Re: [PATCH] ntfs3: validate split-point offset in indx_insert_into_buffer

[Konstantin Komarov <almaz.alexandrovich@paragon-software.com>]

On 4/18/26 00:57, Michael Bommarito wrote:

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>
> 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>
> Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
> Assisted-by: Claude:claude-opus-4-7
> ---
>
>   - FYI, I have a larger refactor variant that migrates
>     hdr_find_split() to the validated hdr_next_de() helper and closes
>     the whole per-entry size-read class for that walker.  Happy to
>     send it as v2 if you prefer the wider change; otherwise this
>     minimal guard is scoped to the actual memmove underflow site and
>     is easier to backport.
>
>   fs/ntfs3/index.c | 14 ++++++++++++++
>   1 file changed, 14 insertions(+)
>
> diff --git a/fs/ntfs3/index.c b/fs/ntfs3/index.c
> index 2c43e7c27861..24add048b4b5 100644
> --- a/fs/ntfs3/index.c
> +++ b/fs/ntfs3/index.c
> @@ -1844,6 +1844,20 @@ indx_insert_into_buffer(struct ntfs_index *indx, struct ntfs_inode *ni,
>          memcpy(up_e, sp, sp_size);
>
>          used1 = le32_to_cpu(hdr1->used);
> +
> +       /*
> +        * hdr_find_split does not validate per-entry sizes, so a crafted
> +        * NTFS_DE whose le16 size field is out of range can place sp such
> +        * that (PtrOffset(hdr1, sp) + sp_size) exceeds used1. Without this
> +        * guard the u32 'used = used1 - to_copy - sp_size' underflows and
> +        * the subsequent memmove count becomes a near-4-GiB value,
> +        * triggering an out-of-bounds kernel write.
> +        */
> +       if (PtrOffset(hdr1, sp) + sp_size > used1) {
> +               err = -EINVAL;
> +               goto out;
> +       }
> +
>          hdr1_saved = kmemdup(hdr1, used1, GFP_NOFS);
>          if (!hdr1_saved) {
>                  err = -ENOMEM;
> --
> 2.53.0
>
Hello,

Sorry for the delay.
Patch is applied. Thank you.

Regards,
Konstantin