linux-kernel - Re: int overflow in io

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Date:	Wed, 6 Jan 2016 13:01:58 -0500
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
	linux-aio <linux-aio@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>
Subject: Re: int overflow in io_getevents

On Wed, Dec 16, 2015 at 07:38:33PM +0100, Dmitry Vyukov wrote:
> > Yup, looks correct. Will you send a patch?
> 
> I've drafted the verification:
> 
> @@ -1269,6 +1269,8 @@ static long read_events(struct kioctx *ctx, long
> min_nr, long nr,
> 
>                 if (unlikely(copy_from_user(&ts, timeout, sizeof(ts))))
>                         return -EFAULT;
> +               if (!timespec_valid_strict(&strict))
> +                       return -EINVAL;
> 
>                 until = timespec_to_ktime(ts);
>         }
> 
> But now I am thinking whether it is the right solution.
> First, user does not know about KTIME_MAX, so it is not unreasonable
> to pass timespec{INT64_MAX, INT64_MAX} as timeout expecting that it
> will block for a long time. And it actually probably mostly works now,
> because after the overflow you still get something large with high
> probability. If we do the fix, then users will need to pass seconds <
> KTIME_MAX, while they don't know KTIME_MAX value.
> Second, there seems to be more serious issue in ktime_set() which
> checks seconds for KTIME_MAX, but on the next line addition still
> overflows int64.
> Thoughts?

I finally had some time to look over this after the holidays, and I 
don't think using timespec_valid_strict() is the right approach here, 
as userspace will have no idea what KTIME_MAX is.  Instead, I think the 
right approach is to -EINVAL for negative values (which should avoid 
the overflow), and to allow too large values to be silently truncated 
by timespec_to_ktime().  The truncation doesn't matter all that much 
given that it's in the hundreds of years ballpark.  I'll push the patch 
below if there are no objections.

		-ben
-- 
"Thought is the essence of where you are now."

commit 4304367826d0df42086ef24428c6c277acd822a9
Author: Benjamin LaHaise <bcrl@...ck.org>
Date:   Wed Jan 6 12:46:12 2016 -0500

    aio: handle integer overflow in io_getevents() timespec usage
    
    Dmitry Vyukov reported an integer overflow in io_getevents() when
    running a fuzzer.  Upon investigation, the triggers appears to be that a
    negative value for the tv_sec or tv_nsec was passed in which is not
    handled by timespec_to_ktime().  This patch fixes that by making
    io_getevents() return -EINVAL when negative timeouts are passed in.
    
    Signed-off-by: Benjamin LaHaise <bcrl@...ck.org>

diff --git a/fs/aio.c b/fs/aio.c
index 155f842..f325ed4 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1269,6 +1269,8 @@ static long read_events(struct kioctx *ctx, long min_nr, long nr,
 
 		if (unlikely(copy_from_user(&ts, timeout, sizeof(ts))))
 			return -EFAULT;
+		if ((ts.tv_sec < 0) || (ts.tv_nsec < 0))
+			return -EINVAL;
 
 		until = timespec_to_ktime(ts);
 	}
--
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