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

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Date:   Tue, 13 Jun 2017 13:10:48 +0200
From:   Jiri Slaby <jslaby@...e.cz>
To:     Jeff Layton <jlayton@...hat.com>
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 06/13/2017, 12:11 PM, Jeff Layton wrote:
> 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.

Of course it will. What was I thinking?

So catch:

a) ==== the single case? ====

if (who == INT_MIN)
  return -EINVAL;

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

b) ==== or all the larger values? ====

if (who == INT_MIN || arg != (unsigned)who)
  return -EINVAL;

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

====

The former added test could be inside the "if (who < 0) { }", alternatively.

thanks,
-- 
js
suse labs