linux-kernel - Re: [PATCH][next] clone3: fix an unsigned args.cgroup comparison to less than zero

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Message-ID: <20200224073157.GB3286@kadam>
Date:   Mon, 24 Feb 2020 10:31:57 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Colin King <colin.king@...onical.com>,
        Christian Brauner <christian@...uner.io>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, kernel-janitors@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH][next] clone3: fix an unsigned args.cgroup comparison to
 less than zero

On Sat, Feb 22, 2020 at 01:18:01PM +0100, Christian Brauner wrote:
> On Sat, Feb 22, 2020 at 12:15:13AM +0000, Colin King wrote:
> > From: Colin Ian King <colin.king@...onical.com>
> > 
> > The less than zero comparison of args.cgroup is aways false because
> > args.cgroup is a u64 and can never be less than zero.  I believe the
> > correct check is to cast args.cgroup to a s64 first to ensure an
> > invalid value is not copied to kargs->cgroup.
> > 
> > Addresses-Coverity: ("Unsigned compared against 0")
> > Fixes: ef2c41cf38a7 ("clone3: allow spawning processes into cgroups")
> > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> 
> Thanks, Colin.
> Dan has reported this issue a few days prior on the janitors list so he
> likely should get a 
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> too.

Colin found it independently so no need for a Reported-by.

> diff --git a/kernel/fork.c b/kernel/fork.c
> index 2diff --git a/kernel/fork.c b/kernel/fork.c
> index 2853e258fe1f..dca4dde3b5b2 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2618,7 +2618,8 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>                      !valid_signal(args.exit_signal)))
>                 return -EINVAL;
> 
> -       if ((args.flags & CLONE_INTO_CGROUP) && args.cgroup < 0)
> +       if ((args.flags & CLONE_INTO_CGROUP) &&
> +           (args.cgroup > INT_MAX || (s64)args.cgroup < 0))

If we're capping it at INT_MAX then the check for negative isn't
required and static analysis tools know it's not so they might complain.

regards,
dan carpenter