Re: CET/IBT support and live-patches

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 23, 2021 at 11:03:20AM -0500, Steven Rostedt wrote:
> On Tue, 23 Nov 2021 12:39:15 +0100 (CET)
> Miroslav Benes <mbenes@suse.cz> wrote:
> 
> > +++ b/kernel/livepatch/patch.c
> > @@ -127,15 +127,18 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> >  /*
> >   * Convert a function address into the appropriate ftrace location.
> >   *
> > - * Usually this is just the address of the function, but on some architectures
> > - * it's more complicated so allow them to provide a custom behaviour.
> > + * Usually this is just the address of the function, but there are some
> > + * exceptions.
> > + *
> > + *   * PPC - live patch works only with -mprofile-kernel. In this case,
> > + *     the ftrace location is always within the first 16 bytes.
> > + *   * x86_64 with CET/IBT enabled - there is ENDBR instruction at +0 offset.
> > + *     __fentry__ follows it.
> >   */
> > -#ifndef klp_get_ftrace_location
> > -static unsigned long klp_get_ftrace_location(unsigned long faddr)
> > +static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
> 
> Why make this the default function? It should only do this for powerpc and
> x86 *if* CET/IBT is enabled.

Well, only this variant of IBT. Once Joao gets his clang patches
together we'll probably have it back at +0.

Something like the below would be more robust, it also gets us something
grep-able for when the IBT code-gen changes yet again.

diff --git a/arch/x86/include/asm/livepatch.h b/arch/x86/include/asm/livepatch.h
index 7c5cc6660e4b..4e683a1aa411 100644
--- a/arch/x86/include/asm/livepatch.h
+++ b/arch/x86/include/asm/livepatch.h
@@ -17,4 +17,13 @@ static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned long ip)
 	ftrace_instruction_pointer_set(fregs, ip);
 }
 
+#define klp_get_ftrace_location klp_get_ftrace_location
+static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
+{
+	unsigned long addr = faddr_location(faddr);
+	if (!addr && IS_ENABLED(CONFIG_X86_IBT))
+		addr = faddr_location(faddr + 4);
+	return addr;
+}
+
 #endif /* _ASM_X86_LIVEPATCH_H */
diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index fe316c021d73..fd295bbbcbc7 100644
--- a/kernel/livepatch/patch.c
+++ b/kernel/livepatch/patch.c
@@ -133,7 +133,7 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 #ifndef klp_get_ftrace_location
 static unsigned long klp_get_ftrace_location(unsigned long faddr)
 {
-	return faddr;
+	return ftrace_location(faddr);
 }
 #endif