Messages in this thread Patch in this message | | | From | Brian Norris <> | Subject | [PATCH v2 4/8] mtd: spi-nor: disallow further writes to SR if WP# is low | Date | Fri, 29 Jan 2016 11:25:33 -0800 |
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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> --- v2: * added clearing the SRWD bit when unlocking the entire flash
drivers/mtd/spi-nor/spi-nor.c | 7 +++++++ 1 file changed, 7 insertions(+)
diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c index 68133b949fe5..54eaf4b5bf05 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; -- 2.7.0.rc3.207.g0ac5344
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