[PATCH v2] fs: hardlink creation restriction cleanup

[Kees Cook <keescook@xxxxxxxxxxxx>]

Clean-up of hardlink restriction logic, as suggested by Andrew Morton.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v2:
 - add eye-strain-reduction-whitespace for Ingo. ;)

---
 fs/namei.c |   62 ++++++++++++++++++++++++++++++++++++++++++-----------------
 1 files changed, 44 insertions(+), 18 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 8ed4e00..f86494c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -693,46 +693,72 @@ static inline int may_follow_link(struct path *link)
 }
 
 /**
+ * safe_hardlink_source - Check for safe hardlink conditions
+ * @inode: the source inode to hardlink from
+ *
+ * Return false if at least one of the following conditions:
+ *    - inode is not a regular file
+ *    - inode is setuid
+ *    - inode is setgid and group-exec
+ *    - access failure for read and write
+ *
+ * Otherwise returns true.
+ */
+static bool safe_hardlink_source(struct inode *inode)
+{
+	mode_t mode = inode->i_mode;
+
+	/* Special files should not get pinned to the filesystem. */
+	if (!S_ISREG(mode))
+		return false;
+
+	/* Setuid files should not get pinned to the filesystem. */
+	if (mode & S_ISUID)
+		return false;
+
+	/* Executable setgid files should not get pinned to the filesystem. */
+	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
+		return false;
+
+	/* Hardlinking to unreadable or unwritable sources is dangerous. */
+	if (inode_permission(inode, MAY_READ | MAY_WRITE))
+		return false;
+
+	return true;
+}
+
+/**
  * may_linkat - Check permissions for creating a hardlink
  * @link: the source to hardlink from
  *
  * Block hardlink when all of:
  *  - sysctl_protected_hardlinks enabled
  *  - fsuid does not match inode
- *  - at least one of:
- *    - inode is not a regular file
- *    - inode is setuid
- *    - inode is setgid and group-exec
- *    - access failure for read and write
+ *  - hardlink source is unsafe (see safe_hardlink_source() above)
  *  - not CAP_FOWNER
  *
  * Returns 0 if successful, -ve on error.
  */
 static int may_linkat(struct path *link)
 {
-	int error = 0;
 	const struct cred *cred;
 	struct inode *inode;
-	int mode;
 
 	if (!sysctl_protected_hardlinks)
 		return 0;
 
 	cred = current_cred();
 	inode = link->dentry->d_inode;
-	mode = inode->i_mode;
-
-	if (cred->fsuid != inode->i_uid &&
-	    (!S_ISREG(mode) || (mode & S_ISUID) ||
-	     ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) ||
-	     (inode_permission(inode, MAY_READ | MAY_WRITE))) &&
-	    !capable(CAP_FOWNER))
-		error = -EPERM;
 
-	if (error)
-		audit_log_link_denied("linkat", link);
+	/* Source inode owner (or CAP_FOWNER) can hardlink all they like,
+	 * otherwise, it must be a safe source.
+	 */
+	if (cred->fsuid == inode->i_uid || safe_hardlink_source(inode) ||
+	    capable(CAP_FOWNER))
+		return 0;
 
-	return error;
+	audit_log_link_denied("linkat", link);
+	return -EPERM;
 }
 #else
 static inline int may_follow_link(struct path *link)
-- 
1.7.0.4

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