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Subject[PATCH v2] fs: hardlink creation restriction cleanup
Clean-up of hardlink restriction logic, as suggested by Andrew Morton.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
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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