[PATCH v5] x86/dom0: disable SMAP for PV domain building only

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

Move the logic that disables SMAP so it's only performed when building a PV
dom0, PVH dom0 builder doesn't require disabling SMAP.

The fixes tag is to account for the wrong usage of cpu_has_smap in
create_dom0(), it should instead have used
boot_cpu_has(X86_FEATURE_XEN_SMAP).  Fix while moving the logic to apply to PV
only.

While there also make cr4_pv32_mask __ro_after_init.

Fixes: 493ab190e5b1 ('xen/sm{e, a}p: allow disabling sm{e, a}p for Xen itself')
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v4:
 - New approach, move the current logic so it's only applied when creating a PV
   dom0.
---
 xen/arch/x86/dom0_build.c        | 17 +++++++++++++++++
 xen/arch/x86/include/asm/setup.h |  2 ++
 xen/arch/x86/setup.c             | 19 +------------------
 3 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/xen/arch/x86/dom0_build.c b/xen/arch/x86/dom0_build.c
index 8d56705a0861..31c94b14bb06 100644
--- a/xen/arch/x86/dom0_build.c
+++ b/xen/arch/x86/dom0_build.c
@@ -612,7 +612,24 @@ int __init construct_dom0(struct domain *d, const module_t 
*image,
     if ( is_hvm_domain(d) )
         rc = dom0_construct_pvh(d, image, image_headroom, initrd, cmdline);
     else if ( is_pv_domain(d) )
+    {
+        /*
+         * Temporarily clear SMAP in CR4 to allow user-accesses in
+         * construct_dom0().  This saves a large number of corner cases
+         * interactions with copy_from_user().
+         */
+        if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
+        {
+            cr4_pv32_mask &= ~X86_CR4_SMAP;
+            write_cr4(read_cr4() & ~X86_CR4_SMAP);
+        }
         rc = dom0_construct_pv(d, image, image_headroom, initrd, cmdline);
+        if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
+        {
+            write_cr4(read_cr4() | X86_CR4_SMAP);
+            cr4_pv32_mask |= X86_CR4_SMAP;
+        }
+    }
     else
         panic("Cannot construct Dom0. No guest interface available\n");
 
diff --git a/xen/arch/x86/include/asm/setup.h b/xen/arch/x86/include/asm/setup.h
index d75589178b91..8f7dfefb4dcf 100644
--- a/xen/arch/x86/include/asm/setup.h
+++ b/xen/arch/x86/include/asm/setup.h
@@ -64,6 +64,8 @@ extern bool opt_dom0_verbose;
 extern bool opt_dom0_cpuid_faulting;
 extern bool opt_dom0_msr_relaxed;
 
+extern unsigned long cr4_pv32_mask;
+
 #define max_init_domid (0)
 
 #endif
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index eee20bb1753c..eb0fcb6c8767 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -80,7 +80,7 @@ int8_t __initdata opt_probe_port_aliases = -1;
 boolean_param("probe-port-aliases", opt_probe_port_aliases);
 
 /* Only used in asm code and within this source file */
-unsigned long asmlinkage __read_mostly cr4_pv32_mask;
+unsigned long asmlinkage __ro_after_init cr4_pv32_mask;
 
 /* **** Linux config option: propagated to domain0. */
 /* "acpi=off":    Sisables both ACPI table parsing and interpreter. */
@@ -955,26 +955,9 @@ static struct domain *__init create_dom0(const module_t 
*image,
         }
     }
 
-    /*
-     * Temporarily clear SMAP in CR4 to allow user-accesses in 
construct_dom0().
-     * This saves a large number of corner cases interactions with
-     * copy_from_user().
-     */
-    if ( cpu_has_smap )
-    {
-        cr4_pv32_mask &= ~X86_CR4_SMAP;
-        write_cr4(read_cr4() & ~X86_CR4_SMAP);
-    }
-
     if ( construct_dom0(d, image, headroom, initrd, cmdline) != 0 )
         panic("Could not construct domain 0\n");
 
-    if ( cpu_has_smap )
-    {
-        write_cr4(read_cr4() | X86_CR4_SMAP);
-        cr4_pv32_mask |= X86_CR4_SMAP;
-    }
-
     return d;
 }
 
-- 
2.46.0