[PATCH] fs/ntfs3: validate ef->size covers the record's name and value

[Weiming Shi <bestswngs@gmail.com>]

When an EA record has a non-zero ef->size, ntfs_read_ea() only checks
that the record fits in the remaining buffer (ea_size > bytes), not that
ef->size is large enough to hold the record's own name_len + 1 + elength.

A crafted image can pass validation with, e.g., ef->size = 24 but
elength = 0xffff. ntfs_get_ea() then trusts elength and copies it out of
the undersized record, reading past the kmalloc(info->size) allocation
and leaking heap memory to userspace via getxattr():

 BUG: KASAN: slab-out-of-bounds in ntfs_get_ea (fs/ntfs3/xattr.c:302)
 Read of size 65535 at addr ffff888100794550 by task exploit
  __asan_memcpy (mm/kasan/shadow.c:105)
  ntfs_get_ea (fs/ntfs3/xattr.c:302)
  ntfs_getxattr (fs/ntfs3/xattr.c:848)
  __vfs_getxattr (fs/xattr.c:441)
  vfs_getxattr (fs/xattr.c:474)
  do_getxattr (fs/xattr.c:800)
  path_getxattrat (fs/xattr.c:868)
  do_syscall_64 (arch/x86/entry/syscall_64.c:94)

 The buggy address is located 80 bytes inside of
  allocated 84-byte region in cache kmalloc-96

Compute the size the record needs and require ef->size to cover it.

Fixes: 0e8235d28f3a ("fs/ntfs3: Check fields while reading")
Reported-by: Xiang Mei <xmei5@asu.edu>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 fs/ntfs3/xattr.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/fs/ntfs3/xattr.c b/fs/ntfs3/xattr.c
index 9eeac0ab2b71..9097ded7d7dc 100644
--- a/fs/ntfs3/xattr.c
+++ b/fs/ntfs3/xattr.c
@@ -146,26 +146,29 @@ 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;
 
+		/* 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));
+
 		if (ef->size) {
 			ea_size = le32_to_cpu(ef->size);
-			if (ea_size > bytes)
+			/* ef->size must fit the list and cover the record. */
+			if (ea_size > bytes || ea_size < need)
 				goto out1;
 			continue;
 		}
 
-		/* Check if we can use fields ef->name_len and ef->elength. */
-		if (bytes < offsetof(struct EA_FULL, name))
-			goto out1;
-
-		ea_size = ALIGN(struct_size(ef, name,
-					    1 + ef->name_len +
-						    le16_to_cpu(ef->elength)),
-				4);
+		ea_size = ALIGN(need, 4);
 		if (ea_size > bytes)
 			goto out1;
 	}
-- 
2.43.0