lib/bch: fix possible stack overrun

The previous patch introduced very large kernel stack usage and a Makefile
change to hide the warning about it.

From what I can tell, a number of things went wrong here:

- The BCH_MAX_T constant was set to the maximum value for 'n',
  not the maximum for 't', which is much smaller.

- The stack usage is actually larger than the entire kernel stack
  on some architectures that can use 4KB stacks (m68k, sh, c6x), which
  leads to an immediate overrun.

- The justification in the patch description claimed that nothing
  changed, however that is not the case even without the two points above:
  the configuration is machine specific, and most boards  never use the
  maximum BCH_ECC_WORDS() length but instead have something much smaller.
  That maximum would only apply to machines that use both the maximum
  block size and the maximum ECC strength.

The largest value for 't' that I could find is '32', which in turn leads
to a 60 byte array instead of 2048 bytes. With that changed, the warning
can be enabled again.

Fixes: 02361bc77888 ("lib/bch: Remove VLA usage")
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: stable@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
Please review carefully to ensure my logic is correct. I spent a
long time trying to understand what is going on here, but I'm not
too familiar with this algorithm, and it's possible I still got it
wrong as well.
In particular, I'm not 100% sure if '32' is the maximum ECC strength
we can support, or if a larger one (which?) might be possible.
---
 lib/Makefile |  1 -
 lib/bch.c    | 10 ++++++----
 2 files changed, 6 insertions(+), 5 deletions(-)

On Fri,  5 Oct 2018 17:56:51 +0200
Arnd Bergmann <arnd@arndb.de> wrote:

> The previous patch introduced very large kernel stack usage and a Makefile
> change to hide the warning about it.
> 
> From what I can tell, a number of things went wrong here:
> 
> - The BCH_MAX_T constant was set to the maximum value for 'n',
>   not the maximum for 't', which is much smaller.
> 
> - The stack usage is actually larger than the entire kernel stack
>   on some architectures that can use 4KB stacks (m68k, sh, c6x), which
>   leads to an immediate overrun.
> 
> - The justification in the patch description claimed that nothing
>   changed, however that is not the case even without the two points above:
>   the configuration is machine specific, and most boards  never use the
>   maximum BCH_ECC_WORDS() length but instead have something much smaller.
>   That maximum would only apply to machines that use both the maximum
>   block size and the maximum ECC strength.
> 
> The largest value for 't' that I could find is '32', which in turn leads
> to a 60 byte array instead of 2048 bytes. With that changed, the warning
> can be enabled again.
> 
> Fixes: 02361bc77888 ("lib/bch: Remove VLA usage")
> Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: stable@vger.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> Please review carefully to ensure my logic is correct. I spent a
> long time trying to understand what is going on here, but I'm not
> too familiar with this algorithm, and it's possible I still got it
> wrong as well.
> In particular, I'm not 100% sure if '32' is the maximum ECC strength
> we can support, or if a larger one (which?) might be possible.

Well, the ECC strength (T) is dynamically configurable through the
nand-ecc-strength param, so it's theoretically not limited to 32. This
being said, I doubt people are using soft BCH with more strength higher
than 32.

> ---
>  lib/Makefile |  1 -
>  lib/bch.c    | 10 ++++++----
>  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/Makefile b/lib/Makefile
> index 8c9647fa271a..12c479dd46e0 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -122,7 +122,6 @@ obj-$(CONFIG_ZLIB_INFLATE) += zlib_inflate/
>  obj-$(CONFIG_ZLIB_DEFLATE) += zlib_deflate/
>  obj-$(CONFIG_REED_SOLOMON) += reed_solomon/
>  obj-$(CONFIG_BCH) += bch.o
> -CFLAGS_bch.o := $(call cc-option,-Wframe-larger-than=4500)
>  obj-$(CONFIG_LZO_COMPRESS) += lzo/
>  obj-$(CONFIG_LZO_DECOMPRESS) += lzo/
>  obj-$(CONFIG_LZ4_COMPRESS) += lz4/
> diff --git a/lib/bch.c b/lib/bch.c
> index 7b0f2006698b..3ef1a3467e7b 100644
> --- a/lib/bch.c
> +++ b/lib/bch.c
> @@ -79,20 +79,19 @@
>  #define GF_T(_p)               (CONFIG_BCH_CONST_T)
>  #define GF_N(_p)               ((1 << (CONFIG_BCH_CONST_M))-1)
>  #define BCH_MAX_M              (CONFIG_BCH_CONST_M)
> +#define BCH_MAX_T	       (CONFIG_BCH_CONST_T)
>  #else
>  #define GF_M(_p)               ((_p)->m)
>  #define GF_T(_p)               ((_p)->t)
>  #define GF_N(_p)               ((_p)->n)
> -#define BCH_MAX_M              15
> +#define BCH_MAX_M              15 /* 2KB */
> +#define BCH_MAX_T              32 /* 32 bit correction */

I'd recommend adding a test on t here [1] (t > BCH_MAX_T), so that you
fail at init time if t is too big.

[1]https://elixir.bootlin.com/linux/v4.19-rc6/source/lib/bch.c#L1280 

>  #endif
>  
> -#define BCH_MAX_T              (((1 << BCH_MAX_M) - 1) / BCH_MAX_M)
> -
>  #define BCH_ECC_WORDS(_p)      DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 32)
>  #define BCH_ECC_BYTES(_p)      DIV_ROUND_UP(GF_M(_p)*GF_T(_p), 8)
>  
>  #define BCH_ECC_MAX_WORDS      DIV_ROUND_UP(BCH_MAX_M * BCH_MAX_T, 32)
> -#define BCH_ECC_MAX_BYTES      DIV_ROUND_UP(BCH_MAX_M * BCH_MAX_T, 8)
>  
>  #ifndef dbg
>  #define dbg(_fmt, args...)     do {} while (0)
> @@ -202,6 +201,9 @@ void encode_bch(struct bch_control *bch, const uint8_t *data,
>  	const uint32_t * const tab3 = tab2 + 256*(l+1);
>  	const uint32_t *pdata, *p0, *p1, *p2, *p3;
>  
> +	if (WARN_ON(r_bytes > sizeof(r)))
> +		return;
> +
>  	if (ecc) {
>  		/* load ecc parity bytes into internal 32-bit buffer */
>  		load_ecc8(bch, bch->ecc_buf, ecc);