[PATCH -V26 14/16] fs: limit linkat syscall with null relative name to CAP_DAC_READ_SEARCH

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

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>
---
 fs/namei.c |   21 +++++++++++++++++++++
 1 files changed, 21 insertions(+), 0 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 990b155..5c4902c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3408,6 +3408,18 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
 	return error;
 }
 
+static int null_name(const char __user *name)
+{
+	int retval = 0;
+	char *tmp = getname_null(name);
+	if (!IS_ERR(tmp)) {
+		if (*tmp == 0)
+			retval = 1;
+	}
+	putname(tmp);
+	return retval;
+}
+
 /*
  * Hardlinks are often used in delicate situations.  We avoid
  * security-related surprises by not following symlinks on the
@@ -3428,6 +3440,15 @@ SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,
 
 	if ((flags & ~AT_SYMLINK_FOLLOW) != 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 (null_name(oldname)) {
+		if (!capable(CAP_DAC_READ_SEARCH))
+			return -ENOENT;
+	}
 
 	error = user_path_at(olddfd, oldname,
 			     flags & AT_SYMLINK_FOLLOW ? LOOKUP_FOLLOW : 0,
-- 
1.7.1