linux-kernel - Re: [PATCH v2 2/2] fs/fcntl: f_setown, avoid undefined behaviour

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Date:   Tue, 13 Jun 2017 06:11:29 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     linux-kernel@...r.kernel.org,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] fs/fcntl: f_setown, avoid undefined behaviour

On Tue, 2017-06-13 at 11:22 +0200, Jiri Slaby wrote:
> fcntl(0, F_SETOWN, 0x80000000) triggers:
> UBSAN: Undefined behaviour in fs/fcntl.c:118:7
> negation of -2147483648 cannot be represented in type 'int':
> CPU: 1 PID: 18261 Comm: syz-executor Not tainted 4.8.1-0-syzkaller #1
> ...
> Call Trace:
> ...
>  [<ffffffffad8f0868>] ? f_setown+0x1d8/0x200
>  [<ffffffffad8f19a9>] ? SyS_fcntl+0x999/0xf30
>  [<ffffffffaed1fb00>] ? entry_SYSCALL_64_fastpath+0x23/0xc1
> 
> Fix that by checking the arg parameter properly (against INT_MAX) before
> "who = -who". And return immediatelly with -EINVAL in case it is wrong.
> Note that according to POSIX we can return EINVAL:
>     http://pubs.opengroup.org/onlinepubs/9699919799/functions/fcntl.html
> 
>     [EINVAL]
>         The cmd argument is F_SETOWN and the value of the argument
>         is not valid as a process or process group identifier.
> 
> [v2] returns an error, v1 used to fail silently
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Cc: Jeff Layton <jlayton@...chiereds.net>
> Cc: "J. Bruce Fields" <bfields@...ldses.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: linux-fsdevel@...r.kernel.org
> ---
>  fs/fcntl.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 313eba860346..db853670e22f 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -114,6 +114,10 @@ int f_setown(struct file *filp, unsigned long arg, int force)
>  	enum pid_type type;
>  	struct pid *pid;
>  	int who = arg;
> +
> +	if (arg > INT_MAX)
> +		return -EINVAL;
> +
>  	type = PIDTYPE_PID;
>  	if (who < 0) {
>  		type = PIDTYPE_PGID;

The next part here says:

        if (who < 0) {                                                          
                type = PIDTYPE_PGID;                                            
                who = -who;                                                     
        }                                                                       

Won't this break the ability to pass in a pgid? Valid negative values
will end up getting back -EINVAL here, AFAICT.
-- 
Jeff Layton <jlayton@...hat.com>