fs/ntfs3: validate the EA value length in ntfs_read_ea()

ntfs_read_ea() checks the $EA list for consistency, but it only
recomputes the packed size of an entry when the entry's own size field is
zero:

	for (off = 0; off < size; off += ea_size) {
		const struct EA_FULL *ef = Add2Ptr(ea_p, off);
		u32 bytes = size - off;
		...
		if (ef->size) {
			ea_size = le32_to_cpu(ef->size);
			if (ea_size > bytes)
				goto out1;
			continue;              /* <- elength never checked */
		}
		...
		ea_size = ALIGN(struct_size(ef, name,
					    1 + ef->name_len +
						    le16_to_cpu(ef->elength)),
				4);
		if (ea_size > bytes)
			goto out1;
	}

ef->size is non-zero for every entry except the last one, and it is what
ntfs3 itself writes (ntfs_set_ea() stores exactly that aligned packed
size), so in practice elength is not validated at all.  find_ea() then
uses unpacked_ea_size(), which also prefers ea->size when it is set, so
it agrees and hands the entry back.

ntfs_get_ea() copies elength bytes with no further bound of its own:

	len = le16_to_cpu(ea->elength);
	...
	if (len > size) {              /* size is the *caller's* buffer */
		err = -ERANGE;
		...
	}
	memcpy(buffer, ea->name + ea->name_len + 1, len);

so a caller that supplies a large enough buffer receives up to 64 KiB of
whatever follows the small $EA allocation -- straight to userspace via
getxattr(2).

Compute the packed size unconditionally and require it to fit in the
entry.  Hoist the "bytes < offsetof(struct EA_FULL, name)" test above the
ef->size branch as well: name_len and elength were being read there
without it, which is itself a small over-read of the kmalloc'd buffer
when fewer than 8 bytes are left.  The old "bytes < sizeof(ef->size)"
test is subsumed by it.

  # mkntfs image; create /mnt/ntfs/poc with user.ntfs6 set, then rewrite
  # that EA_FULL's elength from 0xf to 0xff00 on the raw device
  mount -t ntfs3 -o ro /dev/vda /mnt/ntfs
  getxattr("/mnt/ntfs/poc", "user.ntfs6", buf, 128 * 1024);

  BUG: KASAN: slab-out-of-bounds in ntfs_get_ea+0x294/0x3e0
  Read of size 65280 at addr ffff0000cca53acf by task xattr_probe/142
   __asan_memcpy+0x3c/0xa0
   ntfs_get_ea+0x294/0x3e0
   ntfs_getxattr+0x284/0x318
   __vfs_getxattr+0x104/0x160
   vfs_getxattr+0x1b4/0x1e0
   do_getxattr+0xcc/0x230
   path_getxattrat+0x184/0x270
   __arm64_sys_getxattr+0x64/0x80
  Allocated by task 142:
   __kmalloc_noprof+0x294/0x668
   ntfs_read_ea+0x1e4/0x420
   ntfs_get_ea+0x2c8/0x3e0
  The buggy address belongs to the object at ffff0000cca53a80
   which belongs to the cache kmalloc-96 of size 96

The object being over-read is the one ntfs_read_ea() just allocated, and
it is 96 bytes.

Entries written by ntfs3 and by Windows set size to the aligned packed
size, so the new test does not reject anything a working filesystem
produces.

Fixes: be71b5cba2 ("fs/ntfs3: Add attrib operations")
Cc: stable@vger.kernel.org
Signed-off-by: Yuejie Shi <syjcnss@gmail.com>
[almaz.alexandrovich@paragon-software.com: applied as-is, logic is the same but the code is cleaner]
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>

fs/ntfs3/xattr.c

@@ -146,49 +146,32 @@ static int ntfs_read_ea(struct ntfs_inode *ni, struct EA_FULL **ea,
 	for (off = 0; off < size; off += ea_size) {
 		const struct EA_FULL *ef = Add2Ptr(ea_p, off);
 		u32 bytes = size - off;
-		size_t need;
-
-		/* Check if we can use field ea->size. */
-		if (bytes < sizeof(ef->size))
-			goto out1;
+		u32 packed_size;
 
 		/* Check if we can use fields ef->name_len and ef->elength. */
 		if (bytes < offsetof(struct EA_FULL, name))
 			goto out1;
 
-		/* Size needed to hold this record's name and value. */
-		need = struct_size(ef, name,
-				   1 + ef->name_len + le16_to_cpu(ef->elength));
+		/*
+		 * The name and the value have to fit in the entry whether or
+		 * not ef->size is set, because ntfs_get_ea() copies elength
+		 * bytes out of this buffer and checks them only against the
+		 * caller supplied buffer size.
+		 */
+		packed_size =
+			ALIGN(struct_size(ef, name,
+					  1 + ef->name_len +
+						  le16_to_cpu(ef->elength)),
+			      4);
 
 		if (ef->size) {
 			ea_size = le32_to_cpu(ef->size);
-			/* ef->size must fit the list and cover the record. */
-			if (ea_size > bytes || ea_size < need)
-				goto out1;
-			if (bytes < offsetof(struct EA_FULL, name))
-				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)
+			if (ea_size > bytes || packed_size > ea_size)
 				goto out1;
 			continue;
 		}
 
-		ea_size = ALIGN(need, 4);
+		ea_size = packed_size;
 		if (ea_size > bytes)
 			goto out1;
 	}