[xen staging] x86/spec-ctrl: Protect against CALL/JMP straight-line speculation

[patchbot@xxxxxxx]

commit 3b7dab93f2401b08c673244c9ae0f92e08bd03ba
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Jul 1 12:39:59 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Jul 1 17:01:24 2020 +0100

    x86/spec-ctrl: Protect against CALL/JMP straight-line speculation
    
    Some x86 CPUs speculatively execute beyond indirect CALL/JMP instructions.
    
    With CONFIG_INDIRECT_THUNK / Retpolines, indirect CALL/JMP instructions are
    converted to direct CALL/JMP's to __x86_indirect_thunk_REG(), leaving just a
    handful of indirect JMPs implementing those stubs.
    
    There is no architectrual execution beyond an indirect JMP, so use INT3 as
    recommended by vendors to halt speculative execution.  This is shorter than
    LFENCE (which would also work fine), but also shows up in logs if we do
    unexpected execute them.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Release-acked-by: Paul Durrant <paul@xxxxxxx>
---
 xen/arch/x86/indirect-thunk.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/xen/arch/x86/indirect-thunk.S b/xen/arch/x86/indirect-thunk.S
index 3c17f75c23..7392aee127 100644
--- a/xen/arch/x86/indirect-thunk.S
+++ b/xen/arch/x86/indirect-thunk.S
@@ -24,10 +24,12 @@
 .macro IND_THUNK_LFENCE reg:req
         lfence
         jmp *%\reg
+        int3 /* Halt straight-line speculation */
 .endm
 
 .macro IND_THUNK_JMP reg:req
         jmp *%\reg
+        int3 /* Halt straight-line speculation */
 .endm
 
 /*
--
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