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Subject[PATCH 4/6] xen/x86: adjust handling of the L3 user vsyscall special page table
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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@suse.com>

--- 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.
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