Re: KMSAN: uninit-value in snapshot_compat_ioctl

[Eric Biggers]

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().

> 
> > 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!
> 

- Eric