[PATCH] x86/cet: Force -fno-jump-tables for CET-IBT

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Both GCC and Clang have a (mis)feature where, even with
-fcf-protection=branch, jump tables are created using a notrack jump rather
than using endbr's in each case statement.

This is incompatible with the safety properties we want in Xen, and enforced
by not setting MSR_S_CET.NOTRACK_EN.  The consequence is a fatal #CP[endbr].

-fno-jump-tables is generally active as a side effect of
CONFIG_INDIRECT_THUNK (retpoline), but as of c/s 95d9ab461436 ("x86/Kconfig:
introduce option to select retpoline usage"), we explicitly support turning
retpoline off.

Fixes: 3667f7f8f7c4 ("x86: Introduce support for CET-IBT")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
---
 xen/arch/x86/arch.mk | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/arch.mk b/xen/arch/x86/arch.mk
index f6fc852b5767..8e57476d6573 100644
--- a/xen/arch/x86/arch.mk
+++ b/xen/arch/x86/arch.mk
@@ -51,7 +51,10 @@ CFLAGS-$(CONFIG_CC_IS_CLANG) += -mretpoline-external-thunk
 endif
 
 ifdef CONFIG_XEN_IBT
-CFLAGS += -fcf-protection=branch -mmanual-endbr
+# Force -fno-jump-tables to work around
+#   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104816
+#   https://github.com/llvm/llvm-project/issues/54247
+CFLAGS += -fcf-protection=branch -mmanual-endbr -fno-jump-tables
 $(call cc-option-add,CFLAGS,CC,-fcf-check-attribute=no)
 else
 $(call cc-option-add,CFLAGS,CC,-fcf-protection=none)
-- 
2.11.0