[PATCH] x86/PV: guest_get_eff_kern_l1e() may still need to switch page tables

[PATCH] x86/PV: guest_get_eff_kern_l1e() may still need to switch page tables

[Jan Beulich]

While indeed unnecessary for pv_ro_page_fault(), pv_map_ldt_shadow_page()
may run when guest user mode is active, and hence may need to switch to
the kernel page tables in order to retrieve an LDT page mapping.

Fixes: 9ff970564764 ("x86/mm: drop guest_get_eff_l1e()")
Reported-by: Manuel Bouyer <bouyer@antioche.eu.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
Manuel, could you test this again, just to be on the safe side
before we throw it in (at which point we could then also again
add a Tested-by)? Thanks.

--- a/xen/arch/x86/pv/mm.h
+++ b/xen/arch/x86/pv/mm.h
@@ -11,10 +11,15 @@ int new_guest_cr3(mfn_t mfn);
  */
 static inline l1_pgentry_t guest_get_eff_kern_l1e(unsigned long linear)
 {
+    struct vcpu *curr = current;
+    bool user_mode = !(curr->arch.flags & TF_kernel_mode);
     l1_pgentry_t l1e;
 
-    ASSERT(!paging_mode_translate(current->domain));
-    ASSERT(!paging_mode_external(current->domain));
+    ASSERT(!paging_mode_translate(curr->domain));
+    ASSERT(!paging_mode_external(curr->domain));
+
+    if ( user_mode )
+        toggle_guest_pt(curr);
 
     if ( unlikely(!__addr_ok(linear)) ||
          __copy_from_user(&l1e,
@@ -22,6 +27,9 @@ static inline l1_pgentry_t guest_get_eff
                           sizeof(l1_pgentry_t)) )
         l1e = l1e_empty();
 
+    if ( user_mode )
+        toggle_guest_pt(curr);
+
     return l1e;
 }
 

Re: [PATCH] x86/PV: guest_get_eff_kern_l1e() may still need to switch page tables

[Andrew Cooper]

On 14/12/2020 13:57, Jan Beulich wrote:
> While indeed unnecessary for pv_ro_page_fault(), pv_map_ldt_shadow_page()
> may run when guest user mode is active, and hence may need to switch to
> the kernel page tables in order to retrieve an LDT page mapping.
>
> Fixes: 9ff970564764 ("x86/mm: drop guest_get_eff_l1e()")
> Reported-by: Manuel Bouyer <bouyer@antioche.eu.org>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> Manuel, could you test this again, just to be on the safe side
> before we throw it in (at which point we could then also again
> add a Tested-by)? Thanks.

I've got a repro of the issue (literally - just booting the
netinstaller), and this does fix it.

~Andrew

Re: [PATCH] x86/PV: guest_get_eff_kern_l1e() may still need to switch page tables

[Manuel Bouyer]

On Mon, Dec 14, 2020 at 02:57:53PM +0100, Jan Beulich wrote:
> While indeed unnecessary for pv_ro_page_fault(), pv_map_ldt_shadow_page()
> may run when guest user mode is active, and hence may need to switch to
> the kernel page tables in order to retrieve an LDT page mapping.
> 
> Fixes: 9ff970564764 ("x86/mm: drop guest_get_eff_l1e()")
> Reported-by: Manuel Bouyer <bouyer@antioche.eu.org>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> Manuel, could you test this again, just to be on the safe side
> before we throw it in (at which point we could then also again
> add a Tested-by)? Thanks.

Yes, this works for me. thanks !

-- 
Manuel Bouyer <bouyer@antioche.eu.org>
     NetBSD: 26 ans d'experience feront toujours la difference
--