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

[Stephen Smalley <sds@tycho.nsa.gov>]

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@hp.com>
> ---
>  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().

>  
>  	sclass = unmap_class(orig_sclass);
>  	*sid = SECINITSID_UNLABELED;
> @@ -2336,11 +2339,27 @@ int security_genfs_sid(const char *fstype,
>  	*sid = c->sid[0];
>  	rc = 0;
>  out:
> -	read_unlock(&policy_rwlock);
> +	if (!locked)
> +		read_unlock(&policy_rwlock);
>  	return rc;
>  }
>  
>  /**
> + * security_genfs_sid - 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
> + */
> +int security_genfs_sid(const char *fstype,
> +		       char *path,
> +		       u16 orig_sclass,
> +		       u32 *sid)
> +{
> +	return __security_genfs_sid(fstype, path, orig_sclass, sid, false);
> +}
> +
> +/**
>   * security_fs_use - Determine how to handle labeling for a filesystem.
>   * @sb: superblock in question
>   */
> @@ -2370,7 +2389,8 @@ int security_fs_use(struct super_block *sb)
>  		}
>  		sbsec->sid = c->sid[0];
>  	} else {
> -		rc = security_genfs_sid(fstype, "/", SECCLASS_DIR, &sbsec->sid);
> +		rc = __security_genfs_sid(fstype, "/", SECCLASS_DIR,
> +					  &sbsec->sid, true);
>  		if (rc) {
>  			sbsec->behavior = SECURITY_FS_USE_NONE;
>  			rc = 0;
>