[Xen-changelog] [xen master] x86: disable CR0.WP while applying alternatives

[patchbot@xxxxxxx]

commit 36c672b559d35e088b181c67538b1ed5bacb0da5
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Feb 25 13:06:16 2016 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Feb 25 13:06:16 2016 +0100

    x86: disable CR0.WP while applying alternatives
    
    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>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 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 9d54df1..0a0ce15 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)
--
generated by git-patchbot for /home/xen/git/xen.git#master

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