[Xen-devel] [PATCH] xen/x86: Disable CR0.WP while applying alternatives

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

In preparation for marking .text as read-only, care needs to be taken not to
fault while applying alternatives.

Swapping back to RW mappings is a possibility, but would require additional
TLB management.  A temporary disabling of CR0.WP is cleaner.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>

New in v2.  (The one downside of my very-quick-to-reboot test box is that it
is sufficiently old to not have any alternatives needing patching.)
---
 xen/arch/x86/alternative.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/xen/arch/x86/alternative.c b/xen/arch/x86/alternative.c
index 46ac0fd..d123fa7 100644
--- a/xen/arch/x86/alternative.c
+++ b/xen/arch/x86/alternative.c
@@ -147,11 +147,15 @@ static void __init apply_alternatives(struct alt_instr 
*start, struct alt_instr
     struct alt_instr *a;
     u8 *instr, *replacement;
     u8 insnbuf[MAX_PATCH_LEN];
+    unsigned long cr0 = read_cr0();
 
     ASSERT(!local_irq_is_enabled());
 
     printk(KERN_INFO "alt table %p -> %p\n", start, end);
 
+    /* Disable WP to allow application of alternatives to read-only pages. */
+    write_cr0(cr0 & ~X86_CR0_WP);
+
     /*
      * The scan order should be from start to end. A later scanned
      * alternative code can overwrite a previous scanned alternative code.
@@ -181,6 +185,9 @@ static void __init apply_alternatives(struct alt_instr 
*start, struct alt_instr
                  a->instrlen - a->replacementlen);
         text_poke_early(instr, insnbuf, a->instrlen);
     }
+
+    /* Reinstate WP. */
+    write_cr0(cr0);
 }
 
 void __init alternative_instructions(void)
-- 
2.1.4


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