linux-kernel - [PATCH v2 03/14] x86,smap: Fix smap

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Message-Id: <20200428191659.558899462@infradead.org>
Date:   Tue, 28 Apr 2020 21:11:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     jpoimboe@...hat.com, alexandre.chartre@...cle.com
Cc:     linux-kernel@...r.kernel.org, jthierry@...hat.com,
        tglx@...utronix.de, x86@...nel.org, mbenes@...e.cz,
        peterz@...radead.org
Subject: [PATCH v2 03/14] x86,smap: Fix smap_{save,restore}() alternatives

As reported by objtool:

  lib/ubsan.o: warning: objtool: .altinstr_replacement+0x0: alternative modifies stack
  lib/ubsan.o: warning: objtool: .altinstr_replacement+0x7: alternative modifies stack

the smap_{save,restore}() alternatives violate (the newly enforced)
rule on stack invariance. That is, due to there only being a single
ORC table it must be valid to any alternative. These alternatives
violate this with the direct result that unwinds will not be correct
in between these calls.

[ In specific, since we force SMAP on for objtool, running on !SMAP
hardware will observe a different stack-layout and the ORC unwinder
will stumble. ]

So rewrite the functions to unconditionally save/restore the flags,
which gives an invariant stack layout irrespective of the SMAP state.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/smap.h |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

--- a/arch/x86/include/asm/smap.h
+++ b/arch/x86/include/asm/smap.h
@@ -57,16 +57,19 @@ static __always_inline unsigned long sma
 {
 	unsigned long flags;
 
-	asm volatile (ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC,
-				  X86_FEATURE_SMAP)
-		      : "=rm" (flags) : : "memory", "cc");
+	asm volatile ("# smap_save\n\t"
+		      "pushf; pop %0"
+		      : "=rm" (flags) : : "memory");
+
+	clac();
 
 	return flags;
 }
 
 static __always_inline void smap_restore(unsigned long flags)
 {
-	asm volatile (ALTERNATIVE("", "push %0; popf", X86_FEATURE_SMAP)
+	asm volatile ("# smap_restore\n\t"
+		      "push %0; popf"
 		      : : "g" (flags) : "memory", "cc");
 }