On 3/16/2021 8:49 AM, Dave Hansen wrote:
On 3/16/21 8:13 AM, Yu-cheng Yu wrote:
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -392,3 +392,21 @@ For 32-bit we have the following conventions - kernel is built with
.endm
#endif /* CONFIG_SMP */
+/*
+ * ENDBR is an instruction for the Indirect Branch Tracking (IBT) component
+ * of CET. IBT prevents attacks by ensuring that (most) indirect branches
+ * function calls may only land at ENDBR instructions. Branches that don't
+ * follow the rules will result in control flow (#CF) exceptions.
+ * ENDBR is a noop when IBT is unsupported or disabled. Most ENDBR
+ * instructions are inserted automatically by the compiler, but branch
+ * targets written in assembly must have ENDBR added manually.
+ */
+.macro ENDBR
+#ifdef CONFIG_X86_CET
+#ifdef __i386__
+ endbr32
+#else
+ endbr64
+#endif
+#endif
+.endm
Is "#ifdef __i386__" the right thing to use here? I guess ENDBR only
ends up getting used in the VDSO, but there's a lot of
non-userspace-exposed stuff in calling.h. It seems a bit weird to have
the normally userspace-only __i386__ in there.
I don't see any existing direct use of __i386__ in arch/x86/entry/vdso.
Good point. My thought was, __i386__ comes from the compiler having the
-m32 command-line option, and it is not dependent on anything else.
Alternatively, there is another compiler-defined macro _CET_ENDBR that
can be used. We can put the following in calling.h:
#ifdef __CET__
#include <cet.h>
#else
#define _CET_ENDBR
#endif
and then use _CET_ENDBR in other files. How is that?
In the future, in case we have kernel-mode IBT, ENDBR macros are also
needed for other assembly files.
Thanks,
Yu-cheng
