Re: [RFC PATCH v2 17/19] heki: x86: Update permissions counters during text patching

[Peter Zijlstra]

On Mon, Nov 27, 2023 at 10:48:29AM -0600, Madhavan T. Venkataraman wrote:
> Apologies for the late reply. I was on vacation. Please see my response below:
> 
> On 11/13/23 02:19, Peter Zijlstra wrote:
> > On Sun, Nov 12, 2023 at 09:23:24PM -0500, Mickaël Salaün wrote:
> >> From: Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx>
> >>
> >> X86 uses a function called __text_poke() to modify executable code. This
> >> patching function is used by many features such as KProbes and FTrace.
> >>
> >> Update the permissions counters for the text page so that write
> >> permissions can be temporarily established in the EPT to modify the
> >> instructions in that page.
> >>
> >> Cc: Borislav Petkov <bp@xxxxxxxxx>
> >> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> >> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> >> Cc: Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx>
> >> Cc: Mickaël Salaün <mic@xxxxxxxxxxx>
> >> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> >> Cc: Sean Christopherson <seanjc@xxxxxxxxxx>
> >> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> >> Cc: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >> Signed-off-by: Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx>
> >> ---
> >>
> >> Changes since v1:
> >> * New patch
> >> ---
> >>  arch/x86/kernel/alternative.c |  5 ++++
> >>  arch/x86/mm/heki.c            | 49 +++++++++++++++++++++++++++++++++++
> >>  include/linux/heki.h          | 14 ++++++++++
> >>  3 files changed, 68 insertions(+)
> >>
> >> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> >> index 517ee01503be..64fd8757ba5c 100644
> >> --- a/arch/x86/kernel/alternative.c
> >> +++ b/arch/x86/kernel/alternative.c
> >> @@ -18,6 +18,7 @@
> >>  #include <linux/mmu_context.h>
> >>  #include <linux/bsearch.h>
> >>  #include <linux/sync_core.h>
> >> +#include <linux/heki.h>
> >>  #include <asm/text-patching.h>
> >>  #include <asm/alternative.h>
> >>  #include <asm/sections.h>
> >> @@ -1801,6 +1802,7 @@ static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t l
> >>  	 */
> >>  	pgprot = __pgprot(pgprot_val(PAGE_KERNEL) & ~_PAGE_GLOBAL);
> >>  
> >> +	heki_text_poke_start(pages, cross_page_boundary ? 2 : 1, pgprot);
> >>  	/*
> >>  	 * The lock is not really needed, but this allows to avoid open-coding.
> >>  	 */
> >> @@ -1865,7 +1867,10 @@ static void *__text_poke(text_poke_f func, void *addr, const void *src, size_t l
> >>  	}
> >>  
> >>  	local_irq_restore(flags);
> >> +
> >>  	pte_unmap_unlock(ptep, ptl);
> >> +	heki_text_poke_end(pages, cross_page_boundary ? 2 : 1, pgprot);
> >> +
> >>  	return addr;
> >>  }
> > 
> > This makes no sense, we already use a custom CR3 with userspace alias
> > for the actual pages to write to, why are you then frobbing permissions
> > on that *again* ?
> 
> Today, the permissions for a guest page in the extended page table
> (EPT) are RWX (unless permissions are restricted for some specific
> reason like for shadow page table pages). In this Heki feature, we
> don't allow RWX by default in the EPT. We only allow those permissions
> in the EPT that the guest page actually needs.  E.g., for a text page,
> it is R_X in both the guest page table and the EPT.

To what end? If you always mirror what the guest does, you've not
actually gained anything.

> For text patching, the above code establishes an alternate mapping in
> the guest page table that is RW_ so that the text can be patched. That
> needs to be reflected in the EPT so that the EPT permissions will
> change from R_X to RWX. In other words, RWX is allowed only as
> necessary. At the end of patching, the EPT permissions are restored to
> R_X.
> 
> Does that address your comment?

No, if you want to mirror the native PTEs why don't you hook into the
paravirt page-table muck and get all that for free?

Also, this is the user range, are you saying you're also playing these
daft games with user maps?