Re: [PATCH] parisc: Fix csum_ipv6_magic on 64-bit systems

Charlie Jenkins
2024-02-14T20:22:38Z

On Tue, Feb 13, 2024 at 03:46:31PM -0800, Guenter Roeck wrote:
> hppa 64-bit systems calculates the IPv6 checksum using 64-bit add
> operations. The last add folds protocol and length fields into the 64-bit
> result. While unlikely, this operation can overflow. The overflow can be
> triggered with a code sequence such as the following.
>
> /* try to trigger massive overflows */
> memset(tmp_buf, 0xff, sizeof(struct in6_addr));
> csum_result = csum_ipv6_magic((struct in6_addr *)tmp_buf,
> (struct in6_addr *)tmp_buf,
> 0xffff, 0xff, 0xffffffff);
>
> Fix the problem by adding any overflows from the final add operation into
> the calculated checksum. Fortunately, we can do this without additional
> cost by replacing the add operation used to fold the checksum into 32 bit
> with "add,dc" to add in the missing carry.
>
> Cc: Charlie Jenkins <charlie@rivosinc.com>
> Cc: Palmer Dabbelt <palmer@rivosinc.com>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> ---
> This patch does not completely fix the problems with csum_ipv6_magic seen
> when running 64-bit parisc images with the C3700 emulation in qemu. That
> is due to unaligned 64-bit load operations which (presumably as part of
> unaligned trap handling) generate bad carry flags. It is unknown if that
> is a problem with the qemu emulation or with the Linux kernel, so it is not
> addressed here.
>
> arch/parisc/include/asm/checksum.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/parisc/include/asm/checksum.h b/arch/parisc/include/asm/checksum.h
> index e619e67440db..c949aa20fa16 100644
> --- a/arch/parisc/include/asm/checksum.h
> +++ b/arch/parisc/include/asm/checksum.h
> @@ -137,8 +137,8 @@ static __inline__ __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
> " add,dc %3, %0, %0\n" /* fold in proto+len | carry bit */
> " extrd,u %0, 31, 32, %4\n"/* copy upper half down */
> " depdi 0, 31, 32, %0\n"/* clear upper half */
> -" add %4, %0, %0\n" /* fold into 32-bits */
> -" addc 0, %0, %0\n" /* add carry */
> +" add,dc %4, %0, %0\n" /* fold into 32-bits, plus carry */
> +" addc 0, %0, %0\n" /* add final carry */
>
> #else
>
> --
> 2.39.2
>

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>