Re: [RFC PATCH -next V2 5/7] arm64: add get_user to machine check safe

[Mark Rutland]

On Thu, Apr 07, 2022 at 10:38:04PM +0800, Tong Tiangen wrote:
> 在 2022/4/6 19:22, Mark Rutland 写道:
> > On Wed, Apr 06, 2022 at 09:13:09AM +0000, Tong Tiangen wrote:
> > > Add scenarios get_user to machine check safe. The processing of
> > > EX_TYPE_UACCESS_ERR_ZERO and EX_TYPE_UACCESS_ERR_ZERO_UCE_RECOVERY is same
> > > and both return -EFAULT.
> > 
> > Which uaccess cases do we expect to *not* be recoverable?
> > 
> > Naively I would assume that if we're going to treat a memory error on a uaccess
> > as fatal to userspace we should be able to do that for *any* uacesses.
> > 
> > The commit message should explain why we need the distinction between a
> > recoverable uaccess and a non-recoverable uaccess.
> > 
> > Thanks,
> > Mark.
> 
> Currently, any memory error consumed in kernel mode will lead to panic
> (do_sea()).
> 
> My idea is that not all memory errors consumed in kernel mode are fatal,
> such as copy_ from_ user/get_ user is a memory error consumed when
> reading user data in the process context. In this case, we can not let the
> kernel panic, just kill the process without affecting the operation
> of the system.

I understood this part.

> However, not all uaccess can be recovered without affecting the normal
> operation of the system. The key is not whether it is uaccess, but whether
> there are key data affecting the normal operation of the system in the read
> page.

Ok. Can you give an example of such a case where the a uaccess that hits
a memory error must be fatal?

I think you might be trying to say that for copy_{to,from}_user() we can
make that judgement, but those are combined user+kernel access
primitives, and the *uaccess* part should never be reading from a page
with "key data affecting the normal operation of the system", since
that's userspace memory.

Is there any *userspace access* (e.g. where we use LDTR/STTR today)
where we must treat a memory error as fatal to the system?

Thanks,
Mark.