[PATCH v2 1/2] x86/head: check base address alignment

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Ensure that the base address is 2M aligned, or else the page table
entries created would be corrupt as reserved bits on the PDE end up
set.

We have encountered a broken firmware where grub2 would end up loading
Xen at a non 2M aligned region when using the multiboot2 protocol, and
that caused a very difficult to debug triple fault.

If the alignment is not as required by the page tables print an error
message and stop the boot.  Also add a build time check that the
calculation of symbol offsets don't break alignment of passed
addresses.

The check could be performed earlier, but so far the alignment is
required by the page tables, and hence feels more natural that the
check lives near to the piece of code that requires it.

Note that when booted as an EFI application from the PE entry point
the alignment check is already performed by
efi_arch_load_addr_check(), and hence there's no need to add another
check at the point where page tables get built in
efi_arch_memory_setup().

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Use test instead of and instruction.
 - Add a build time check for sym_offs correctness.
 - Reword part of the commit message.
---
 xen/arch/x86/boot/head.S | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 0fb7dd3029f2..b9c9447df9df 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -1,3 +1,4 @@
+#include <xen/lib.h>
 #include <xen/multiboot.h>
 #include <xen/multiboot2.h>
 #include <public/xen.h>
@@ -121,6 +122,7 @@ multiboot2_header:
 .Lbad_ldr_nst: .asciz "ERR: EFI SystemTable is not provided by bootloader!"
 .Lbad_ldr_nih: .asciz "ERR: EFI ImageHandle is not provided by bootloader!"
 .Lbad_efi_msg: .asciz "ERR: EFI IA-32 platforms are not supported!"
+.Lbag_alg_msg: .asciz "ERR: Xen must be loaded at a 2Mb boundary!"
 
         .section .init.data, "aw", @progbits
         .align 4
@@ -146,6 +148,9 @@ bad_cpu:
 not_multiboot:
         add     $sym_offs(.Lbad_ldr_msg),%esi   # Error message
         jmp     .Lget_vtb
+not_aligned:
+        add     $sym_offs(.Lbag_alg_msg),%esi   # Error message
+        jmp     .Lget_vtb
 .Lmb2_no_st:
         /*
          * Here we are on EFI platform. vga_text_buffer was zapped earlier
@@ -670,6 +675,15 @@ trampoline_setup:
         cmp     %edi, %eax
         jb      1b
 
+        .if !IS_ALIGNED(sym_offs(0), 1 << L2_PAGETABLE_SHIFT)
+        .error "Symbol offset calculation breaks alignment"
+        .endif
+
+        /* Check that the image base is aligned. */
+        lea     sym_esi(_start), %eax
+        test    $(1 << L2_PAGETABLE_SHIFT) - 1, %eax
+        jnz     not_aligned
+
         /* Map Xen into the higher mappings using 2M superpages. */
         lea     _PAGE_PSE + PAGE_HYPERVISOR_RWX + sym_esi(_start), %eax
         mov     $sym_offs(_start),   %ecx   /* %eax = PTE to write ^      */
-- 
2.40.0