From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Biggers Subject: Re: [PATCH] HID: uhid: prevent uhid_char_write() under KERNEL_DS Date: Wed, 14 Nov 2018 13:54:35 -0800 Message-ID: <20181114215434.GB87768@gmail.com> References: <20181114171447.GA87768@gmail.com> <20181114180217.195917-1-ebiggers@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Jann Horn Cc: dh.herrmann@googlemail.com, Jiri Kosina , benjamin.tissoires@redhat.com, linux-input@vger.kernel.org, kernel list , syzkaller-bugs@googlegroups.com, Dmitry Vyukov , dtor@google.com, syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com, stable@vger.kernel.org, Andy Lutomirski List-Id: linux-input@vger.kernel.org On Wed, Nov 14, 2018 at 07:18:39PM +0100, 'Jann Horn' via syzkaller-bugs wrote: > +cc Andy > > On Wed, Nov 14, 2018 at 7:03 PM Eric Biggers wrote: > > When a UHID_CREATE command is written to the uhid char device, a > > copy_from_user() is done from a user pointer embedded in the command. > > When the address limit is KERNEL_DS, e.g. as is the case during > > sendfile(), this can read from kernel memory. Therefore, UHID_CREATE > > must not be allowed in this case. > > > > For consistency and to make sure all current and future uhid commands > > are covered, apply the restriction to uhid_char_write() as a whole > > rather than to UHID_CREATE specifically. > > > > Thanks to Dmitry Vyukov for adding uhid definitions to syzkaller and to > > Jann Horn for commit 9da3f2b740544 ("x86/fault: BUG() when uaccess > > helpers fault on kernel addresses"), allowing this bug to be found. > > > > Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com > > Wheeeee, it found something! :) > > > Fixes: d365c6cfd337 ("HID: uhid: add UHID_CREATE and UHID_DESTROY events") > > Cc: # v3.6+ > > Cc: Jann Horn > > Signed-off-by: Eric Biggers > > --- > > drivers/hid/uhid.c | 6 ++++++ > > 1 file changed, 6 insertions(+) > > > > diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c > > index 3c55073136064..e94c5e248b56e 100644 > > --- a/drivers/hid/uhid.c > > +++ b/drivers/hid/uhid.c > > @@ -705,6 +705,12 @@ static ssize_t uhid_char_write(struct file *file, const char __user *buffer, > > int ret; > > size_t len; > > > > + if (uaccess_kernel()) { /* payload may contain a __user pointer */ > > + pr_err_once("%s: process %d (%s) called from kernel context, this is not allowed.\n", > > + __func__, task_tgid_vnr(current), current->comm); > > + return -EACCES; > > + } > > If this file can conceivably be opened by a process that doesn't have > root privileges, this check should be something along the lines of > ib_safe_file_access() or sg_check_file_access(). > > Checking for uaccess_kernel() prevents the symptom that syzkaller > notices - a user being able to cause a kernel memory access -, but it > doesn't deal with the case where a user opens a file descriptor to > this device and tricks a more privileged process into writing into it > (e.g. by passing it to a suid binary as stdout or stderr). > Yep, I'll do that. > Looking closer, I wonder whether this kind of behavior is limited to > the UHID_CREATE request, which has a comment on it saying "/* > Obsolete! Use UHID_CREATE2. */". If we could keep this kind of ugly > kludge away from the code paths you're supposed to be using, that > would be nice... > I wanted to be careful, but yes AFAICS it can be limited to UHID_CREATE only, so I'll do that instead. - Eric