[PATCH] x86/boot: Restrict directmap permissions for .text/.rodata

[Andrew Cooper <andrew.cooper3@citrix.com>]

While we've been diligent to ensure that the main text/data/rodata mappings
have suitable restrictions, their aliases via the directmap were left fully
RW.  Worse, we even had pieces of code making use of this as a feature.

Restrict the permissions, as we have no legitimate need for writeability of
these areas via the directmap alias.

Note that the pagetables and cpu0_stack do get written through their directmap
alias, so we can't just read-only the whole image.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>

Ever so slightly RFC, as it has only had light testing.

Notes:
 * The stubs are still have RX via one alias, RW via another, and these need
   to stay.  Hardening options include splitting the stubs so the SYSCALL ones
   can be read-only after setup, and/or expanding the stub size to 4k per CPU
   so we really can keep the writeable alias as not present when the stub
   isn't in active use.
 * Future CPUs with Protection Key Supervisor (Sapphire Rapids and later)
   would be able to inhibit writeability outside of a permitted region, and
   because the protection key is per logical thread, we woulnd't need to
   expand the stubs to 4k per CPU.
 * At the time of writing, PV Shim still makes use of .rodata's read/write
   alias in the directmap to patch the hypercall table, but that runs earlier
   on boot.  Also, there are patches out to address this.
 * For backporting, this patch depends on c/s e7f147bf4ac7 ("x86/crash: Drop
   manual hooking of exception_table[]"), and nothing would break at compile
   time if the dependency was missing.
---
 xen/arch/x86/setup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index f40a9fe5d351..c8641c227d9a 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1566,6 +1566,14 @@ void __init noreturn __start_xen(unsigned long mbi_p)
         destroy_xen_mappings((unsigned long)&__2M_rwdata_end,
                              ROUNDUP((unsigned long)&__2M_rwdata_end, MB(2)));
 
+    /*
+     * Mark all of .text and .rodata as RO in the directmap - we don't want
+     * these sections writeable via any alias.
+     */
+    modify_xen_mappings((unsigned long)__va(__pa(_start)),
+                        (unsigned long)__va(__pa(__2M_rodata_end)),
+                        PAGE_HYPERVISOR_RO);
+
     nr_pages = 0;
     for ( i = 0; i < e820.nr_map; i++ )
         if ( e820.map[i].type == E820_RAM )
-- 
2.11.0