[xen staging-4.18] x86/emul: Fix extable registration in invoke_stub()

[patchbot@xxxxxxx]

commit 438bb1285f470e2f385c0ea917ac9787d56aa8d3
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu May 15 19:01:33 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Jul 1 12:58:12 2025 +0100

    x86/emul: Fix extable registration in invoke_stub()
    
    For exception recovery in the stubs, the registered address for fixup is the
    return address of the CALL entering the stub.
    
    In invoke_stub(), the '.Lret%=:' label is the wrong side of the 'post'
    parameter.  The 'post' parameter is non-empty in cases where the arithmetic
    flags of the operation need recovering.
    
    Split the line to separate 'pre' and 'post', making it more obvious that the
    return address label was in the wrong position.
    
    However, in the case that an exception did occur, we want to skip 'post' as
    it's logically part of the operation which had already failed.  Therefore, 
add
    a new skip label and use that for the exception recovery path.
    
    This is XSA-470 / CVE-2025-27465
    
    Fixes: 79903e50dba9 ("x86emul: catch exceptions occurring in stubs")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit b9f83119750ffa0e2e925d74e6e5f38925094883)
---
 xen/arch/x86/x86_emulate/private.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/private.h 
b/xen/arch/x86/x86_emulate/private.h
index 719dad59cd..da6f833bdd 100644
--- a/xen/arch/x86/x86_emulate/private.h
+++ b/xen/arch/x86/x86_emulate/private.h
@@ -699,12 +699,15 @@ struct stub_exn {
     stub_exn.info = (union stub_exception_token) { .raw = ~0 };         \
     stub_exn.line = __LINE__; /* Utility outweighs livepatching cost */ \
     block_speculation(); /* SCSB */                                     \
-    asm volatile ( pre "\n\tINDIRECT_CALL %[stub]\n\t" post "\n"        \
+    asm volatile ( pre "\n\t"                                           \
+                   "INDIRECT_CALL %[stub]\n"                            \
                    ".Lret%=:\n\t"                                       \
+                   post "\n\t"                                          \
+                   ".Lskip%=:\n\t"                                      \
                    ".pushsection .fixup,\"ax\"\n"                       \
                    ".Lfix%=:\n\t"                                       \
                    "pop %[exn]\n\t"                                     \
-                   "jmp .Lret%=\n\t"                                    \
+                   "jmp .Lskip%=\n\t"                                   \
                    ".popsection\n\t"                                    \
                    _ASM_EXTABLE(.Lret%=, .Lfix%=)                       \
                    : [exn] "+g" (stub_exn.info) ASM_CALL_CONSTRAINT,    \
--
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