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

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Message-ID: <CAG_fn=X8UkYx5=3ARUtW3+asc+3tEdeBg=1NKS9VzChSCp33Yg@mail.gmail.com>
Date:   Mon, 9 Mar 2020 12:53:28 +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

> > 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.

> Although, it also looks like the problematic code can just be removed, since
> always sizeof(compat_loff_t) == sizeof(loff_t).  I'll send a patch to do that...

Thanks!

-- 
Alexander Potapenko
Software Engineer

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