[PATCH 3/3] SELinux: special dontaudit for access checks

[Eric Paris <eparis-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Currently there are a number of applications (nautilus being the main one) which
calls access() on files in order to determine how they should be displayed.  It
is normal and expected that nautilus will want to see if files are executable
or if they are really read/write-able.  access() should return the real
permission.  SELinux policy checks are done in access() and can result in lots
of AVC denials as policy denies RWX on files which DAC allows.  Currently
SELinux must dontaudit actual attempts to read/write/execute a file in
order to silence these messages (and not flood the logs.)  But dontaudit rules
like that can hide real attacks.  This patch addes a new common file
permission audit_access.  This permission is special in that it is meaningless
and should never show up in an allow rule.  Instead the only place this
permission has meaning is in a dontaudit rule like so:

dontaudit nautilus_t sbin_t:file audit_access

With such a rule if nautilus just checks access() we will still get denied and
thus userspace will still get the correct answer but we will not log the denial.
If nautilus attempted to actually perform one of the forbidden actions
(rather than just querying access(2) about it) we would still log a denial.
This type of dontaudit rule should be used sparingly, as it could be a
method for an attacker to probe the system permissions without detection.

Signed-off-by: Eric Paris <eparis-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---

 security/selinux/hooks.c            |   46 ++++++++++++++++++++++++++++++-----
 security/selinux/include/classmap.h |    2 +-
 2 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 344ba62..34e9d1b 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2696,19 +2696,51 @@ static int selinux_inode_follow_link(struct dentry *dentry, struct nameidata *na
 	return dentry_has_perm(cred, NULL, dentry, FILE__READ);
 }
 
-static int selinux_inode_permission(struct inode *inode, int mask)
+static int selinux_inode_permission(struct inode *inode, int in_mask)
 {
 	const struct cred *cred = current_cred();
+	struct inode_security_struct *isec;
+	struct common_audit_data ad;
+	struct av_decision avd;
+	u32 sid, perms;
+	int rc, mask;
 
-	mask &= (MAY_READ|MAY_WRITE|MAY_EXEC|MAY_APPEND);
+	mask = in_mask & (MAY_READ|MAY_WRITE|MAY_EXEC|MAY_APPEND);
 
-	if (!mask) {
-		/* No permission to check.  Existence test. */
+	/* No permission to check.  Existence test. */
+	if (!mask)
+		return 0;
+
+	validate_creds(cred);
+
+	if (unlikely(IS_PRIVATE(inode)))
 		return 0;
-	}
 
-	return inode_has_perm(cred, inode,
-			      file_mask_to_av(inode->i_mode, mask), NULL);
+	sid = cred_sid(cred);
+	isec = inode->i_security;
+
+	COMMON_AUDIT_DATA_INIT(&ad, FS);
+	ad.u.fs.inode = inode;
+
+	perms = file_mask_to_av(inode->i_mode, mask);
+
+	rc = avc_has_perm_noaudit(sid, isec->sid, isec->sclass, perms, 0, &avd);
+	/*
+	 * We want to audit if this call was not from access(2).
+	 * We also want to audit if the call was from access(2)
+	 * but the magic FILE__AUDIT_ACCESS permission was in the auditdeny
+	 * vector.
+	 *
+	 * aka there is a not dontaudit rule for file__audit_access.  This
+	 * might make more sense as a test inside avc_audit, but then we would
+	 * have to push the MAY_ACCESS flag down to avc_audit and I think we
+	 * already have enough stuff down there.
+	 */
+	if (!(in_mask & MAY_ACCESS) ||
+	    (avd.auditdeny & FILE__AUDIT_ACCESS))
+		avc_audit(sid, isec->sid, isec->sclass, perms, &avd, rc, &ad);
+
+	return rc;
 }
 
 static int selinux_inode_setattr(struct dentry *dentry, struct iattr *iattr)
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index 8b32e95..d64603e 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -2,7 +2,7 @@
     "getattr", "setattr", "lock", "relabelfrom", "relabelto", "append"
 
 #define COMMON_FILE_PERMS COMMON_FILE_SOCK_PERMS, "unlink", "link", \
-    "rename", "execute", "swapon", "quotaon", "mounton"
+    "rename", "execute", "swapon", "quotaon", "mounton", "audit_access"
 
 #define COMMON_SOCK_PERMS COMMON_FILE_SOCK_PERMS, "bind", "connect", \
     "listen", "accept", "getopt", "setopt", "shutdown", "recvfrom",  \