[PATCH] fs/ntfs3: prevent attribute-list pointer underflow

["Cen Zhang (Microsoft)" <blbllhy@gmail.com>]

attr_set_size_ex() and attr_collapse_range() use the on-disk size of a
removed ATTR_LIST_ENTRY to step a pointer backward. A crafted entry whose
size exceeds its offset from the attribute-list buffer start underflows the
pointer, and the next entry dereference reads outside the allocation.

BUG: KASAN: slab-out-of-bounds in attr_set_size_ex
  attr_set_size_ex fs/ntfs3/attrib.c:847
  ntfs_file_release fs/ntfs3/file.c:1427
  __fput fs/file_table.c:512
  __x64_sys_close fs/open.c:1496

Before each backward step, reject an entry whose on-disk size exceeds its
offset from the attribute-list buffer start. This prevents pointer
underflow in both the shrink and collapse-range paths while preserving
their existing fail-stop error handling.

Commit be71b5cba2e6 ("fs/ntfs3: Add attrib operations") introduced the
vulnerable implementation. Commit 12dad495eaab ("fs/ntfs3: Add Kconfig,
Makefile and doc") added the local build files. Commit 6e5be40d32fb
("fs/ntfs3: Add NTFS3 in fs/Kconfig and fs/Makefile") wired them into
the top-level build, making NTFS3 triggerable in standard kernels.

Fixes: 6e5be40d32fb ("fs/ntfs3: Add NTFS3 in fs/Kconfig and fs/Makefile")
Reported-by: AutonomousCodeSecurity@microsoft.com
Reported-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Reported-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
 fs/ntfs3/attrib.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/ntfs3/attrib.c b/fs/ntfs3/attrib.c
index c621a4c582f9..562bfcf3561f 100644
--- a/fs/ntfs3/attrib.c
+++ b/fs/ntfs3/attrib.c
@@ -779,12 +779,15 @@ int attr_set_size_ex(struct ntfs_inode *ni, enum ATTR_TYPE type,
 			u16 le_sz = le16_to_cpu(le->size);
 
 			/*
-			 * NOTE: List entries for one attribute are always
-			 * the same size. We deal with last entry (vcn==0)
-			 * and it is not first in entries array
-			 * (list entry for std attribute always first).
-			 * So it is safe to step back.
+			 * List entries for one attribute are expected to have
+			 * the same size. Validate the on-disk size before using
+			 * it to step back.
 			 */
+			if (le_sz > PtrOffset(ni->attr_list.le, le)) {
+				err = -EINVAL;
+				goto bad_inode;
+			}
+
 			mi_remove_attr(NULL, mi, attr);
 
 			if (!al_remove_le(ni, le)) {
@@ -2216,6 +2219,12 @@ int attr_collapse_range(struct ntfs_inode *ni, u64 vbo, u64 bytes)
 				}
 				continue;
 			}
+
+			if (le_sz > PtrOffset(ni->attr_list.le, le)) {
+				err = -EINVAL;
+				goto out;
+			}
+
 			le = (struct ATTR_LIST_ENTRY *)((u8 *)le - le_sz);
 		}
 
-- 
2.52.0