Re: [PATCH 04/30] x86, kaiser: disable global pages by default with KAISER

[Ingo Molnar]

* Rik van Riel <riel@xxxxxxxxxx> wrote:

> On Fri, 2017-11-10 at 11:31 -0800, Dave Hansen wrote:
> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > 
> > Global pages stay in the TLB across context switches.  Since all
> > contexts
> > share the same kernel mapping, these mappings are marked as global
> > pages
> > so kernel entries in the TLB are not flushed out on a context switch.
> > 
> > But, even having these entries in the TLB opens up something that an
> > attacker can use [1].
> > 
> > That means that even when KAISER switches page tables on return to
> > user
> > space the global pages would stay in the TLB cache.
> > 
> > Disable global pages so that kernel TLB entries can be flushed before
> > returning to user space. This way, all accesses to kernel addresses
> > from
> > userspace result in a TLB miss independent of the existence of a
> > kernel
> > mapping.
> > 
> > Replace _PAGE_GLOBAL by __PAGE_KERNEL_GLOBAL and keep _PAGE_GLOBAL
> > available so that it can still be used for a few selected kernel
> > mappings
> > which must be visible to userspace, when KAISER is enabled, like the
> > entry/exit code and data.
> 
> Nice changelog.
> 
> Why am I pointing this out?
> 
> > +++ b/arch/x86/include/asm/pgtable_types.h	2017-11-10
> > 11:22:06.626244956 -0800
> > @@ -179,8 +179,20 @@ enum page_cache_mode {
> >  #define PAGE_READONLY_EXEC	__pgprot(_PAGE_PRESENT |
> > _PAGE_USER |	\
> >  					 _PAGE_ACCESSED)
> >  
> > +/*
> > + * Disable global pages for anything using the default
> > + * __PAGE_KERNEL* macros.  PGE will still be enabled
> > + * and _PAGE_GLOBAL may still be used carefully.
> > + */
> > +#ifdef CONFIG_KAISER
> > +#define __PAGE_KERNEL_GLOBAL	0
> > +#else
> > +#define __PAGE_KERNEL_GLOBAL	_PAGE_GLOBAL
> > +#endif
> > +					
> 
> The comment above could use a little more info
> on why things are done that way, though :)

Good point - I've updated these comments to say:

/*
 * Disable global pages for anything using the default
 * __PAGE_KERNEL* macros.
 *
 * PGE will still be enabled and _PAGE_GLOBAL may still be used carefully
 * for a few selected kernel mappings which must be visible to userspace,
 * when KAISER is enabled, like the entry/exit code and data.
 */
#ifdef CONFIG_KAISER
#define __PAGE_KERNEL_GLOBAL	0
#else
#define __PAGE_KERNEL_GLOBAL	_PAGE_GLOBAL
#endif

... and I've added your Reviewed-by tag which I assume now applies?

Thanks,

	Ingo