linux-kernel - Re: [PATCH] KEYS: fix refcount

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Date:   Fri, 26 May 2017 12:05:48 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     David Howells <dhowells@...hat.com>, keyrings@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Windsor <dwindsor@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        James Morris <james.l.morris@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] KEYS: fix refcount_inc() on zero

On Fri, May 26, 2017 at 10:37 AM, Mark Rutland <mark.rutland@....com> wrote:
> If a key's refcount is dropped to zero between key_lookup() peeking at
> the refcount and subsequently attempting to increment it, refcount_inc()
> will see a zero refcount. Here, refcount_inc() will WARN_ONCE(), and
> will *not* increment the refcount, which will remain zero.
>
> Once key_lookup() drops key_serial_lock, it is possible for the key to
> be freed behind our back.
>
> This patch uses refcount_inc_not_zero() to perform the peek and
> increment atomically. A helper with lockdep annotation is added to
> document why this is safe.
>
> Fixes: fff292914d3a2f1e ("security, keys: convert key.usage from atomic_t to refcount_t")
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Cc: David Howells <dhowells@...hat.com>
> Cc: David Windsor <dwindsor@...il.com>
> Cc: Elena Reshetova <elena.reshetova@...el.com>
> Cc: Hans Liljestrand <ishkamiel@...il.com>
> Cc: James Morris <james.l.morris@...cle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Peter Zijlstra <peterz@...radead.org>

Thanks for catching this!

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  security/keys/key.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/security/keys/key.c b/security/keys/key.c
> index 455c04d..150f51d 100644
> --- a/security/keys/key.c
> +++ b/security/keys/key.c
> @@ -632,6 +632,12 @@ void key_put(struct key *key)
>  }
>  EXPORT_SYMBOL(key_put);
>
> +static bool key_get_not_free(struct key *key)
> +{
> +       lockdep_assert_held(&key_serial_lock);
> +       return refcount_inc_not_zero(&key->usage);
> +}
> +
>  /*
>   * Find a key by its serial number.
>   */
> @@ -660,14 +666,12 @@ struct key *key_lookup(key_serial_t id)
>         goto error;
>
>  found:
> -       /* pretend it doesn't exist if it is awaiting deletion */
> -       if (refcount_read(&key->usage) == 0)
> -               goto not_found;
> -
> -       /* this races with key_put(), but that doesn't matter since key_put()
> -        * doesn't actually change the key
> +       /*
> +        * Pretend it doesn't exist if it is awaiting deletion. This races with
> +        * key_put(), but we can peek at the key until we drop key_serial_lock.
>          */
> -       __key_get(key);
> +       if (!key_get_not_free(key))
> +               goto not_found;
>
>  error:
>         spin_unlock(&key_serial_lock);
> --
> 1.9.1
>



-- 
Kees Cook
Pixel Security