linux-kernel - Re: [PATCH] sysctl: Fix conversion of INT

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Date:	Sat, 11 Jul 2015 20:15:50 +0300
From:	Ilya Dryomov <idryomov@...il.com>
To:	Robert Xiao <brx@...cmu.edu>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mikulas Patocka <mikulas@...bright.com>,
	Robert Xiao <nneonneo@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] sysctl: Fix conversion of INT_MIN for LP64 systems

On Sat, Jul 11, 2015 at 10:35 AM, Robert Xiao <brx@...cmu.edu> wrote:
> On LP64 systems, reading a sysctl file containing an INT_MIN (-2147483648)
> could incorrectly show -18446744071562067968 due to an incorrect conversion
> in do_proc_dointvec_conv. This patch fixes the edge case by converting to
> unsigned int first to avoid sign extending INT_MIN to unsigned long.
>
> Test:
>
> root:/proc/sys/kernel# echo -2147483648 0 0 0 > printk
> root:/proc/sys/kernel# cat printk
>
> Without patch, produces -18446744071562067968 0 0 0.
> With patch, should produce -2147483648 0 0 0.
>
> Signed-off-by: Robert Xiao <brx@...cmu.edu>
> ---
>  kernel/sysctl.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 19b62b5..464df36 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1995,10 +1995,10 @@ static int do_proc_dointvec_conv(bool *negp, unsigned long *lvalp,
>                 int val = *valp;
>                 if (val < 0) {
>                         *negp = true;
> -                       *lvalp = (unsigned long)-val;
> +                       *lvalp = (unsigned int)-val;
>                 } else {
>                         *negp = false;
> -                       *lvalp = (unsigned long)val;
> +                       *lvalp = (unsigned int)val;
>                 }
>         }
>         return 0;

I don't know why am I CC'ed on this - CC'ing Andrew along with Eric and
Kees who seem to have worked directly on sysctl.c not too long ago.

That said, I took a look at this and I think this patch is wrong.
Casting to unsigned int instead of unsigned long *after* the negation
is bogus, because we have

    if (val < 0)
        ...
        *lvalp = (unsigned long)-val;

and the compiler is free to assume -val to be positive and use the
sign-extend instruction.  On gcc (GCC) 4.8.3 20140911 (Red Hat 4.8.3-7)
that I have here the cast to unsigned int works only with -O1, with -O2
it goes to town and uses cltq which sign-extends:

    neg    %eax
    movb   $0x1,(%rdi)
    cltq

IMO the right way to do this would be to first cast to unsigned long
and then negate - that way we will first sign-extend and then negate an
unsigned, which is well defined.  Also, this needs to be done not just
for do_proc_dointvec_conv(), but for do_proc_dointvec_minmax_conv() and
jiffies functions as well (although it's probably virtually impossible
to set val to exactly INT_MIN through jiffies write branches).

Speaking of write branches, only do_proc_dointvec_conv() does check
it's input properly, so that's something to look at.

Thanks,

                Ilya
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