ntfs3: validate split-point offset in indx_insert_into_buffer

ntfs3: validate split-point offset in indx_insert_into_buffer

indx_insert_into_buffer() computes

    used = used1 - to_copy - sp_size;
    memmove(de_t, Add2Ptr(sp, sp_size), used - le32_to_cpu(hdr1->de_off));

where sp and sp_size come from hdr_find_split().  hdr_find_split()
walks entries by le16_to_cpu(e->size) without validating that each
step stays within hdr->used or that the size field is at least
sizeof(struct NTFS_DE).  index_hdr_check(), the on-load gatekeeper,
only validates header-level fields (used, total, de_off) and does
not walk per-entry sizes.

A crafted NTFS image whose leaf INDEX_HDR reports used == total but
contains one interior NTFS_DE with size = 0xFFF0 therefore passes
validation, descends to indx_insert_into_buffer() through the
ntfs_create() -> indx_insert_entry() path, and makes hdr_find_split()
return an sp whose sp_size (0xFFF0) greatly exceeds the remaining
bytes in the buffer.  The u32 subtraction underflows and the memmove
count becomes a near-4-GiB value, producing an out-of-bounds kernel
write that corrupts adjacent allocations and panics the kernel.

Reproduced on 7.0.0-rc7 with UML + KASAN via a crafted image and a
single 'touch' inside the mounted directory; crash site resolves to
fs/ntfs3/index.c at the memmove.  Trigger requires only local mount
of an attacker-supplied filesystem image (USB, loopback, or removable
media auto-mount).

Reject the split whenever the chosen sp plus its declared size
already extends past hdr1->used.  This is the minimal fix; it
preserves the existing hdr_find_split() contract and relies on the
same out: cleanup path as the pre-existing error returns.

A prior OOB read in the very same indx_insert_into_buffer() memmove
was fixed in commit b8c44949044e ("fs/ntfs3: Fix OOB read in
indx_insert_into_buffer") by tightening hdr_find_e(), but that fix
does not cover the split-point size field path addressed here: sp is
returned by hdr_find_split(), not hdr_find_e(), and the underflow is
driven by sp->size rather than hdr->used exceeding hdr->total.

Fixes: 82cae269cfa9 ("fs/ntfs3: Add initialization of super block")
Cc: stable@vger.kernel.org
Reported-by: Michael Bommarito <michael.bommarito@gmail.com>
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>