[PATCH] fs/ntfs3: bound DeleteIndexEntryAllocation memmove length

[Michael Bommarito <michael.bommarito@gmail.com>]

In do_action()'s DeleteIndexEntryAllocation case, e->size comes
from an on-disk INDEX_BUFFER entry.  When e->size makes
e + e->size point past hdr + hdr->used,
PtrOffset(e1, Add2Ptr(hdr, used)) returns a negative ptrdiff_t
that is silently cast to a quasi-infinite size_t when passed
to memmove().  The memmove then walks past the destination
buffer.

The sibling DeleteIndexEntryRoot case at fslog.c:3540-3543
already carries the corresponding guard:

	if (PtrOffset(e1, Add2Ptr(hdr, used)) < esize ||
	    Add2Ptr(e, esize) > Add2Ptr(lrh, rec_len) ||
	    used + esize > le32_to_cpu(hdr->total)) {
		goto dirty_vol;
	}

Apply the same shape to the allocation-path case.  Also reject
esize == 0: memmove(e, e, ...) is a no-op and leaves
hdr->used unchanged, hiding a malformed entry from the
existing check_index_header() walk.

Reproduced under UML+KASAN on mainline 8d90b09e6741 by
mounting a crafted NTFS image: the unguarded memmove takes a
length of 0xffffffffffffff00 and the kernel oopses in
memmove+0x81/0x1a0 on the do_action+0x36a2 frame.

Fixes: b46acd6a6a62 ("fs/ntfs3: Add NTFS journal")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 fs/ntfs3/fslog.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/ntfs3/fslog.c b/fs/ntfs3/fslog.c
index acfa18b84401e..4b32d08441b8d 100644
--- a/fs/ntfs3/fslog.c
+++ b/fs/ntfs3/fslog.c
@@ -3570,9 +3570,24 @@ static int do_action(struct ntfs_log *log, struct OPEN_ATTR_ENRTY *oe,
 		}
 
 		e1 = Add2Ptr(e, esize);
-		nsize = esize;
 		used = le32_to_cpu(hdr->used);
 
+		/*
+		 * Reject crafted entries whose e->size makes e + esize
+		 * point past the INDEX_HDR's used boundary.  Without this,
+		 * PtrOffset(e1, hdr + used) underflows to a quasi-infinite
+		 * size_t when fed to the memmove() below.
+		 *
+		 * Also reject esize == 0: memmove(e, e, ...) is a no-op and
+		 * leaves hdr->used unchanged, masking the crafted entry.
+		 */
+		if (!esize ||
+		    Add2Ptr(e, esize) > Add2Ptr(hdr, used) ||
+		    PtrOffset(e1, Add2Ptr(hdr, used)) < esize)
+			goto dirty_vol;
+
+		nsize = esize;
+
 		memmove(e, e1, PtrOffset(e1, Add2Ptr(hdr, used)));
 
 		hdr->used = cpu_to_le32(used - nsize);
-- 
2.53.0