mtd: spi-nor: disallow further writes to SR if WP# is low

Locking the flash is most useful if it provides real hardware security.
Otherwise, it's little more than a software permission bit.

A reasonable use case that provides real HW security might be like
follows:

(1) hardware WP# is deasserted
(2) program flash
(3) flash range is protected via status register
(4) hardware WP# is asserted
(5) flash protection range can no longer be changed, until WP# is
    deasserted

In this way, flash protection is co-owned by hardware and software.

Now, one would expect to be able to perform step (3) with
ioctl(MEMLOCK), except that the spi-nor driver does not set the Status
Register Protect bit (a.k.a. Status Register Write Disable (SRWD)), so
even though the range is now locked, it does not satisfy step (5) -- it
can still be changed by a call to ioctl(MEMUNLOCK).

So, let's enable status register protection after the first lock
command, and disable protection only when the flash is fully unlocked.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Tested-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>

drivers/mtd/spi-nor/spi-nor.c

@@ -540,6 +540,9 @@ static int stm_lock(struct spi_nor *nor, loff_t ofs, uint64_t len)
 
 	status_new = (status_old & ~mask) | val;
 
+	/* Disallow further writes if WP pin is asserted */
+	status_new |= SR_SRWD;
+
 	/* Don't bother if they're the same */
 	if (status_new == status_old)
 		return 0;
@@ -605,6 +608,10 @@ static int stm_unlock(struct spi_nor *nor, loff_t ofs, uint64_t len)
 
 	status_new = (status_old & ~mask) | val;
 
+	/* Don't protect status register if we're fully unlocked */
+	if (lock_len == mtd->size)
+		status_new &= ~SR_SRWD;
+
 	/* Don't bother if they're the same */
 	if (status_new == status_old)
 		return 0;