Re: [PATCH v2] selinux: no recursive read_lock of policy_rwlock in security_genfs_sid()

[Waiman Long]

On 06/23/2014 08:34 AM, Stephen Smalley wrote:
> On 06/20/2014 08:30 PM, Waiman Long wrote:
> > v1->v2:
> >   - Add an internal helper to switch on/off lock acquisition instead
> >     of modifying the external API.
> >
> > With introduction of fair queued rwlock, recursive read_lock() may hang
> > the offending process if there is a write_lock() somewhere in between.
> >
> > With recursive read_lock checking enabled, the following error was
> > reported:
> >
> > =============================================
> > [ INFO: possible recursive locking detected ]
> > 3.16.0-rc1 #2 Tainted: G            E
> > ---------------------------------------------
> > load_policy/708 is trying to acquire lock:
> >   (policy_rwlock){.+.+..}, at: [<ffffffff8125b32a>]
> > security_genfs_sid+0x3a/0x170
> >
> > but task is already holding lock:
> >   (policy_rwlock){.+.+..}, at: [<ffffffff8125b48c>]
> > security_fs_use+0x2c/0x110
> >
> > other info that might help us debug this:
> >   Possible unsafe locking scenario:
> >
> >         CPU0
> >         ----
> >    lock(policy_rwlock);
> >    lock(policy_rwlock);
> >
> > This patch fixes the occurrence of recursive read_lock() of
> > policy_rwlock in security_genfs_sid() by adding a helper function
> > which has a 5th argument to indicate if the rwlock has been taken.
> >
> > Signed-off-by: Waiman Long<Waiman.Long@xxxxxx>
> > ---
> >   security/selinux/ss/services.c |   36 ++++++++++++++++++++++++++++--------
> >   1 files changed, 28 insertions(+), 8 deletions(-)
> >
> > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> > index 4bca494..5f4c1f3 100644
> > --- a/security/selinux/ss/services.c
> > +++ b/security/selinux/ss/services.c
> > @@ -2277,20 +2277,22 @@ out:
> >   }
> >
> >   /**
> > - * security_genfs_sid - Obtain a SID for a file in a filesystem
> > + * __security_genfs_sid - Helper to obtain a SID for a file in a filesystem
> >    * @fstype: filesystem type
> >    * @path: path from root of mount
> >    * @sclass: file security class
> >    * @sid: SID for path
> > + * @locked: true if policy_rwlock taken
> >    *
> >    * Obtain a SID to use for a file in a filesystem that
> >    * cannot support xattr or use a fixed labeling behavior like
> >    * transition SIDs or task SIDs.
> >    */
> > -int security_genfs_sid(const char *fstype,
> > -		       char *path,
> > -		       u16 orig_sclass,
> > -		       u32 *sid)
> > +static inline int __security_genfs_sid(const char *fstype,
> > +				       char *path,
> > +				       u16 orig_sclass,
> > +				       u32 *sid,
> > +				       int locked)
> >   {
> >   	int len;
> >   	u16 sclass;
> > @@ -2301,7 +2303,8 @@ int security_genfs_sid(const char *fstype,
> >   	while (path[0] == '/'&&  path[1] == '/')
> >   		path++;
> >
> > -	read_lock(&policy_rwlock);
> > +	if (!locked)
> > +		read_lock(&policy_rwlock);
> I believe that this kind of conditional lock-taking is frowned upon in
> the kernel, although I could be wrong.  I think it would be cleaner to
> instead just unconditionally take and release the lock around the call
> to this helper in security_genfs_sid(), and not do so around the call to
> it from security_fs_use().

Thank for the comments. Will send out a new patch with the suggested change.

-Longman

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/