Re: KMSAN: uninit-value in snapshot_compat_ioctl

Alexander Potapenko
2020-03-13T14:11:01Z

On Mon, Mar 9, 2020 at 7:11 PM Eric Biggers <ebiggers@kernel.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.