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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>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/1048f5b8-b726-dcc1-1216-9d5ac328ce82@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
This commit is contained in:
Jan Beulich 2021-09-30 14:36:16 +02:00 committed by Boris Ostrovsky
parent 4c360db6cc
commit d2a3ef44c2
1 changed files with 11 additions and 1 deletions
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12
arch/x86/xen/mmu_pv.c
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@ -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(struct mm_struct *mm, struct page *page,
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);
}
@ -1762,7 +1766,6 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
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);
@ -1779,6 +1782,13 @@ void __init xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn)
/* 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.