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
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2945,15 +2945,27 @@
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;
+ }
- error = user_path_at(olddfd, oldname,
- flags & AT_SYMLINK_FOLLOW ? LOOKUP_FOLLOW : 0,
- &old_path);
+ if (flags & AT_SYMLINK_FOLLOW)
+ how |= LOOKUP_FOLLOW;
+
+ error = user_path_at(olddfd, oldname, how, &old_path);
if (error)
return error;