[PATCH] fs/ntfs3: fix info-leak on partial LZNT decompress in ni_read_frame()

[Samuel Page <sam@bynar.io>]

ni_read_frame() decompresses an LZNT $DATA frame into the vmapped target
pages and then trusts decompress_lznt()'s return value:

  unc_size = decompress_lznt(frame_ondisk, ondisk_size, frame_mem,
                             frame_size);
  if ((ssize_t)unc_size < 0)        err = unc_size;
  else if (!unc_size || unc_size > frame_size)  err = -EINVAL;

decompress_lznt() stops as soon as the compressed stream is exhausted
(e.g. a zero chunk header) and returns the number of bytes it actually
wrote, which may be far less than frame_size. The bytes between unc_size
and frame_size are never written. The only memset() that follows zeroes
the region beyond i_valid; when the frame lies entirely within the file's
valid size that memset() does not run, so the gap retains whatever was in
the just-vmapped pages. All pages are then marked uptodate and returned
to userspace, disclosing uninitialized (recently-freed) kernel page
memory. A crafted compressed file whose stream decompresses to only a few
bytes leaks the remainder of every frame on a plain read(2), which is
enough to recover kernel pointers and defeat KASLR.

Zero the [unc_size, frame_size) tail immediately after a successful LZNT
decompress so the remainder reads back as zero.

Fixes: 4342306f0f0d ("fs/ntfs3: Add file operations and implementation")
Cc: stable@vger.kernel.org
Assisted-by: Bynario AI
Signed-off-by: Samuel Page <sam@bynar.io>
---
Reproduced on a non-KASAN, KASLR-enabled arm64 guest: a crafted file whose
64 KiB LZNT frames each decompress to 8 bytes leaks the [8, 65536) tail of
every frame on a plain read(2) - up to ~3.3 MiB of uninitialized page memory,
including kernel-text return addresses from freed VMAP_STACK pages, from
which the KASLR base was recovered and verified vs /proc/kallsyms. With the
patch the same read returns zeroes; a zeroed control image (valid_size
truncated so the existing memset runs) leaks nothing, isolating the cause.

 fs/ntfs3/frecord.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/ntfs3/frecord.c b/fs/ntfs3/frecord.c
index 7b035da63c12..cd11df75bdbc 100644
--- a/fs/ntfs3/frecord.c
+++ b/fs/ntfs3/frecord.c
@@ -2443,6 +2443,15 @@ int ni_read_frame(struct ntfs_inode *ni, u64 frame_vbo, struct page **pages,
 			err = unc_size;
 		else if (!unc_size || unc_size > frame_size)
 			err = -EINVAL;
+		else if (unc_size < frame_size) {
+			/*
+			 * Partial decompress: zero the [unc_size, frame_size)
+			 * tail.  decompress_lznt() leaves it untouched, so
+			 * without this the freshly vmapped pages would expose
+			 * uninitialized kernel memory to userspace.
+			 */
+			memset(frame_mem + unc_size, 0, frame_size - unc_size);
+		}
 	}
 	if (!err && valid_size < frame_vbo + frame_size) {
 		size_t ok = valid_size - frame_vbo;
-- 
2.54.0