[Xen-changelog] [xen stable-4.6] x86: use optimal NOPs to fill the SMEP/SMAP placeholders

[patchbot@xxxxxxx]

commit 965929f2f7ddfdfca52be54970bceb2764145d45
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri May 27 14:34:03 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri May 27 14:34:03 2016 +0200

    x86: use optimal NOPs to fill the SMEP/SMAP placeholders
    
    Alternatives patching code picks the most suitable NOPs for the
    running system, so simply use it to replace the pre-populated ones.
    
    Use an arbitrary, always available feature to key off from, but
    hide this behind the new X86_FEATURE_ALWAYS.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    
    x86/compat: correct SMEP/SMAP NOPs patching
    
    Correct the number of single byte NOPs we want to be replaced in case
    neither SMEP nor SMAP are available.
    
    Also simplify the expression adding these NOPs - at that location .
    equals .Lcr4_orig, and removing that part of the expression fixes a
    bogus ".space or fill with negative value, ignored" warning by very old
    gas (which actually is what made me look at those constructs again).
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: 01a0bd0a7d72be638a359db3f8cf551123467d29
    master date: 2016-05-13 18:15:55 +0100
    master commit: f5610009529628314c9d1d52b00715fe855fcf06
    master date: 2016-05-26 17:26:24 +0100
---
 xen/arch/x86/x86_64/compat/entry.S | 11 +++--------
 xen/include/asm-x86/asm_defns.h    |  2 ++
 xen/include/asm-x86/cpufeature.h   |  3 +++
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/x86_64/compat/entry.S 
b/xen/arch/x86/x86_64/compat/entry.S
index 4903773..0e3db7c 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -175,14 +175,7 @@ compat_bad_hypercall:
 ENTRY(compat_restore_all_guest)
         ASSERT_INTERRUPTS_DISABLED
 .Lcr4_orig:
-        ASM_NOP8 /* testb $3,UREGS_cs(%rsp) */
-        ASM_NOP2 /* jpe   .Lcr4_alt_end */
-        ASM_NOP8 /* mov   CPUINFO_cr4...(%rsp), %rax */
-        ASM_NOP6 /* and   $..., %rax */
-        ASM_NOP8 /* mov   %rax, CPUINFO_cr4...(%rsp) */
-        ASM_NOP3 /* mov   %rax, %cr4 */
-        ASM_NOP8 /* cmp   %rax, CPUINFO_cr4...(%rsp) */
-        ASM_NOP2 /* jne   1b */
+        .skip .Lcr4_alt_end - .Lcr4_alt, 0x90
 .Lcr4_orig_end:
         .pushsection .altinstr_replacement, "ax"
 .Lcr4_alt:
@@ -207,6 +200,8 @@ ENTRY(compat_restore_all_guest)
         jne   1b
 .Lcr4_alt_end:
         .section .altinstructions, "a"
+        altinstruction_entry .Lcr4_orig, .Lcr4_orig, X86_FEATURE_ALWAYS, \
+                             (.Lcr4_orig_end - .Lcr4_orig), 0
         altinstruction_entry .Lcr4_orig, .Lcr4_alt, X86_FEATURE_SMEP, \
                              (.Lcr4_orig_end - .Lcr4_orig), \
                              (.Lcr4_alt_end - .Lcr4_alt)
diff --git a/xen/include/asm-x86/asm_defns.h b/xen/include/asm-x86/asm_defns.h
index 788fac4..36fe493 100644
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -176,6 +176,7 @@ void ret_from_intr(void);
         662: __ASM_##op;                                               \
         .popsection;                                                   \
         .pushsection .altinstructions, "a";                            \
+        altinstruction_entry 661b, 661b, X86_FEATURE_ALWAYS, 3, 0;     \
         altinstruction_entry 661b, 662b, X86_FEATURE_SMAP, 3, 3;       \
         .popsection
 
@@ -187,6 +188,7 @@ void ret_from_intr(void);
         .pushsection .altinstr_replacement, "ax";                  \
         668: call cr4_pv32_restore;                                \
         .section .altinstructions, "a";                            \
+        altinstruction_entry 667b, 667b, X86_FEATURE_ALWAYS, 5, 0; \
         altinstruction_entry 667b, 668b, X86_FEATURE_SMEP, 5, 5;   \
         altinstruction_entry 667b, 668b, X86_FEATURE_SMAP, 5, 5;   \
         .popsection
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index 0bf278b..a7e36ae 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -155,6 +155,9 @@
 #define X86_FEATURE_ADX                (7*32+19) /* ADCX, ADOX instructions */
 #define X86_FEATURE_SMAP       (7*32+20) /* Supervisor Mode Access Prevention 
*/
 
+/* An alias of a feature we know is always going to be present. */
+#define X86_FEATURE_ALWAYS      X86_FEATURE_LM
+
 #if !defined(__ASSEMBLY__) && !defined(X86_FEATURES_ONLY)
 #include <xen/bitops.h>
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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