linux-kernel - Re: [PATCH v3 2/2] fs/fcntl: f_setown, avoid undefined behaviour

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Date:   Tue, 13 Jun 2017 21:01:49 +0800
From:   zhong jiang <zhongjiang@...wei.com>
To:     Jeff Layton <jlayton@...hat.com>
CC:     Jiri Slaby <jslaby@...e.cz>, <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 v3 2/2] fs/fcntl: f_setown, avoid undefined behaviour

On 2017/6/13 20:13, Jeff Layton wrote:
> On Tue, 2017-06-13 at 13:35 +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
>> [v3] implement proper check for the bad value INT_MIN
>>
>> 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..693322e28751 100644
>> --- a/fs/fcntl.c
>> +++ b/fs/fcntl.c
>> @@ -116,6 +116,10 @@ int f_setown(struct file *filp, unsigned long arg, int force)
>>  	int who = arg;
>>  	type = PIDTYPE_PID;
>>  	if (who < 0) {
>> +		/* avoid overflow below */
>> +		if (who == INT_MIN)
>> +			return -EINVAL;
>> +
>>  		type = PIDTYPE_PGID;
>>  		who = -who;
>>  	}
> Seems reasonable.
>
> I do somewhat lean toward checking for all larger values, but there
> could be userland programs that leave the upper bits set when they cast
> this to unsigned long. This is probably the safer thing to do.
>
> I'll plan to pick these up for v4.12.
>
> On the other related note...I think we ought to return ESRCH when
> find_vpid returns NULL. I'll take a look at that sometime soon too.
>
> Thanks!
 hi, jeff

 I have sent the patch about find_vpid ,and exist in linux-next branch

 https://patchwork.kernel.org/patch/9766259/

 Thanks
zhongjiang