selinux: fall back to ref-walk if audit is required

commit bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
passed down the rcu flag to the SELinux AVC, but failed to adjust the
test in slow_avc_audit() to also return -ECHILD on LSM_AUDIT_DATA_DENTRY.
Previously, we only returned -ECHILD if generating an audit record with
LSM_AUDIT_DATA_INODE since this was only relevant from inode_permission.
Move the handling of MAY_NOT_BLOCK to avc_audit() and its inlined
equivalent in selinux_inode_permission() immediately after we determine
that audit is required, and always fall back to ref-walk in this case.

Fixes: bda0be7ad994 ("security: make inode_follow_link RCU-walk aware")
Reported-by: Will Deacon <will@kernel.org>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 5 insertions, 19 deletions

diff --git a/security/selinux/avc.c b/security/selinux/avc.c
index 74c43ebe34bb..23dc888ae305 100644
--- a/security/selinux/avc.c
+++ b/security/selinux/avc.c
@@ -424,7 +424,7 @@ static inline int avc_xperms_audit(struct selinux_state *state,
 	if (likely(!audited))
 		return 0;
 	return slow_avc_audit(state, ssid, tsid, tclass, requested,
-			audited, denied, result, ad, 0);
+			audited, denied, result, ad);
 }
 
 static void avc_node_free(struct rcu_head *rhead)
@@ -758,8 +758,7 @@ static void avc_audit_post_callback(struct audit_buffer *ab, void *a)
 noinline int slow_avc_audit(struct selinux_state *state,
 			    u32 ssid, u32 tsid, u16 tclass,
 			    u32 requested, u32 audited, u32 denied, int result,
-			    struct common_audit_data *a,
-			    unsigned int flags)
+			    struct common_audit_data *a)
 {
 	struct common_audit_data stack_data;
 	struct selinux_audit_data sad;
@@ -772,17 +771,6 @@ noinline int slow_avc_audit(struct selinux_state *state,
 		a->type = LSM_AUDIT_DATA_NONE;
 	}
 
-	/*
-	 * When in a RCU walk do the audit on the RCU retry.  This is because
-	 * the collection of the dname in an inode audit message is not RCU
-	 * safe.  Note this may drop some audits when the situation changes
-	 * during retry. However this is logically just as if the operation
-	 * happened a little later.
-	 */
-	if ((a->type == LSM_AUDIT_DATA_INODE) &&
-	    (flags & MAY_NOT_BLOCK))
-		return -ECHILD;
-
 	sad.tclass = tclass;
 	sad.requested = requested;
 	sad.ssid = ssid;
@@ -855,16 +843,14 @@ static int avc_update_node(struct selinux_avc *avc,
 	/*
 	 * If we are in a non-blocking code path, e.g. VFS RCU walk,
 	 * then we must not add permissions to a cache entry
-	 * because we cannot safely audit the denial.  Otherwise,
+	 * because we will not audit the denial.  Otherwise,
 	 * during the subsequent blocking retry (e.g. VFS ref walk), we
 	 * will find the permissions already granted in the cache entry
 	 * and won't audit anything at all, leading to silent denials in
 	 * permissive mode that only appear when in enforcing mode.
 	 *
-	 * See the corresponding handling in slow_avc_audit(), and the
-	 * logic in selinux_inode_follow_link and selinux_inode_permission
-	 * for the VFS MAY_NOT_BLOCK flag, which is transliterated into
-	 * AVC_NONBLOCKING for avc_has_perm_noaudit().
+	 * See the corresponding handling of MAY_NOT_BLOCK in avc_audit()
+	 * and selinux_inode_permission().
 	 */
 	if (flags & AVC_NONBLOCKING)
 		return 0;