[PATCH PTI v3 05/10] x86/vsyscall/64: Explicitly set _PAGE_USER in the pagetable hierarchy

[Andy Lutomirski]

The kernel is very erratic as to which pagetables have _PAGE_USER
set.  The vsyscall page gets lucky: it seems that all of the
relevant pagetables are among the apparently arbitrary ones that set
_PAGE_USER.  Rather than relying on chance, just explicitly set
_PAGE_USER.

This will let us clean up pagetable setup to stop setting
_PAGE_USER.  The added code can also be reused by pagetable
isolation to manage the _PAGE_USER bit in the usermode tables.

Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
 arch/x86/entry/vsyscall/vsyscall_64.c | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c 
b/arch/x86/entry/vsyscall/vsyscall_64.c
index f279ba2643dc..bc88a0540347 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -329,16 +329,47 @@ int in_gate_area_no_mm(unsigned long addr)
        return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
 }
 
+/*
+ * The VSYSCALL page is the only user-accessible page in the kernel address
+ * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
+ * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
+ * are enabled.
+ *
+ * Some day we may create a "minimal" vsyscall mode in which we emulate
+ * vsyscalls but leave the page not present.  If so, we skip calling
+ * this.
+ */
+static void __init set_vsyscall_pgtable_user_bits(void)
+{
+       pgd_t *pgd;
+       p4d_t *p4d;
+       pud_t *pud;
+       pmd_t *pmd;
+
+       pgd = pgd_offset_k(VSYSCALL_ADDR);
+       pgd->pgd |= _PAGE_USER;
+       p4d = p4d_offset(pgd, VSYSCALL_ADDR);
+#if CONFIG_PGTABLE_LEVELS >= 5
+       p4d->p4d |= _PAGE_USER;
+#endif
+       pud = pud_offset(p4d, VSYSCALL_ADDR);
+       pud->pud |= _PAGE_USER;
+       pmd = pmd_offset(pud, VSYSCALL_ADDR);
+       pmd->pmd |= _PAGE_USER;
+}
+
 void __init map_vsyscall(void)
 {
        extern char __vsyscall_page;
        unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
 
-       if (vsyscall_mode != NONE)
+       if (vsyscall_mode != NONE) {
                __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
                             vsyscall_mode == NATIVE
                             ? PAGE_KERNEL_VSYSCALL
                             : PAGE_KERNEL_VVAR);
+               set_vsyscall_pgtable_user_bits();
+       }
 
        BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
                     (unsigned long)VSYSCALL_ADDR);
-- 
2.13.6