[PATCH] mtd: cfi_cmdset_0001: Do not check for OTP outside device

[Nicolas Pitre]

On Tue, 30 May 2023, Linus Walleij wrote:

> 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 flash memory ends at
> 0x11ffffff, so 0x12000000 is outside the range of the MTD
> device.
> 
> Breaking the while loop of we offset outside the size of the
> MTD device fixes the issue.
> 
> Cc: Nicolas Pitre <npitre at baylibre.com>
> Signed-off-by: Linus Walleij <linus.walleij at linaro.org>

Well well... I apparently wrote that code... 23 years ago! Dang!
Obviously I barely remember the details.

Still, I have reservations about your fix which looks like it is 
papering over the real issue. Looking at the original code, we can see 
this from line 2418:

                        /* next OTP region */
                        if (++field == extp->NumProtectionFields)
                                break;

That's where the loop break should be happening. Why isn't it the case 
for you? Especially given that 0x12000000 is completely bogus for sure, 
which is likely random memory content outside the actual extp record. 
Next time that random memory content may well be <= mtd->size and your 
fix won't do anything.

This needs more investigation.

> ---
>  drivers/mtd/chips/cfi_cmdset_0001.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/mtd/chips/cfi_cmdset_0001.c b/drivers/mtd/chips/cfi_cmdset_0001.c
> index 54f92d09d9cf..a979e0316b31 100644
> --- a/drivers/mtd/chips/cfi_cmdset_0001.c
> +++ b/drivers/mtd/chips/cfi_cmdset_0001.c
> @@ -2352,6 +2352,9 @@ static int cfi_intelext_otp_walk(struct mtd_info *mtd, loff_t from, size_t len,
>  			reg_fact_size *= cfi->interleave;
>  			reg_user_size *= cfi->interleave;
>  
> +			if (reg_prot_offset >= mtd->size)
> +				break;
> +
>  			if (user_regs) {
>  				groups = reg_user_groups;
>  				groupsize = reg_user_size;
> -- 
> 2.40.1
> 
>