Commit 2105a927 authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

static_call,x86: Robustify trampoline patching 



Add a few signature bytes after the static call trampoline and verify
those bytes match before patching the trampoline. This avoids patching
random other JMPs (such as CFI jump-table entries) instead.

These bytes decode as:

   d:   53                      push   %rbx
   e:   43 54                   rex.XB push %r12

And happen to spell "SCT".

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211030074758.GT174703@worktop.programming.kicks-ass.net
parent debe436e

arch/x86/include/asm/static_call.h

+1 −0
Original line number Diff line number Diff line
@@ -27,6 +27,7 @@ 
	    ".globl " STATIC_CALL_TRAMP_STR(name) "		\n"	\

	    STATIC_CALL_TRAMP_STR(name) ":			\n"	\

	    insns "						\n"	\

	    ".byte 0x53, 0x43, 0x54				\n"	\

	    ".type " STATIC_CALL_TRAMP_STR(name) ", @function	\n"	\

	    ".size " STATIC_CALL_TRAMP_STR(name) ", . - " STATIC_CALL_TRAMP_STR(name) " \n" \

	    ".popsection					\n")

arch/x86/kernel/static_call.c

+10 −4
Original line number Diff line number Diff line
@@ -56,10 +56,15 @@ static void __ref __static_call_transform(void *insn, enum insn_type type, void 
	text_poke_bp(insn, code, size, emulate);

}



static void __static_call_validate(void *insn, bool tail)

static void __static_call_validate(void *insn, bool tail, bool tramp)

{

	u8 opcode = *(u8 *)insn;



	if (tramp && memcmp(insn+5, "SCT", 3)) {

		pr_err("trampoline signature fail");

		BUG();

	}



	if (tail) {

		if (opcode == JMP32_INSN_OPCODE ||

		    opcode == RET_INSN_OPCODE)
@@ -74,7 +79,8 @@ static void __static_call_validate(void *insn, bool tail) 
	/*

	 * If we ever trigger this, our text is corrupt, we'll probably not live long.

	 */

	WARN_ONCE(1, "unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);

	pr_err("unexpected static_call insn opcode 0x%x at %pS\n", opcode, insn);

	BUG();

}



static inline enum insn_type __sc_insn(bool null, bool tail)
@@ -97,12 +103,12 @@ void arch_static_call_transform(void *site, void *tramp, void *func, bool tail) 
	mutex_lock(&text_mutex);



	if (tramp) {

		__static_call_validate(tramp, true);

		__static_call_validate(tramp, true, true);

		__static_call_transform(tramp, __sc_insn(!func, true), func);

	}



	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE) && site) {

		__static_call_validate(site, tail);

		__static_call_validate(site, tail, false);

		__static_call_transform(site, __sc_insn(!func, tail), func);

	}

tools/objtool/check.c

+3 −0
Original line number Diff line number Diff line
@@ -3310,6 +3310,9 @@ static bool ignore_unreachable_insn(struct objtool_file *file, struct instructio 
	if (!insn->func)

		return false;



	if (insn->func->static_call_tramp)

		return true;



	/*

	 * CONFIG_UBSAN_TRAP inserts a UD2 when it sees

	 * __builtin_unreachable().  The BUG() macro has an unreachable() after