Re: BUG: GPF in non-whitelisted uaccess (non-canonical address?)

[Dmitry Vyukov <dvyukov@google.com>]

On Wed, Nov 14, 2018 at 4:20 AM, David Herrmann <dh.herrmann@gmail.com> wrote:
> Hey
>
> On Wed, Nov 14, 2018 at 1:25 AM syzbot
> <syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com> wrote:
>> syzbot has found a reproducer for the following crash on:
>>
>> HEAD commit:    ccda4af0f4b9 Linux 4.20-rc2
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=13b4e77b400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=4a0a89f12ca9b0f5
>> dashboard link: https://syzkaller.appspot.com/bug?extid=72473edc9bf4eb1c6556
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1646a225400000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=108a6533400000
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+72473edc9bf4eb1c6556@syzkaller.appspotmail.com
>>
> [...]
>> BUG: GPF in non-whitelisted uaccess (non-canonical address?)
>
> This uses sendpage(2) to feed data from a file into a uhid chardev.
> The default behavior of the kernel is to create a temporary pipe, then
> splice from the file into the pipe, and then splice again from the
> pipe into uhid.
>
> The kernel provides default implementations for splicing between files
> and any other file. The default implementation of `.splice_write()`
> uses kmap() to map the page from the pipe and then uses the
> __kernel_write() (which uses .f_op->write()) to push the data into the
> target file. The problem is, __kernel_write() sets the address-space
> to KERNEL_DS `set_fs(get_ds())`, thus granting the UHID request access
> to kernel memory.
>
> I see several ways to fix that, the most simple solution is to simply
> prevent splice/sendpage on uhid (by setting f_op.splice_write to a
> dummy). Alternatively, we can implement a proper splice helper that
> takes the page directly, rather than through the __kernel_write()
> default implementation.

also +dtor for uhid