[PATCH 4/6] xen/x86: adjust handling of the L3 user vsyscall special page table

[Jan Beulich <jbeulich@xxxxxxxx>]

Marking the page tableas pinned without ever actually pinning is was
probably an oversight in the first place. The main reason for the change
is more subtle, though: The write of the one present entry each here and
in the subsequently allocated L2 table engage a code path in the
hypervisor which exists only for thought-to-be-broken guests: An mmu-
update operation to a page which is neither a page table nor marked
writable. The hypervisor merely assumes (or should I say "hopes") that
the fact that a writable reference to the page can be obtained means it
is okay to actually write to that page in response to such a hypercall.

While there make all involved code and data dependent upon
X86_VSYSCALL_EMULATION (some code was already).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -86,8 +86,10 @@
 #include "mmu.h"
 #include "debugfs.h"
 
+#ifdef CONFIG_X86_VSYSCALL_EMULATION
 /* l3 pud for userspace vsyscall mapping */
 static pud_t level3_user_vsyscall[PTRS_PER_PUD] __page_aligned_bss;
+#endif
 
 /*
  * Protects atomic reservation decrease/increase against concurrent increases.
@@ -791,7 +793,9 @@ static void __init xen_mark_pinned(struc
 static void __init xen_after_bootmem(void)
 {
        static_branch_enable(&xen_struct_pages_ready);
+#ifdef CONFIG_X86_VSYSCALL_EMULATION
        SetPagePinned(virt_to_page(level3_user_vsyscall));
+#endif
        xen_pgd_walk(&init_mm, xen_mark_pinned, FIXADDR_TOP);
 }
 
@@ -1761,7 +1765,6 @@ void __init xen_setup_kernel_pagetable(p
        set_page_prot(init_top_pgt, PAGE_KERNEL_RO);
        set_page_prot(level3_ident_pgt, PAGE_KERNEL_RO);
        set_page_prot(level3_kernel_pgt, PAGE_KERNEL_RO);
-       set_page_prot(level3_user_vsyscall, PAGE_KERNEL_RO);
        set_page_prot(level2_ident_pgt, PAGE_KERNEL_RO);
        set_page_prot(level2_kernel_pgt, PAGE_KERNEL_RO);
        set_page_prot(level2_fixmap_pgt, PAGE_KERNEL_RO);
@@ -1778,6 +1781,13 @@ void __init xen_setup_kernel_pagetable(p
        /* Unpin Xen-provided one */
        pin_pagetable_pfn(MMUEXT_UNPIN_TABLE, PFN_DOWN(__pa(pgd)));
 
+#ifdef CONFIG_X86_VSYSCALL_EMULATION
+       /* Pin user vsyscall L3 */
+       set_page_prot(level3_user_vsyscall, PAGE_KERNEL_RO);
+       pin_pagetable_pfn(MMUEXT_PIN_L3_TABLE,
+                         PFN_DOWN(__pa_symbol(level3_user_vsyscall)));
+#endif
+
        /*
         * At this stage there can be no user pgd, and no page structure to
         * attach it to, so make sure we just set kernel pgd.