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

[Tong Tiangen]

在 2022/4/8 23:22, Mark Rutland 写道:
> 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.
> .

I seem to understand what you mean.
Take copy_to_user()/put_user() as an example. If it encounters memory 
error, only related processes will be affected. According to this 
understanding, it seems that all uaccess can be recovered.

Thanks,
Tong.