linux-kernel - [PATCH 4/8] mtd: spi-nor: disallow further writes to SR if WP# is low

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1453960307-10181-5-git-send-email-computersforpeace@gmail.com>
Date:	Wed, 27 Jan 2016 21:51:43 -0800
From:	Brian Norris <computersforpeace@...il.com>
To:	<linux-mtd@...ts.infradead.org>
Cc:	Brian Norris <computersforpeace@...il.com>,
	Rafał Miłecki <zajec5@...il.com>,
	Ezequiel Garcia <ezequiel@...guardiasur.com.ar>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	linux-kernel@...r.kernel.org, Bayi Cheng <bayi.cheng@...iatek.com>,
	Marek Vasut <marex@...x.de>, djkurtz@...omium.org
Subject: [PATCH 4/8] 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.

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

diff --git a/drivers/mtd/spi-nor/spi-nor.c b/drivers/mtd/spi-nor/spi-nor.c
index 3a08aa53c171..46da6bb706fa 100644
--- a/drivers/mtd/spi-nor/spi-nor.c
+++ b/drivers/mtd/spi-nor/spi-nor.c
@@ -518,6 +518,9 @@ 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;
-- 
1.7.9.5