Re: [PATCH] x86/boot: Drop incorrect mapping at l2_xenmap[0]

[Andrew Cooper <amc96@xxxxxxxx>]

On 29/11/2021 17:26, Andrew Cooper wrote:
> It has been 4 years since the default load address changed from 1M to 2M, and
> _stext ceased residing in l2_xenmap[0].  We should not be inserting an unused
> mapping.
>
> To ensure we don't create/remove mappings accidentally, loop from 0 and obey
> _PAGE_PRESENT on all entries.
>
> Fixes: 7ed93f3a0dff ("x86: change default load address from 1 MiB to 2 MiB")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
>
> I ought to have spotted this in c/s 52975142d154 ("x86/boot: Create the
> l2_xenmap[] mappings dynamically") too.
> ---
>  xen/arch/x86/setup.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
> index da47cdea14a1..6f241048425c 100644
> --- a/xen/arch/x86/setup.c
> +++ b/xen/arch/x86/setup.c
> @@ -1279,16 +1279,12 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>  
>              /* The only data mappings to be relocated are in the Xen area. */
>              pl2e = __va(__pa(l2_xenmap));
> -            /*
> -             * Undo the temporary-hooking of the l1_directmap.  
> __2M_text_start
> -             * is contained in this PTE.
> -             */
> +
>              BUG_ON(using_2M_mapping() &&
>                     l2_table_offset((unsigned long)_erodata) ==
>                     l2_table_offset((unsigned long)_stext));
> -            *pl2e++ = l2e_from_pfn(xen_phys_start >> PAGE_SHIFT,
> -                                   PAGE_HYPERVISOR_RX | _PAGE_PSE);
> -            for ( i = 1; i < L2_PAGETABLE_ENTRIES; i++, pl2e++ )
> +
> +            for ( i = 0; i < L2_PAGETABLE_ENTRIES; i++, pl2e++ )
>              {
>                  unsigned int flags;
>  

Actually, on further consideration, this hunk should be included too, to
cross-check that we don't zap any of the dynamically created mappings.

~Andrew

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 6f241048425c..5c8026b30e3e 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1320,7 +1320,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                 }
                 else
                 {
-                    *pl2e = l2e_empty();
+                    ASSERT_UNREACHABLE();
                     continue;
                 }