Re: [PATCH] fs: fcntl, avoid undefined behaviour

[Jeff Layton]

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@xxxxxxx>
> Cc: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
> Cc: "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> ---
>  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...

Thanks,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>