Re: [PATCH] fs/ntfs3: add depth limit to indx_find_buffer to prevent stack overflow

On 4/13/26 15:31, Michael Bommarito wrote:

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>
> indx_find_buffer() recursively descends the B+ tree index with no depth
> limit.  A crafted NTFS image with circular index node references causes
> unbounded recursion, overflowing the kernel stack and panicking the
> system.
>
> This is reachable by mounting a malicious NTFS filesystem (e.g. from a
> USB drive via desktop automount) and deleting a file whose index entry
> triggers the rebalancing fallback path in indx_delete_entry().
>
> Add a depth parameter and bail out with -EINVAL when it reaches the
> fnd->nodes array bound, matching the constraint already enforced by
> fnd_push() in indx_find().
>
> The related function indx_find() was previously patched for a similar
> infinite-loop issue (commit 1732053c8a6b), but indx_find_buffer() was
> missed.
>
> Fixes: 82cae269cfa9 ("fs/ntfs3: Add initialization of super block")
> Cc: [email protected]
> Assisted-by: Claude:claude-opus-4-6
> Assisted-by: Codex:gpt-5-4
> Signed-off-by: Michael Bommarito <[email protected]>
> ---
> Found during a broader arch/um/ and filesystem security audit.
> This is the same class of bug as the one fixed by commit
> 1732053c8a6b ("fs: ntfs3: check return value of indx_find to
> avoid infinite loop"), which added a depth limit to indx_find()
> but missed indx_find_buffer().
>
> Reproduced on UML (ARCH=um) with a crafted NTFS image containing
> a circular B+ tree directory index. Mounting the image and
> deleting a specific file triggers indx_delete_entry() ->
> indx_find_buffer() -> unbounded recursion -> stack overflow:
>
>    Kernel panic - not syncing: Kernel tried to access user memory
>      at addr 0x606128c4, ip 0x6012907e
>    Call Trace:
>     [<60611ec2>] ? indx_read_ra+0x0/0x677
>
> At 168+ bytes per frame, ~97 recursions overflow the 16KB kernel
> stack. Desktop automount (udisks2 + ntfs3) means a crafted USB
> drive can trigger this without privilege.
>
> Note: the pre-existing indx_node allocated by indx_read() during
> the DFS is leaked when the new depth limit fires. This is a
> pre-existing issue (the node was also leaked on any other error
> return from indx_find_buffer); fixing it cleanly requires
> restructuring the node ownership model and is left for a
> follow-up patch.
>
> Reproducer script and crafted image builder available on request.
>
>   fs/ntfs3/index.c | 15 ++++++++++++---
>   1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ntfs3/index.c b/fs/ntfs3/index.c
> index 97f06c26fe1a..2c43e7c27861 100644
> --- a/fs/ntfs3/index.c
> +++ b/fs/ntfs3/index.c
> @@ -2013,13 +2013,21 @@ int indx_insert_entry(struct ntfs_index *indx, struct ntfs_inode *ni,
>   static struct indx_node *indx_find_buffer(struct ntfs_index *indx,
>                                            struct ntfs_inode *ni,
>                                            const struct INDEX_ROOT *root,
> -                                         __le64 vbn, struct indx_node *n)
> +                                         __le64 vbn, struct indx_node *n,
> +                                         int depth)
>   {
>          int err;
>          const struct NTFS_DE *e;
>          struct indx_node *r;
>          const struct INDEX_HDR *hdr = n ? &n->index->ihdr : &root->ihdr;
>
> +       /*
> +        * Limit recursion depth to prevent stack overflow from crafted
> +        * images.  Use the same bound as the fnd->nodes array (20).
> +        */
> +       if (depth > ARRAY_SIZE(((struct ntfs_fnd *)NULL)->nodes))
> +               return ERR_PTR(-EINVAL);
> +
>          /* Step 1: Scan one level. */
>          for (e = hdr_first_de(hdr);; e = hdr_next_de(hdr, e)) {
>                  if (!e)
> @@ -2040,7 +2048,8 @@ static struct indx_node *indx_find_buffer(struct ntfs_index *indx,
>                          if (err)
>                                  return ERR_PTR(err);
>
> -                       r = indx_find_buffer(indx, ni, root, vbn, n);
> +                       r = indx_find_buffer(indx, ni, root, vbn, n,
> +                                            depth + 1);
>                          if (r)
>                                  return r;
>                  }
> @@ -2446,7 +2455,7 @@ int indx_delete_entry(struct ntfs_index *indx, struct ntfs_inode *ni,
>
>                  fnd_clear(fnd);
>
> -               in = indx_find_buffer(indx, ni, root, sub_vbn, NULL);
> +               in = indx_find_buffer(indx, ni, root, sub_vbn, NULL, 0);
>                  if (IS_ERR(in)) {
>                          err = PTR_ERR(in);
>                          goto out;
> --
> 2.53.0
>
Hello,

Sorry for the delay.
Your patch was applied, thanks.

Regards,
Konstantin