x86/boot: Don't map 0 during boot

In particular, it causes accidental NULL pointer dereferences to go unnoticed.

The majority of the early operation takes place either in Real mode, or
Protected Unpaged mode.  The only bit which requires pagetable mappings is the
trampoline transition into Long mode and jump to the higher mappings, so there
is no need for the whole bottom 2M to be mapped.

Introduce a new l1_bootmap in .init.data, and use it instead of l1_identmap.
The EFI boot path doesn't pass through the trampoline, so doesn't need any
adjustment.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 0b75d33a25c8d57cf8e75df442848e7d338dbc85..d246e374f1b6218939e89bb25ca58d92c84cdb1b 100644 (file)
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -689,12 +689,14 @@ trampoline_setup:
         sub     $(L2_PAGETABLE_ENTRIES*8),%eax
         loop    1b
 
-        /*
-         * During boot, hook 4kB mappings of first 2MB of memory into L2.
-         * This avoids mixing cachability for the legacy VGA region.
-         */
-        lea     __PAGE_HYPERVISOR+sym_esi(l1_identmap),%edi
-        mov     %edi,sym_fs(l2_bootmap)
+        /* Map the permanent trampoline page into l{1,2}_bootmap[]. */
+        mov     sym_esi(trampoline_phys), %ecx
+        lea     __PAGE_HYPERVISOR_RX(%ecx), %edx /* %edx = PTE to write  */
+        shr     $PAGE_SHIFT, %ecx                /* %ecx = Slot to write */
+        mov     %edx, sym_offs(l1_bootmap)(%esi, %ecx, 8)
+
+        lea     __PAGE_HYPERVISOR + sym_esi(l1_bootmap), %edx
+        mov     %edx, sym_esi(l2_bootmap)
 
         /* Apply relocations to bootstrap trampoline. */
         mov     sym_fs(trampoline_phys),%edx

diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index de555f87f45b6372069ae344a79fb07a95971f9c..af62850589a76db590f0c3ea6100e9c61ec4c367 100644 (file)
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -155,6 +155,10 @@ GLOBAL(__page_tables_end)
         .section .init.data, "aw", @progbits
         .align PAGE_SIZE, 0
 
+l1_bootmap:
+        .fill L1_PAGETABLE_ENTRIES, 8, 0
+        .size l1_bootmap, . - l1_bootmap
+
 GLOBAL(l2_bootmap)
         .fill 4 * L2_PAGETABLE_ENTRIES, 8, 0
         .size l2_bootmap, . - l2_bootmap