linux-kernel - Re: KMSAN: uninit-value in snapshot_compat

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Date:   Fri, 13 Mar 2020 15:10:45 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     syzbot <syzbot+af962bf9e7e27bccd025@...kaller.appspotmail.com>,
        len.brown@...el.com, LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Pavel Machek <pavel@....cz>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: KMSAN: uninit-value in snapshot_compat_ioctl

On Mon, Mar 9, 2020 at 7:11 PM Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Mon, Mar 09, 2020 at 12:53:28PM +0100, 'Alexander Potapenko' via syzkaller-bugs wrote:
> > > > Looks like a KMSAN false positive?  As far as I can tell, the memory is being
> > > > initialized by put_user() called under set_fs(KERNEL_DS).
> >
> > Why? put_user() doesn't write to kernel memory, instead it copies a
> > value to the userspace.
> > That's why KMSAN performs kmsan_check_memory() on it.
> > It would actually be better if KMSAN printed an kernel-infoleak warning instead.
>
> When under set_fs(KERNEL_DS), the userspace access functions like put_user() and
> copy_to_user() can write to kernel memory.  It's discouraged and people have
> been trying to get rid of uses of set_fs(), but a lot still remain, since
> sometimes it's useful to allow code to operate on both user and kernel memory.
> A common example is kernel_read().

Ah, you're right. We can simply check that the target address is in
the userspace before actually reporting the error.