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Date:   Sun, 31 Oct 2021 17:24:13 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Mark Rutland <mark.rutland@....com>, X86 ML <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] static_call,x86: Robustify trampoline patching

On Sat, 30 Oct 2021 at 20:55, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Sat, 30 Oct 2021 at 20:03, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Sat, Oct 30, 2021 at 07:19:53PM +0200, Ard Biesheuvel wrote:
> > > I just realized that arm64 has the exact same problem, which is not
> > > being addressed by my v5 of the static call support patch.
> >
> > Yeah, it would.
> >
> > > As it turns out, the v11 Clang that I have been testing with is broken
> > > wrt BTI landing pads, and omits them from the jump table entries.
> > > Clang 12+ adds them properly, which means that both the jump table
> > > entry and the static call trampoline may start with BTI C + direct
> > > branch, and we also need additional checks to disambiguate.
> >
> > I'm not sure, why would the static_call trampoline need a BTI C ? The
> > whole point of static_call() is to be a direct call, we should never
> > have an indirect call to the trampoline, that would defeat the whole
> > purpose.
>
> This might happen when the distance between the caller and the
> trampoline is more than 128 MB, in which case we emit a veneer that
> uses an indirect call as well. So we definitely need the landing pad
> in the trampoline.

Something like the below seems to work to prevent getting the wrong
trampoline address into arch_static_call_transform:

diff --git a/arch/x86/include/asm/static_call.h
b/arch/x86/include/asm/static_call.h
index cbb67b6030f9..c3704ea21bee 100644
--- a/arch/x86/include/asm/static_call.h
+++ b/arch/x86/include/asm/static_call.h
@@ -25,7 +25,9 @@
        asm(".pushsection .static_call.text, \"ax\"             \n"     \
            ".align 4                                           \n"     \
            ".globl " STATIC_CALL_TRAMP_STR(name) "             \n"     \
+           ".globl " STATIC_CALL_TRAMP_P_STR(name) "           \n"     \
            STATIC_CALL_TRAMP_STR(name) ":                      \n"     \
+           STATIC_CALL_TRAMP_P_STR(name) ":                    \n"     \
            insns "                                             \n"     \
            ".type " STATIC_CALL_TRAMP_STR(name) ", @function   \n"     \
            ".size " STATIC_CALL_TRAMP_STR(name) ", . - "
STATIC_CALL_TRAMP_STR(name) " \n" \
diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index 19dc210214c0..46777a3395d3 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -143,7 +143,7 @@
  */
 extern void arch_static_call_transform(void *site, void *tramp, void
*func, bool tail);

-#define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP(name)
+#define STATIC_CALL_TRAMP_ADDR(name) &STATIC_CALL_TRAMP_P(name)

 #else
 #define STATIC_CALL_TRAMP_ADDR(name) NULL
diff --git a/include/linux/static_call_types.h
b/include/linux/static_call_types.h
index 5a00b8b2cf9f..98a448f5ae45 100644
--- a/include/linux/static_call_types.h
+++ b/include/linux/static_call_types.h
@@ -18,6 +18,8 @@
 #define STATIC_CALL_TRAMP(name)
__PASTE(STATIC_CALL_TRAMP_PREFIX, name)
 #define STATIC_CALL_TRAMP_STR(name)    __stringify(STATIC_CALL_TRAMP(name))

+#define STATIC_CALL_TRAMP_P(name)      __PASTE(STATIC_CALL_TRAMP(name), _p)
+#define STATIC_CALL_TRAMP_P_STR(name)  __stringify(STATIC_CALL_TRAMP_P(name))
 /*
  * Flags in the low bits of static_call_site::key.
  */
@@ -36,7 +38,8 @@ struct static_call_site {

 #define DECLARE_STATIC_CALL(name, func)
         \
        extern struct static_call_key STATIC_CALL_KEY(name);            \
-       extern typeof(func) STATIC_CALL_TRAMP(name);
+       extern typeof(func) STATIC_CALL_TRAMP(name);                    \
+       extern u8 STATIC_CALL_TRAMP_P(name);

 #ifdef CONFIG_HAVE_STATIC_CALL

That leaves the 'func' argument, which ideally should not go through
the jump table either, but at least it is not terminally broken there.

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