fs: add link restrictions

[mirror_ubuntu-zesty-kernel.git] / fs / namei.c

diff --git a/fs/namei.c b/fs/namei.c
index afa087649ddb162b14f3e9fae6cb77593037d5a4..3861d85f848807dc2a7c97b58920c1fb3969e176 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -650,6 +650,119 @@ static inline void put_link(struct nameidata *nd, struct path *link, void *cooki
        path_put(link);
 }
 
+int sysctl_protected_symlinks __read_mostly = 1;
+int sysctl_protected_hardlinks __read_mostly = 1;
+
+/**
+ * may_follow_link - Check symlink following for unsafe situations
+ * @link: The path of the symlink
+ *
+ * In the case of the sysctl_protected_symlinks sysctl being enabled,
+ * CAP_DAC_OVERRIDE needs to be specifically ignored if the symlink is
+ * in a sticky world-writable directory. This is to protect privileged
+ * processes from failing races against path names that may change out
+ * from under them by way of other users creating malicious symlinks.
+ * It will permit symlinks to be followed only when outside a sticky
+ * world-writable directory, or when the uid of the symlink and follower
+ * match, or when the directory owner matches the symlink's owner.
+ *
+ * Returns 0 if following the symlink is allowed, -ve on error.
+ */
+static inline int may_follow_link(struct path *link, struct nameidata *nd)
+{
+       const struct inode *inode;
+       const struct inode *parent;
+
+       if (!sysctl_protected_symlinks)
+               return 0;
+
+       /* Allowed if owner and follower match. */
+       inode = link->dentry->d_inode;
+       if (current_cred()->fsuid == inode->i_uid)
+               return 0;
+
+       /* Allowed if parent directory not sticky and world-writable. */
+       parent = nd->path.dentry->d_inode;
+       if ((parent->i_mode & (S_ISVTX|S_IWOTH)) != (S_ISVTX|S_IWOTH))
+               return 0;
+
+       /* Allowed if parent directory and link owner match. */
+       if (parent->i_uid == inode->i_uid)
+               return 0;
+
+       path_put_conditional(link, nd);
+       path_put(&nd->path);
+       return -EACCES;
+}
+
+/**
+ * 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)
+{
+       umode_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
+ *  - 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)
+{
+       const struct cred *cred;
+       struct inode *inode;
+
+       if (!sysctl_protected_hardlinks)
+               return 0;
+
+       cred = current_cred();
+       inode = link->dentry->d_inode;
+
+       /* 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 -EPERM;
+}
+
 static __always_inline int
 follow_link(struct path *link, struct nameidata *nd, void **p)
 {
@@ -1818,6 +1931,9 @@ static int path_lookupat(int dfd, const char *name,
                while (err > 0) {
                        void *cookie;
                        struct path link = path;
+                       err = may_follow_link(&link, nd);
+                       if (unlikely(err))
+                               break;
                        nd->flags |= LOOKUP_PARENT;
                        err = follow_link(&link, nd, &cookie);
                        if (err)
@@ -2778,6 +2894,9 @@ static struct file *path_openat(int dfd, const char *pathname,
                        error = -ELOOP;
                        break;
                }
+               error = may_follow_link(&link, nd);
+               if (unlikely(error))
+                       break;
                nd->flags |= LOOKUP_PARENT;
                nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL);
                error = follow_link(&link, nd, &cookie);
@@ -3421,6 +3540,9 @@ SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,
        error = -EXDEV;
        if (old_path.mnt != new_path.mnt)
                goto out_dput;
+       error = may_linkat(&old_path);
+       if (unlikely(error))
+               goto out_dput;
        error = security_path_link(old_path.dentry, &new_path, new_dentry);
        if (error)
                goto out_dput;