Re: [PATCH] s390/extable: Fix mvcos instruction decoding

[Sven Schnelle <svens@linux.ibm.com>]

Heiko Carstens <hca@linux.ibm.com> writes:

> If either the b1 or b2 field of the mvcos instruction is zero, register
> zero is not used for address computation; zero is used instead, according
> to the architecture. ex_handler_ua_mvcos() incorrectly uses the real
> contents of register zero. Fix this.
>
> This shouldn't be a problem for any generated code, since the compiler
> would only generate such code if it is possible to prove at compile time
> that either the source or destination address is within the first 4kb of
> the designated address space. Such code should not exist (also confirmed
> by an allyesconfig build and decoding all ~7000 mvcos instructions).
>
> Fixes: c488f5187a24 ("s390/uaccess: Shorten raw_copy_from_user() / raw_copy_to_user() inline assemblies")
> Cc: stable@vger.kernel.org
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
> ---
>  arch/s390/mm/extable.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/s390/mm/extable.c b/arch/s390/mm/extable.c
> index 7498e858c401..848855da9a70 100644
> --- a/arch/s390/mm/extable.c
> +++ b/arch/s390/mm/extable.c
> @@ -106,9 +106,9 @@ static bool ex_handler_ua_mvcos(const struct exception_table_entry *ex,
>  	regs->psw.addr = extable_fixup(ex);
>  	insn = (struct insn_ssf *)regs->psw.addr;
>  	if (from)
> -		uaddr = regs->gprs[insn->b2] + insn->d2;
> +		uaddr = (insn->b2 ? regs->gprs[insn->b2] : 0) + insn->d2;
>  	else
> -		uaddr = regs->gprs[insn->b1] + insn->d1;
> +		uaddr = (insn->b1 ? regs->gprs[insn->b1] : 0) + insn->d1;
>  	remainder = PAGE_SIZE - (uaddr & (PAGE_SIZE - 1));
>  	if (regs->gprs[insn->r3] <= remainder)
>  		remainder = 0;

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>