[PATCH] fs/ntfs3: validate rec->used in journal-replay file record check

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

check_file_record() validates rec->total against the record size but
never validates rec->used.  The do_action() journal-replay handlers read
rec->used from disk and use it to compute memmove lengths:

  DeleteAttribute:    memmove(attr, ..., used - asize - roff)
  CreateAttribute:    memmove(..., attr, used - roff)
  change_attr_size:   memmove(..., used - PtrOffset(rec, next))

When rec->used is smaller than the offset of a validated attribute, or
larger than the record size, these subtractions can underflow allowing
us to copy huge amounts of memory in to a 4kb buffer, generally
considered a bad idea overall.

This requires a corrupted filesystem, which isn't a threat model the
kernel really needs to worry about, but checking for such an obvious
out-of-bounds value is good to keep things robust, especially on journal
replay

Fix this up by bounding rec->used correctly.

This is much like commit b2bc7c44ed17 ("fs/ntfs3: Fix slab-out-of-bounds
read in DeleteIndexEntryRoot") which checked different values in this
same switch statement.

Cc: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
Fixes: b46acd6a6a62 ("fs/ntfs3: Add NTFS journal")
Cc: stable <stable@kernel.org>
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/ntfs3/fslog.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/ntfs3/fslog.c b/fs/ntfs3/fslog.c
index 272e45276143..037df47fa9f3 100644
--- a/fs/ntfs3/fslog.c
+++ b/fs/ntfs3/fslog.c
@@ -2791,13 +2791,14 @@ static inline bool check_file_record(const struct MFT_REC *rec,
 	u16 fn = le16_to_cpu(rec->rhdr.fix_num);
 	u16 ao = le16_to_cpu(rec->attr_off);
 	u32 rs = sbi->record_size;
+	u32 used = le32_to_cpu(rec->used);
 
 	/* Check the file record header for consistency. */
 	if (rec->rhdr.sign != NTFS_FILE_SIGNATURE ||
 	    fo > (SECTOR_SIZE - ((rs >> SECTOR_SHIFT) + 1) * sizeof(short)) ||
 	    (fn - 1) * SECTOR_SIZE != rs || ao < MFTRECORD_FIXUP_OFFSET_1 ||
 	    ao > sbi->record_size - SIZEOF_RESIDENT || !is_rec_inuse(rec) ||
-	    le32_to_cpu(rec->total) != rs) {
+	    le32_to_cpu(rec->total) != rs || used > rs || used < ao) {
 		return false;
 	}
 
@@ -2809,6 +2810,15 @@ static inline bool check_file_record(const struct MFT_REC *rec,
 		return false;
 	}
 
+	/*
+	 * The do_action() handlers compute memmove lengths as
+	 * "rec->used - <offset of validated attr>", which underflows when
+	 * rec->used is smaller than the attribute walk reached.  At this
+	 * point attr is the ATTR_END marker; rec->used must cover it.
+	 */
+	if (used < PtrOffset(rec, attr) + sizeof(attr->type))
+		return false;
+
 	return true;
 }
 
-- 
2.53.0