[PATCH] 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(+)

diff --git a/fs/ntfs3/xattr.c b/fs/ntfs3/xattr.c
index 04814dd29375..57114fb726f7 100644
--- a/fs/ntfs3/xattr.c
+++ b/fs/ntfs3/xattr.c
@@ -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