| Message ID | 1495820254-6651-1-git-send-email-mark.rutland@arm.com |
|---|---|
| State | New |
| Headers | show |
On Fri, May 26, 2017 at 10:37 AM, Mark Rutland <mark.rutland@arm.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@arm.com> > Cc: David Howells <dhowells@redhat.com> > Cc: David Windsor <dwindsor@gmail.com> > Cc: Elena Reshetova <elena.reshetova@intel.com> > Cc: Hans Liljestrand <ishkamiel@gmail.com> > Cc: James Morris <james.l.morris@oracle.com> > Cc: Kees Cook <keescook@chromium.org> > Cc: Peter Zijlstra <peterz@infradead.org> Thanks for catching this! Reviewed-by: Kees Cook <keescook@chromium.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
Mark Rutland <mark.rutland@arm.com> wrote: > This patch uses refcount_inc_not_zero() to perform the peek and > increment atomically. I think the helper is unnecessary. Better to adjust the comment if you really want to explain it. Anyone editing the code should be that this is inside a critical section. > A helper with lockdep annotation is added to document why this is safe. This doesn't explain why this is safe. > + /* > + * 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. > */ With your change, there is no race with key_put() - so the second sentence is unnecessary. I've adjusted your patch - see attached. David --- commit f66bf831c45306ebbc28aecd407e238983457251 Author: Mark Rutland <mark.rutland@arm.com> Date: Fri May 26 18:37:34 2017 +0100 KEYS: fix refcount_inc() on zero 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. Fixes: fff292914d3a2f1e ("security, keys: convert key.usage from atomic_t to refcount_t") Signed-off-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: David Howells <dhowells@redhat.com> Cc: David Windsor <dwindsor@gmail.com> Cc: Elena Reshetova <elena.reshetova@intel.com> Cc: Hans Liljestrand <ishkamiel@gmail.com> Cc: James Morris <james.l.morris@oracle.com> Cc: Kees Cook <keescook@chromium.org> Cc: Peter Zijlstra <peterz@infradead.org>diff --git a/security/keys/key.c b/security/keys/key.c index 455c04d80bbb..d84ee2a87da6 100644 --- a/security/keys/key.c +++ b/security/keys/key.c @@ -660,14 +660,11 @@ 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 + /* A key is allowed to be looked up only if someone still owns a + * reference to it - otherwise it's awaiting the gc. */ - __key_get(key); + if (!refcount_inc_not_zero(&key->usage)) + goto not_found; error: spin_unlock(&key_serial_lock);
On Wed, May 31, 2017 at 01:20:44PM +0100, David Howells wrote: > Mark Rutland <mark.rutland@arm.com> wrote: > > > This patch uses refcount_inc_not_zero() to perform the peek and > > increment atomically. > > I think the helper is unnecessary. Better to adjust the comment if you really > want to explain it. Anyone editing the code should be that this is inside a > critical section. > > > A helper with lockdep annotation is added to document why this is safe. > > This doesn't explain why this is safe. > > > + /* > > + * 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. > > */ > > With your change, there is no race with key_put() - so the second sentence is > unnecessary. Fair enough, on all counts. > I've adjusted your patch - see attached. That looks fine to me, thanks! Mark. > > David > --- > commit f66bf831c45306ebbc28aecd407e238983457251 > Author: Mark Rutland <mark.rutland@arm.com> > Date: Fri May 26 18:37:34 2017 +0100 > > KEYS: fix refcount_inc() on zero > > 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. > > Fixes: fff292914d3a2f1e ("security, keys: convert key.usage from atomic_t to refcount_t") > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Signed-off-by: David Howells <dhowells@redhat.com> > Cc: David Windsor <dwindsor@gmail.com> > Cc: Elena Reshetova <elena.reshetova@intel.com> > Cc: Hans Liljestrand <ishkamiel@gmail.com> > Cc: James Morris <james.l.morris@oracle.com> > Cc: Kees Cook <keescook@chromium.org> > Cc: Peter Zijlstra <peterz@infradead.org> > > diff --git a/security/keys/key.c b/security/keys/key.c > index 455c04d80bbb..d84ee2a87da6 100644 > --- a/security/keys/key.c > +++ b/security/keys/key.c > @@ -660,14 +660,11 @@ 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 > + /* A key is allowed to be looked up only if someone still owns a > + * reference to it - otherwise it's awaiting the gc. > */ > - __key_get(key); > + if (!refcount_inc_not_zero(&key->usage)) > + goto not_found; > > error: > spin_unlock(&key_serial_lock);
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);
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@arm.com> Cc: David Howells <dhowells@redhat.com> Cc: David Windsor <dwindsor@gmail.com> Cc: Elena Reshetova <elena.reshetova@intel.com> Cc: Hans Liljestrand <ishkamiel@gmail.com> Cc: James Morris <james.l.morris@oracle.com> Cc: Kees Cook <keescook@chromium.org> Cc: Peter Zijlstra <peterz@infradead.org> --- security/keys/key.c | 18 +++++++++++------- 1 file changed, 11 insertions(+), 7 deletions(-) -- 1.9.1