret = ashmem_pin(asma, pgstart, pgend);
> +               ret = ashmem_pin(asma, pgstart, pgend, &range);
>                 break;
>         case ASHMEM_UNPIN:
> -               ret = ashmem_unpin(asma, pgstart, pgend);
> +               ret = ashmem_unpin(asma, pgstart, pgend, &range);
>                 break;
>         case ASHMEM_GET_PIN_STATUS:
>                 ret = ashmem_get_pin_status(asma, pgstart, pgend);
> @@ -763,6 +767,8 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
>
>  out_unlock:
>         mutex_unlock(&ashmem_mutex);
> +       if (range)
> +               kmem_cache_free(ashmem_range_cachep, range);
>
>         return ret;
>  }
> --
> 1.8.3.1
>