linux-kernel - Re: [PATCH] fs: fcntl, avoid undefined behaviour

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Date:   Fri, 14 Oct 2016 09:38:46 -0400
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Jeff Layton <jlayton@...hat.com>
Cc:     Jiri Slaby <jslaby@...e.cz>, viro@...iv.linux.org.uk,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: fcntl, avoid undefined behaviour

On Fri, Oct 14, 2016 at 07:48:15AM -0400, Jeff Layton wrote:
> On Fri, 2016-10-14 at 11:23 +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) and
> > return immediatelly in case it is wrong. No error is returned, the
> > same as in other cases.
> > 
> > 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 350a2c8cfd28..bfc3b040d956 100644
> > --- a/fs/fcntl.c
> > +++ b/fs/fcntl.c
> > @@ -112,6 +112,10 @@ void 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;
> > +
> >  	type = PIDTYPE_PID;
> >  	if (who < 0) {
> >  		type = PIDTYPE_PGID;
> 
> Might it be better to change f_setown to return int there, so you can
> return -EINVAL in that case? The other caller (sock_ioctl) can also
> handle an int return there too...

That might also be worth a note in the RETURN VALUE section of fcntl(2),
which goes into surprising detail about the EINVAL cases for different
commands.

--b.