From 47b8edbf0d43dcb9fda83833c05470edf59c31e3 Mon Sep 17 00:00:00 2001 From: Brian Norris Date: Fri, 29 Jan 2016 11:25:33 -0800 Subject: 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 Tested-by: Ezequiel Garcia --- drivers/mtd/spi-nor/spi-nor.c | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'drivers/mtd') diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c index 680bc1510475..6fcd9d7589f8 100644 --- a/drivers/mtd/spi-nor/spi-nor.c +++ b/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; -- cgit v1.2.3