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

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Date:   Mon, 24 Oct 2016 11:15:53 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...hat.com>
Cc:     viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: fcntl, avoid undefined behaviour

On 10/14/2016, 03:38 PM, J. Bruce Fields wrote:
> 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.

Yes, I checked POSIX before I sent the patch and it does not explicitly
document EINVAL, neither an error from SETOWN. So I am not sure whether
at this point we can start returning an error without breaking userspace?

thanks,
-- 
js
suse labs