fs: allow AT_EMPTY_PATH in linkat(), limit that to CAP_DAC_READ_SEARCH

We don't want to allow creation of private hardlinks by different application
using the fd passed to them via SCM_RIGHTS. So limit the null relative name
usage in linkat syscall to CAP_DAC_READ_SEARCH

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

diff --git a/fs/namei.c b/fs/namei.c
index 9d4f327..c9b7f5b 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2945,15 +2945,27 @@ SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,
        struct dentry *new_dentry;
        struct nameidata nd;
        struct path old_path;
+       int how = 0;
        int error;
        char *to;
 
-       if ((flags & ~AT_SYMLINK_FOLLOW) != 0)
+       if ((flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
                return -EINVAL;
+       /*
+        * To use null names we require CAP_DAC_READ_SEARCH
+        * This ensures that not everyone will be able to create
+        * handlink using the passed filedescriptor.
+        */
+       if (flags & AT_EMPTY_PATH) {
+               if (!capable(CAP_DAC_READ_SEARCH))
+                       return -ENOENT;
+               how = LOOKUP_EMPTY;
+       }
+
+       if (flags & AT_SYMLINK_FOLLOW)
+               how |= LOOKUP_FOLLOW;
 
-       error = user_path_at(olddfd, oldname,
-                            flags & AT_SYMLINK_FOLLOW ? LOOKUP_FOLLOW : 0,
-                            &old_path);
+       error = user_path_at(olddfd, oldname, how, &old_path);
        if (error)
                return error;