The per-token range check in ntfs_decompress() uses

     if (cb >= cb_sb_end || dp_addr > dp_sb_end)
         break;

so dp_addr == dp_sb_end falls through to the symbol copy
`*dp_addr++ = *cb++`, writing one byte past the destination page. Since
NTFS_SB_SIZE == PAGE_SIZE the destination is a single page, so the byte
lands in the adjacent page, and *dest_ofs is left one past the sub-block
end (the later `*dest_ofs &= ~PAGE_MASK` then yields 1, not 0, so the page
is never finalized and later sub-blocks keep writing further past it).  A
corrupted compressed $DATA attribute then produces a bounded run of
out-of-bounds writes when the file is read.

The fix is to break as soon as dp_addr reaches dp_sb_end; a full
sub-block still completes, as its final copy advances dp_addr to exactly
dp_sb_end.

Fixes: 1e9ea7e04472 ("Revert \"fs: Remove NTFS classic\"")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Dennis Tighe <dennis.tighe@gmail.com>
---
Reproducer is available on request if needed for validation. I was able to
craft an imagine that would exercise this scenario leading to KASAN UAF
errors on read-only mounts.

  fs/ntfs/compress.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ntfs/compress.c b/fs/ntfs/compress.c
index 76bd806b41e..dc5ad951dc6 100644
--- a/fs/ntfs/compress.c
+++ b/fs/ntfs/compress.c
@@ -352,7 +352,7 @@ static int ntfs_decompress(struct page 
*dest_pages[], int completed_pages[],
          u8 *dp_back_addr;

          /* Check if we are done / still in range. */
-        if (cb >= cb_sb_end || dp_addr > dp_sb_end)
+        if (cb >= cb_sb_end || dp_addr >= dp_sb_end)
              break;

          /* Determine token type and parse appropriately.*/
-- 
2.47.3

2026년 8월 18일 (화) 오전 12:18, Dennis Tighe <dennis.tighe@gmail.com>님이 작성:
>
> The per-token range check in ntfs_decompress() uses
>
>      if (cb >= cb_sb_end || dp_addr > dp_sb_end)
>          break;
>
> so dp_addr == dp_sb_end falls through to the symbol copy
> `*dp_addr++ = *cb++`, writing one byte past the destination page. Since
> NTFS_SB_SIZE == PAGE_SIZE the destination is a single page, so the byte
> lands in the adjacent page, and *dest_ofs is left one past the sub-block
> end (the later `*dest_ofs &= ~PAGE_MASK` then yields 1, not 0, so the page
> is never finalized and later sub-blocks keep writing further past it).  A
> corrupted compressed $DATA attribute then produces a bounded run of
> out-of-bounds writes when the file is read.
>
> The fix is to break as soon as dp_addr reaches dp_sb_end; a full
> sub-block still completes, as its final copy advances dp_addr to exactly
> dp_sb_end.
>
> Fixes: 1e9ea7e04472 ("Revert \"fs: Remove NTFS classic\"")
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Dennis Tighe <dennis.tighe@gmail.com>

This patch looks good to me, but I could not
apply this patch:
  warning: Patch sent with format=flowed; space at
    the end of lines might be lost.

Could you send this patch again?

Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>

> ---
> Reproducer is available on request if needed for validation. I was able to
> craft an imagine that would exercise this scenario leading to KASAN UAF
> errors on read-only mounts.
>
>   fs/ntfs/compress.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ntfs/compress.c b/fs/ntfs/compress.c
> index 76bd806b41e..dc5ad951dc6 100644
> --- a/fs/ntfs/compress.c
> +++ b/fs/ntfs/compress.c
> @@ -352,7 +352,7 @@ static int ntfs_decompress(struct page
> *dest_pages[], int completed_pages[],
>           u8 *dp_back_addr;
>
>           /* Check if we are done / still in range. */
> -        if (cb >= cb_sb_end || dp_addr > dp_sb_end)
> +        if (cb >= cb_sb_end || dp_addr >= dp_sb_end)
>               break;
>
>           /* Determine token type and parse appropriately.*/
> --
> 2.47.3
>
>


-- 
Thanks,
Hyunchul