[PATCH v7 2/2] badblocks: validate sector range and shift before rounding

[Ramesh Adhikari <adhikari.resume@gmail.com>]

_badblocks_set(), _badblocks_clear() and badblocks_check() round
the caller-supplied [s, s+sectors) range to the current bb->shift
block size before touching the bad block table. That rounding
was not defensive against a few cases:

- s + sectors can overflow sector_t (u64), wrapping the range
  end before it is ever compared against s.

- bb->shift is a plain 'int' field, populated in one case
  (drivers/md/md.c, from the on-disk superblock's bblog_shift)
  straight from an unvalidated byte with no upper bound. Shifting
  by an amount >= the width of the shifted type is undefined
  behaviour in C; "1 << bb->shift" was shifting an int literal,
  so this was already undefined for bb->shift >= 32, let alone
  the full 0-255 range bblog_shift allows.

- round_up()/round_down() rounding a value near ULLONG_MAX can
  itself wrap back to a small value, so even with a valid shift
  the rounded end of the range could end up smaller than the
  rounded start, silently turning a small range into a huge one
  (in _badblocks_clear()/badblocks_check(), which round the end
  up) or losing the range entirely.

Add an explicit s+sectors overflow check and cast the shift
operand to sector_t so the shift itself is never performed on a
32-bit int, and detect post-rounding wrap by comparing the
rounded result back against the pre-rounding value.

badblocks.c does not itself bound bb->shift: every caller except
drivers/md/md.c always leaves it at 0, so the one caller that
populates it from untrusted on-disk data is responsible for
bounding it before assigning it, per struct badblocks's shift
field documentation in include/linux/badblocks.h. That md.c-side
bound is being sent as a separate patch.

badblocks_check() returns 0 rather than -EINVAL on the wrap case,
matching its existing "0: no known bad blocks in the range"
return convention instead of introducing a new error path callers
don't expect.

Suggested-by: Coly Li <colyli@fygo.io>
Fixes: aa511ff8218b ("badblocks: switch to the improved badblock handling code")
Cc: stable@vger.kernel.org
Signed-off-by: Ramesh Adhikari <adhikari.resume@gmail.com>
Reviewed-by: Coly Li <colyli@fygo.io>
---
Changes in v7 (per Coly Li's review of v6):
 - Simplify the overflow check in all three call sites from
   "s > ULLONG_MAX - sectors" to "(s + sectors) < s".
 - Drop the "bb->shift >= BITS_PER_LONG_LONG" guards in badblocks.c.
   Only drivers/md/md.c ever sets a nonzero bb->shift, so the bound
   belongs where bblog_shift is read off the on-disk superblock, not
   scattered through the badblocks API. Documented the caller
   requirement on struct badblocks.shift in badblocks.h instead; the
   md.c-side bound follows as a separate patch.
 - badblocks_check() now returns 0 instead of -EINVAL on the wrap
   case, consistent with its existing return convention.

 block/badblocks.c         | 40 ++++++++++++++++++++++++++++++++-------
 include/linux/badblocks.h |  6 +++++-
 2 files changed, 38 insertions(+), 8 deletions(-)

diff --git a/block/badblocks.c b/block/badblocks.c
index 1f786b193fb..728b59a1a5d 100644
--- a/block/badblocks.c
+++ b/block/badblocks.c
@@ -853,12 +853,19 @@ static bool _badblocks_set(struct badblocks *bb, sector_t s, sector_t sectors,
 		/* Invalid sectors number */
 		return false;

+	if ((s + sectors) < s)
+		/* Range wraps past the end of sector_t */
+		return false;
+
 	if (bb->shift) {
 		/* round the start down, and the end up */
 		sector_t next = s + sectors;

-		s = round_down(s, 1 << bb->shift);
-		next = round_up(next, 1 << bb->shift);
+		s = round_down(s, (sector_t)1 << bb->shift);
+		next = round_up(next, (sector_t)1 << bb->shift);
+		if (next <= s)
+			/* Rounding wrapped past the end of sector_t */
+			return false;
 		sectors = next - s;
 	}

@@ -1061,7 +1068,12 @@ static bool _badblocks_clear(struct badblocks *bb, sector_t s, sector_t sectors)
 		/* Invalid sectors number */
 		return false;

+	if ((s + sectors) < s)
+		/* Range wraps past the end of sector_t */
+		return false;
+
 	if (bb->shift) {
+		sector_t orig_s = s;
 		sector_t target;

 		/* When clearing we round the start up and the end down.
@@ -1071,9 +1083,16 @@ static bool _badblocks_clear(struct badblocks *bb, sector_t s, sector_t sectors)
 		 * isn't than to think a block is not bad when it is.
 		 */
 		target = s + sectors;
-		s = round_up(s, 1 << bb->shift);
-		target = round_down(target, 1 << bb->shift);
-		sectors = target - s;
+		s = round_up(s, (sector_t)1 << bb->shift);
+		target = round_down(target, (sector_t)1 << bb->shift);
+		if (s < orig_s || target < s)
+			/* Rounding wrapped, or range collapsed */
+			sectors = 0;
+		else
+			sectors = target - s;
+
+		if (sectors == 0)
+			return false;
 	}

 	write_seqlock_irq(&bb->lock);
@@ -1303,12 +1322,19 @@ int badblocks_check(struct badblocks *bb, sector_t s, sector_t sectors,

 	WARN_ON(bb->shift < 0 || sectors == 0);

+	if ((s + sectors) < s)
+		/* Range wraps past the end of sector_t */
+		return 0;
+
 	if (bb->shift > 0) {
 		/* round the start down, and the end up */
 		sector_t target = s + sectors;

-		s = round_down(s, 1 << bb->shift);
-		target = round_up(target, 1 << bb->shift);
+		s = round_down(s, (sector_t)1 << bb->shift);
+		target = round_up(target, (sector_t)1 << bb->shift);
+		if (target <= s)
+			/* Rounding wrapped past the end of sector_t */
+			return 0;
 		sectors = target - s;
 	}

diff --git a/include/linux/badblocks.h b/include/linux/badblocks.h
index 996493917f3..5d88992b55a 100644
--- a/include/linux/badblocks.h
+++ b/include/linux/badblocks.h
@@ -34,7 +34,11 @@ struct badblocks {
 				 */
 	int shift;		/* shift from sectors to block size
 				 * a -ve shift means badblocks are
-				 * disabled.*/
+				 * disabled. Callers that set this from
+				 * untrusted/on-disk data are responsible
+				 * for bounding it so 1 << shift does not
+				 * overflow a sector_t.
+				 */
 	u64 *page;		/* badblock list */
 	int changed;
 	seqlock_t lock;
--
2.43.0