netdev - Re: [PATCH] sysctl: fix min/max handling in __do_proc_doulongvec

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Date:	Mon, 4 Oct 2010 18:35:45 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	Robin Holt <holt@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Willy Tarreau <w@....eu>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	"Pekka Savola (ipv6)" <pekkas@...core.fi>,
	Patrick McHardy <kaber@...sh.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Subject: Re: [PATCH] sysctl: fix min/max handling in
 __do_proc_doulongvec_minmax()

On Mon, Oct 04, 2010 at 12:10:30PM +0200, Eric Dumazet wrote:
>Le lundi 04 octobre 2010 à 17:34 +0800, Américo Wang a écrit :
>> On Mon, Oct 04, 2010 at 11:04:18AM +0200, Eric Dumazet wrote:
>> >Le lundi 04 octobre 2010 à 03:59 -0500, Robin Holt a écrit :
>> >> On Sat, Oct 02, 2010 at 03:17:49PM +0200, Eric Dumazet wrote:
>> >> > When proc_doulongvec_minmax() is used with an array of longs,
>> >> > and no min/max check requested (.extra1 or .extra2 being NULL), we
>> >> > dereference a NULL pointer for the second element of the array.
>> >> > 
>> >> > Noticed while doing some changes in network stack for the "16TB problem"
>> >> > 
>> >> > Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
>> >> > ---
>> >> >  kernel/sysctl.c |    3 ++-
>> >> >  1 file changed, 2 insertions(+), 1 deletion(-)
>> >> > 
>> >> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> >> > index f88552c..4fba86d 100644
>> >> > --- a/kernel/sysctl.c
>> >> > +++ b/kernel/sysctl.c
>> >> > @@ -2500,7 +2500,8 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
>> >> >  				break;
>> >> >  			if (neg)
>> >> >  				continue;
>> >> > -			if ((min && val < *min) || (max && val > *max))
>> >> > +			if ((table->extra1 && val < *min) ||
>> >> > +			    (table->extra2 && val > *max))
>> >> 
>> >> How about changing:
>> >>         for (; left && vleft--; i++, min++, max++, first=0) {
>> >> into:
>> >>         for (; left && vleft--; i++, min = min ? min + 1 : NULL, max = max ? max + 1: NULL, first=0) {
>> >> 
>> >> That would make min and max correct and reduce the chances somebody in
>> >> the future overlooks the fact they are currently filled with garbage.
>> >
>> >I prefer my solution, because the check is done only in the 'write'
>> >case, while its done also for 'read' in your solution, not counting the
>> >for (;;) is really ugly...
>> >
>> 
>> Sorry, I still don't get the point here, min and max
>> are pointers, they are already checked before dereferenced.
>> After your patch, min and max will be still increased, while
>> you are still checking ->extra{1,2} which is wrong.
>> 
>> I see no problem with the original code, or I must have missed something?
>
>Please re-read again. I had crashes, so original code is bugyy.
>
>Say you call __do_proc_doulongvec_minmax() with an array of three
>elements. And .extra1 = NULL, .extra2 = NULL (no range checks, this is a
>valid use case)
>
>First element, min = NULL OK. no problem so far.
>
>Second element, min = (long *)(NULL + sizeof(long))   -> BUG 
>
>Third element, min = (long *)(NULL + 2*sizeof(long)) -> BUG 
>
>After my patch, min/max increases normally (they are only pointers after
>all)
>
>But they are _dereferenced_ only if they were _not_ NULL at the
>beginning (extra1 not NULL for *min, extra2 not NULL for *max)
>

Hmm, I see, thanks for explanation.

Your patch does fix the problem, but seems not a good solution,
we should skip all min/max checking if ->extra(1|2) is NULL,
instead of checking it every time within the loop.

Thanks.

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