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

[Stephen Smalley]

On 06/20/2014 01:45 PM, Waiman Long wrote:
> 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 5th argument to
> indicate if the rwlock has been taken.
> 
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>

Thanks, but I'd prefer to instead create a static helper function in
services.c that does not take the lock at all, use that function from
security_fs_use, and leave the extern function unmodified.

> ---
>  security/selinux/hooks.c            |    2 +-
>  security/selinux/include/security.h |    2 +-
>  security/selinux/selinuxfs.c        |    3 ++-
>  security/selinux/ss/services.c      |   13 +++++++++----
>  4 files changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 83d06db..430035a 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -1248,7 +1248,7 @@ static int selinux_proc_get_sid(struct dentry *dentry,
>  			path[1] = '/';
>  			path++;
>  		}
> -		rc = security_genfs_sid("proc", path, tclass, sid);
> +		rc = security_genfs_sid("proc", path, tclass, sid, false);
>  	}
>  	free_page((unsigned long)buffer);
>  	return rc;
> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> index ce7852c..6bc5b2f 100644
> --- a/security/selinux/include/security.h
> +++ b/security/selinux/include/security.h
> @@ -180,7 +180,7 @@ int security_get_allow_unknown(void);
>  int security_fs_use(struct super_block *sb);
>  
>  int security_genfs_sid(const char *fstype, char *name, u16 sclass,
> -	u32 *sid);
> +	u32 *sid, int locked);
>  
>  #ifdef CONFIG_NETLABEL
>  int security_netlbl_secattr_to_sid(struct netlbl_lsm_secattr *secattr,
> diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
> index c71737f..405799e 100644
> --- a/security/selinux/selinuxfs.c
> +++ b/security/selinux/selinuxfs.c
> @@ -1273,7 +1273,8 @@ static int sel_make_bools(void)
>  			goto out;
>  
>  		isec = (struct inode_security_struct *)inode->i_security;
> -		ret = security_genfs_sid("selinuxfs", page, SECCLASS_FILE, &sid);
> +		ret = security_genfs_sid("selinuxfs", page, SECCLASS_FILE,
> +					&sid, false);
>  		if (ret)
>  			goto out;
>  
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index 4bca494..2b23c2c 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -2282,6 +2282,7 @@ out:
>   * @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
> @@ -2290,7 +2291,8 @@ out:
>  int security_genfs_sid(const char *fstype,
>  		       char *path,
>  		       u16 orig_sclass,
> -		       u32 *sid)
> +		       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);
>  
>  	sclass = unmap_class(orig_sclass);
>  	*sid = SECINITSID_UNLABELED;
> @@ -2336,7 +2339,8 @@ 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;
>  }
>  
> @@ -2370,7 +2374,8 @@ 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;
> 

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