On Fri, May 26, 2017 at 10:37 AM, Mark Rutland <[email protected]> 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 <[email protected]> > Cc: David Howells <[email protected]> > Cc: David Windsor <[email protected]> > Cc: Elena Reshetova <[email protected]> > Cc: Hans Liljestrand <[email protected]> > Cc: James Morris <[email protected]> > Cc: Kees Cook <[email protected]> > Cc: Peter Zijlstra <[email protected]>
Thanks for catching this! Reviewed-by: Kees Cook <[email protected]> -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
