[PATCH v2] mtd: cfi_cmdset_0001: Byte swap OTP info

[Linus Walleij]

Currently the offset into the device when looking for OTP
bits can go outside of the address of the MTD NOR devices,
and if that memory isn't readable, bad things happen
on the IXP4xx (added prints that illustrate the problem before
the crash):

cfi_intelext_otp_walk walk OTP on chip 0 start at reg_prot_offset 0x00000100
ixp4xx_copy_from copy from 0x00000100 to 0xc880dd78
cfi_intelext_otp_walk walk OTP on chip 0 start at reg_prot_offset 0x12000000
ixp4xx_copy_from copy from 0x12000000 to 0xc880dd78
8<--- cut here ---
Unable to handle kernel paging request at virtual address db000000
[db000000] *pgd=00000000
(...)

This happens in this case because the IXP4xx is big endian and
the 32- and 16-bit fields in the struct cfi_intelext_otpinfo are not
properly byteswapped. Compare to how the code in read_pri_intelext()
byteswaps the fields in struct cfi_pri_intelext.

Adding some byte swapping after casting the &extp->extra[0] into
a struct cfi_intelext_otpinfo * pointer, and the crash goes away.

The problem went unnoticed for many years until I enabled
CONFIG_MTD_OTP on the IXP4xx as well, triggering the bug.

Cc: Nicolas Pitre <npitre at baylibre.com>
Cc: stable at vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
---
ChangeLog v1->v2:
- Drill deeper and discover a big endian compatibility issue.
---
 drivers/mtd/chips/cfi_cmdset_0001.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/mtd/chips/cfi_cmdset_0001.c b/drivers/mtd/chips/cfi_cmdset_0001.c
index 54f92d09d9cf..7603b0052a16 100644
--- a/drivers/mtd/chips/cfi_cmdset_0001.c
+++ b/drivers/mtd/chips/cfi_cmdset_0001.c
@@ -2336,6 +2336,11 @@ static int cfi_intelext_otp_walk(struct mtd_info *mtd, loff_t from, size_t len,
 		chip = &cfi->chips[chip_num];
 		otp = (struct cfi_intelext_otpinfo *)&extp->extra[0];
 
+		/* Do some byteswapping if necessary */
+		otp->ProtRegAddr = le32_to_cpu(otp->ProtRegAddr);
+		otp->FactGroups = le16_to_cpu(otp->FactGroups);
+		otp->UserGroups = le16_to_cpu(otp->UserGroups);
+
 		/* first OTP region */
 		field = 0;
 		reg_prot_offset = extp->ProtRegAddr;
-- 
2.40.1