Btrfs: avoid buffer overrun in mount option handling

There is an off-by-one error: allocating room for a maximal result
string but without room for a trailing NUL.  That, can lead to
returning a transformed string that is not NUL-terminated, and
then to a caller reading beyond end of the malloc'd buffer.

Rewrite to s/kzalloc/kmalloc/, remove unwarranted use of strncpy
(the result is guaranteed to fit), remove dead strlen at end, and
change a few variable names and comments.

Reviewed-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Jim Meyering <meyering@redhat.com>

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 46b2665..96eb9fe 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -923,63 +923,48 @@ static inline int is_subvolume_inode(struct inode *inode)
  */
 static char *setup_root_args(char *args)
 {
-       unsigned copied = 0;
-       unsigned len = strlen(args) + 2;
-       char *pos;
-       char *ret;
+       unsigned len = strlen(args) + 2 + 1;
+       char *src, *dst, *buf;
 
        /*
-        * We need the same args as before, but minus
+        * We need the same args as before, but with this substitution:
+        * s!subvol=[^,]+!subvolid=0!
         *
-        * subvol=a
-        *
-        * and add
-        *
-        * subvolid=0
-        *
-        * which is a difference of 2 characters, so we allocate strlen(args) +
-        * 2 characters.
+        * Since the replacement string is up to 2 bytes longer than the
+        * original, allocate strlen(args) + 2 + 1 bytes.
         */
-       ret = kzalloc(len * sizeof(char), GFP_NOFS);
-       if (!ret)
-               return NULL;
-       pos = strstr(args, "subvol=");
 
+       src = strstr(args, "subvol=");
        /* This shouldn't happen, but just in case.. */
-       if (!pos) {
-               kfree(ret);
+       if (!src)
+               return NULL;
+
+       buf = dst = kmalloc(len, GFP_NOFS);
+       if (!buf)
                return NULL;
-       }
 
        /*
-        * The subvol=<> arg is not at the front of the string, copy everybody
-        * up to that into ret.
+        * If the subvol= arg is not at the start of the string,
+        * copy whatever precedes it into buf.
         */
-       if (pos != args) {
-               *pos = '\0';
-               strcpy(ret, args);
-               copied += strlen(args);
-               pos++;
+       if (src != args) {
+               *src++ = '\0';
+               strcpy(buf, args);
+               dst += strlen(args);
        }
 
-       strncpy(ret + copied, "subvolid=0", len - copied);
-
-       /* Length of subvolid=0 */
-       copied += 10;
+       strcpy(dst, "subvolid=0");
+       dst += strlen("subvolid=0");
 
        /*
-        * If there is no , after the subvol= option then we know there's no
-        * other options and we can just return.
+        * If there is a "," after the original subvol=... string,
+        * copy that suffix into our buffer.  Otherwise, we're done.
         */
-       pos = strchr(pos, ',');
-       if (!pos)
-               return ret;
+       src = strchr(src, ',');
+       if (src)
+               strcpy(dst, src);
 
-       /* Copy the rest of the arguments into our buffer */
-       strncpy(ret + copied, pos, len - copied);
-       copied += strlen(pos);
-
-       return ret;
+       return buf;
 }
 
 static struct dentry *mount_subvol(const char *subvol_name, int flags,