[PATCH 2/8] mtd: spi-nor: guard against underflows in stm_is_locked_sr

[Brian Norris <computersforpeace@gmail.com>]

Users of stm_is_locked_sr() might do arithmetic that could result in a
negative offset. For example, when stm_unlock() tries to check the
status of the eraseblock below the range, it doesn't check for:

  ofs - mtd->erasesize < 0

Instead of forcing callers to be extra careful, let's just make
stm_is_locked_sr() do the right thing and report errors for invalid
ranges.

Also, fixup the calculations in stm_unlock(), so we:
(a) can handle non-eraseblock-aligned offsets and
(b) don't look for a negative offset when checking the first block

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
---
 drivers/mtd/spi-nor/spi-nor.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
index ef89bed1e5ea..c19674573eec 100644
--- a/drivers/mtd/spi-nor/spi-nor.c
+++ b/drivers/mtd/spi-nor/spi-nor.c
@@ -447,6 +447,9 @@ static int stm_is_locked_sr(struct spi_nor *nor, loff_t ofs, uint64_t len,
 	loff_t lock_offs;
 	uint64_t lock_len;
 
+	if (ofs < 0 || ofs + len > nor->mtd.size)
+		return -EINVAL;
+
 	stm_get_locked_range(nor, sr, &lock_offs, &lock_len);
 
 	return (ofs + len <= lock_offs + lock_len) && (ofs >= lock_offs);
@@ -543,9 +546,13 @@ static int stm_unlock(struct spi_nor *nor, loff_t ofs, uint64_t len)
 	if (status_old < 0)
 		return status_old;
 
-	/* Cannot unlock; would unlock larger region than requested */
-	if (stm_is_locked_sr(nor, ofs - mtd->erasesize, mtd->erasesize,
-			     status_old))
+	/*
+	 * Check the eraseblock next to us; if locked, then this would unlock
+	 * larger region than requested
+	 */
+	if (ofs > 0 && stm_is_locked_sr(nor, ALIGN(ofs - mtd->erasesize,
+					mtd->erasesize), mtd->erasesize,
+					status_old))
 		return -EINVAL;
 
 	/*
-- 
1.7.9.5