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[xen staging] x86/boot: Don't map 0 during boot
commit faa85d4fb39554b382ec3d6d5ec82a9ffc9c63f5
Author: Andrew Cooper <andrew.cooper3@citrix.com>
AuthorDate: Mon Jan 6 13:37:41 2020 +0000
Commit: Andrew Cooper <andrew.cooper3@citrix.com>
CommitDate: Fri Jan 10 17:45:30 2020 +0000

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>
---
xen/arch/x86/boot/head.S | 14 ++++++++------
xen/arch/x86/boot/x86_64.S | 4 ++++
2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 0b75d33a25..d246e374f1 100644
--- 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 de555f87f4..af62850589 100644
--- 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
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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