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