fs/ntfs3: validate that EA entry size covers its name and value

From: Aldo Ariel Panzardo <[email protected]>

ntfs_read_ea() walks the on-disk $EA attribute and checks each EA_FULL entry for consistency. For an entry with a non-zero ef->size (the offset to the next entry) the loop only verifies that this stride fits in the remaining buffer, and then continues:

         if (ef->size) {
                 ea_size = le32_to_cpu(ef->size);
                 if (ea_size > bytes)
                         goto out1;
                 continue;
         }

It never verifies that the entry's own name (ef->name_len) and value (ef->elength) actually fit inside ef->size. A later reader trusts ef->elength unconditionally; ntfs_get_ea() does:

         len = le16_to_cpu(ea->elength);         /* up to 0xffff */
         ...
         if (len > size)                         /* size is the user buffer */
                 return -ERANGE;
         memcpy(buffer, ea->name + ea->name_len + 1, len);

A crafted image with a small but valid ef->size (e.g. 24) and ef->elength == 0xffff therefore passes validation, and a getxattr() with a large enough user buffer copies up to 64 KiB starting just past the short entry -- an out-of-bounds read of the ea_all allocation that leaks adjacent kernel heap memory to userspace.

   BUG: KASAN: slab-out-of-bounds in ntfs_get_ea+0x2b8/0x3f0
   Read of size 65535 ...
    ntfs_get_ea -> ntfs_getxattr -> vfs_getxattr -> do_getxattr

The zero-ef->size branch already computes the same struct_size() to derive the stride; mirror that in the non-zero branch and reject the entry when its declared size cannot hold the name and value.

Fixes 0e8235d28f3a ("fs/ntfs3: Check fields while reading")

Cc [email protected]

Signed-off-by Aldo Ariel Panzardo <[email protected]>

---
  fs/ntfs3/xattr.c | 18 ++++++++++++++++++
  1 file changed, 18 insertions(+)

fs/ntfs3/xattr.c +18 -1

index 04814dd29375..57114fb726f7 100644
@@ -155,6 +155,24 @@ static int ntfs_read_ea(struct ntfs_inode *ni, struct EA_FULL **ea,
                         ea_size = le32_to_cpu(ef->size);
                         if (ea_size > bytes)
                                 goto out1;
+
+                       /* Check if we can use fields ef->name_len and ef->elength. */
+                       if (bytes < offsetof(struct EA_FULL, name))
+                               goto out1;
+
+                       /*
+                        * The declared entry size (ef->size) must be large
+                        * enough to hold the name and value.  Otherwise a later
+                        * reader such as ntfs_get_ea() copies ef->elength bytes
+                        * starting past the entry, reading out of bounds of the
+                        * ea buffer and leaking adjacent heap memory to
+                        * userspace via getxattr().
+                        */
+                       if (struct_size(ef, name,
+                                       1 + ef->name_len +
+                                               le16_to_cpu(ef->elength)) >
+                           ea_size)
+                               goto out1;
                         continue;
                 }
 
--

2.43.0