security: make inode_follow_link RCU-walk aware

inode_follow_link now takes an inode and rcu flag as well as the
dentry.

inode is used in preference to d_backing_inode(dentry), particularly
in RCU-walk mode.

selinux_inode_follow_link() gets dentry_has_perm() and
inode_has_perm() open-coded into it so that it can call
avc_has_perm_flags() in way that is safe if LOOKUP_RCU is set.

Calling avc_has_perm_flags() with rcu_read_lock() held means
that when avc_has_perm_noaudit calls avc_compute_av(), the attempt
to rcu_read_unlock() before calling security_compute_av() will not
actually drop the RCU read-lock.

However as security_compute_av() is completely in a read_lock()ed
region, it should be safe with the RCU read-lock held.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 33b655d..0fa7af2 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -881,8 +881,9 @@ const char *get_link(struct nameidata *nd)
 
        touch_atime(&last->link);
 
-       error = security_inode_follow_link(dentry);
-       if (error)
+       error = security_inode_follow_link(dentry, inode,
+                                          nd->flags & LOOKUP_RCU);
+       if (unlikely(error))
                return ERR_PTR(error);
 
        nd->last_type = LAST_BIND;

diff --git a/include/linux/security.h b/include/linux/security.h
index 62a6620..52febde 100644 (file)
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -476,6 +476,8 @@ static inline void security_free_mnt_opts(struct security_mnt_opts *opts)
  * @inode_follow_link:
  *     Check permission to follow a symbolic link when looking up a pathname.
  *     @dentry contains the dentry structure for the link.
+ *     @inode contains the inode, which itself is not stable in RCU-walk
+ *     @rcu indicates whether we are in RCU-walk mode.
  *     Return 0 if permission is granted.
  * @inode_permission:
  *     Check permission before accessing an inode.  This hook is called by the
@@ -1551,7 +1553,8 @@ struct security_operations {
        int (*inode_rename) (struct inode *old_dir, struct dentry *old_dentry,
                             struct inode *new_dir, struct dentry *new_dentry);
        int (*inode_readlink) (struct dentry *dentry);
-       int (*inode_follow_link) (struct dentry *dentry);
+       int (*inode_follow_link) (struct dentry *dentry, struct inode *inode,
+                                 bool rcu);
        int (*inode_permission) (struct inode *inode, int mask);
        int (*inode_setattr)    (struct dentry *dentry, struct iattr *attr);
        int (*inode_getattr) (const struct path *path);
@@ -1837,7 +1840,8 @@ int security_inode_rename(struct inode *old_dir, struct dentry *old_dentry,
                          struct inode *new_dir, struct dentry *new_dentry,
                          unsigned int flags);
 int security_inode_readlink(struct dentry *dentry);
-int security_inode_follow_link(struct dentry *dentry);
+int security_inode_follow_link(struct dentry *dentry, struct inode *inode,
+                              bool rcu);
 int security_inode_permission(struct inode *inode, int mask);
 int security_inode_setattr(struct dentry *dentry, struct iattr *attr);
 int security_inode_getattr(const struct path *path);
@@ -2239,7 +2243,9 @@ static inline int security_inode_readlink(struct dentry *dentry)
        return 0;
 }
 
-static inline int security_inode_follow_link(struct dentry *dentry)
+static inline int security_inode_follow_link(struct dentry *dentry,
+                                            struct inode *inode,
+                                            bool rcu)
 {
        return 0;
 }

diff --git a/security/capability.c b/security/capability.c
index fae99db..7d3f38f 100644 (file)
--- a/security/capability.c
+++ b/security/capability.c
@@ -209,7 +209,8 @@ static int cap_inode_readlink(struct dentry *dentry)
        return 0;
 }
 
-static int cap_inode_follow_link(struct dentry *dentry)
+static int cap_inode_follow_link(struct dentry *dentry, struct inode *inode,
+                                bool rcu)
 {
        return 0;
 }

diff --git a/security/security.c b/security/security.c
index d7c30b0..04c8fec 100644 (file)
--- a/security/security.c
+++ b/security/security.c
@@ -581,11 +581,12 @@ int security_inode_readlink(struct dentry *dentry)
        return security_ops->inode_readlink(dentry);
 }
 
-int security_inode_follow_link(struct dentry *dentry)
+int security_inode_follow_link(struct dentry *dentry, struct inode *inode,
+                              bool rcu)
 {
-       if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
+       if (unlikely(IS_PRIVATE(inode)))
                return 0;
-       return security_ops->inode_follow_link(dentry);
+       return security_ops->inode_follow_link(dentry, inode, rcu);
 }
 
 int security_inode_permission(struct inode *inode, int mask)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d56a829..ffa5a64 100644 (file)
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2861,11 +2861,23 @@ static int selinux_inode_readlink(struct dentry *dentry)
        return dentry_has_perm(cred, dentry, FILE__READ);
 }
 
-static int selinux_inode_follow_link(struct dentry *dentry)
+static int selinux_inode_follow_link(struct dentry *dentry, struct inode *inode,
+                                    bool rcu)
 {
        const struct cred *cred = current_cred();
+       struct common_audit_data ad;
+       struct inode_security_struct *isec;
+       u32 sid;
 
-       return dentry_has_perm(cred, dentry, FILE__READ);
+       validate_creds(cred);
+
+       ad.type = LSM_AUDIT_DATA_DENTRY;
+       ad.u.dentry = dentry;
+       sid = cred_sid(cred);
+       isec = inode->i_security;
+
+       return avc_has_perm_flags(sid, isec->sid, isec->sclass, FILE__READ, &ad,
+                                 rcu ? MAY_NOT_BLOCK : 0);
 }
 
 static noinline int audit_inode_permission(struct inode *inode,