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This is not a normal data I/O hot path. The single in-tree caller is
the $LogFile emptying path used during read-write mount/remount, and
the bug only becomes visible on NTFS volumes whose cluster_size is
strictly smaller than the kernel's PAGE_SIZE (typically 4 KiB on
x86_64). Per Microsoft's format command documentation, NTFS supports
allocation unit sizes starting at 512 bytes, so 512 B, 1 KiB and 2 KiB
clusters are uncommon but valid on-disk configurations. When
cluster_size >= PAGE_SIZE every "start" passed in is page-aligned and
the buggy "from != 0" path is never taken.
ntfs_bdev_write() splits the write across one or more block-device
folios. Inside the loop, "to" is computed as the *end byte offset*
within the current page (0..PAGE_SIZE), and "from" is the start byte
offset within the page (reset to 0 from the second iteration onward).
The copy length should therefore be "to - from", but the current code
uses "to" directly:
to = min_t(u32, end - offset, PAGE_SIZE);
memcpy_to_folio(folio, from, buf + buf_off, to);
buf_off += to;
When "from != 0" (i.e. "start" is not page-aligned) memcpy_to_folio()
copies "from" extra bytes:
- it reads "from" bytes past the source buffer into kernel heap;
- it writes "from" bytes past the requested range into the next part
of the block-device page (or, if "from + to > PAGE_SIZE", past the
folio boundary entirely, which trips the VM_BUG_ON inside
memcpy_to_folio() on CONFIG_DEBUG_VM=y kernels).
"buf_off" is then advanced by the wrong amount, so every subsequent
iteration also reads the source buffer at the wrong offset and writes
the wrong content to disk.
ntfs_empty_logfile() calls
ntfs_bdev_write(sb, empty_buf, NTFS_CLU_TO_B(vol, lcn),
vol->cluster_size);
with empty_buf sized to vol->cluster_size. On a sub-PAGE_SIZE-cluster
volume, any $LogFile run whose LCN is not aligned to
PAGE_SIZE / cluster_size reaches the non-page-aligned path. The
over-copy can read beyond empty_buf and overwrite the sectors following
the requested cluster in the block-device page with unrelated kernel
heap contents while $LogFile is being emptied.
A userspace reducer of the same arithmetic and copy loop confirms the
bug under AddressSanitizer: ASan reports a heap-buffer-overflow read
past the source buffer for the buggy length, and the fixed version is
ASan-clean.
Compute the copy length as "to - from" and advance buf_off by the same
amount.
Fixes: 5218cd102aec ("ntfs: update misc operations")
Link: https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/format
Signed-off-by: DaeMyung Kang <charsyam@gmail.com>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
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