x86/dom0: disable SMAP for PV domain building only

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: 493ab190e5 ('xen/sm{e, a}p: allow disabling sm{e, a}p for Xen itself')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: fb1658221a
master date: 2024-08-28 19:59:07 +0100

xen/arch/x86/include/asm/setup.h

@@ -60,6 +60,8 @@ 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

xen/arch/x86/pv/dom0_build.c

@@ -354,11 +354,11 @@ static struct page_info * __init alloc_chunk(struct domain *d,
     return page;
 }
 
-int __init dom0_construct_pv(struct domain *d,
-                             const module_t *image,
-                             unsigned long image_headroom,
-                             module_t *initrd,
-                             const char *cmdline)
+static int __init dom0_construct(struct domain *d,
+                                 const module_t *image,
+                                 unsigned long image_headroom,
+                                 module_t *initrd,
+                                 const char *cmdline)
 {
     int i, rc, order, machine;
     bool compatible, compat;
@@ -1049,6 +1049,36 @@ out:
     return rc;
 }
 
+int __init dom0_construct_pv(struct domain *d,
+                             const module_t *image,
+                             unsigned long image_headroom,
+                             module_t *initrd,
+                             const char *cmdline)
+{
+    int rc;
+
+    /*
+     * Clear SMAP in CR4 to allow user-accesses in construct_dom0().  This
+     * prevents us needing to rewrite construct_dom0() in terms of
+     * copy_{to,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(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;
+    }
+
+    return rc;
+}
+
 /*
  * Local variables:
  * mode: C

xen/arch/x86/setup.c

@@ -75,7 +75,7 @@ static bool __initdata opt_invpcid = true;
 boolean_param("invpcid", opt_invpcid);
 bool __read_mostly use_invpcid;
 
-unsigned long __read_mostly cr4_pv32_mask;
+unsigned long __ro_after_init cr4_pv32_mask;
 
 /* **** Linux config option: propagated to domain0. */
 /* "acpi=off":    Sisables both ACPI table parsing and interpreter. */
@@ -946,26 +946,9 @@ static struct domain *__init create_dom0(const module_t *image,
         cmdline = dom0_cmdline;
     }
 
-    /*
-     * 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;
 }