linux-kernel - [PATCH] x86/paravirt: Apply paravirt instructions in consistent order during boot/module load

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Date:   Mon,  7 Mar 2022 12:03:38 -0600
From:   Alex Thorlton <alex.thorlton@...cle.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Alex Thorlton <alex.thorlton@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        stable@...r.kernel.org
Subject: [PATCH] x86/paravirt: Apply paravirt instructions in consistent order during boot/module load

Commit 4e6292114c74 ("x86/paravirt: Add new features for paravirt
patching") changed the order in which altinstructions and paravirt
instructions are patched at boot time.  However, no analogous change was
made in module_finalize, where we apply altinstructions and
parainstructions during module load.

As a result, any code that generates "stacked up" altinstructions and
parainstructions (i.e. local_irq_save/restore) will produce different
results when used in built-in kernel code vs. kernel modules.  This also
makes it possible to inadvertently replace altinstructions in the booted
kernel with their parainstruction counterparts when using
livepatch/kpatch.

To fix this, re-order the processing in module_finalize, so that we do
things in this order:

 1. apply_paravirt
 2. apply_retpolines
 3. apply_alternatives
 4. alternatives_smp_module_add

This is the same ordering that is used at boot time in
alternative_instructions.

Fixes: 4e6292114c74 ("x86/paravirt: Add new features for paravirt patching")
Signed-off-by: Alex Thorlton <alex.thorlton@...cle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Kefeng Wang <wangkefeng.wang@...wei.com>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org # 5.13+
---
 arch/x86/kernel/module.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 95fa745e310a5..4edc9c87ad0bc 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -273,6 +273,10 @@ int module_finalize(const Elf_Ehdr *hdr,
 			retpolines = s;
 	}
 
+	if (para) {
+		void *pseg = (void *)para->sh_addr;
+		apply_paravirt(pseg, pseg + para->sh_size);
+	}
 	if (retpolines) {
 		void *rseg = (void *)retpolines->sh_addr;
 		apply_retpolines(rseg, rseg + retpolines->sh_size);
@@ -290,11 +294,6 @@ int module_finalize(const Elf_Ehdr *hdr,
 					    tseg, tseg + text->sh_size);
 	}
 
-	if (para) {
-		void *pseg = (void *)para->sh_addr;
-		apply_paravirt(pseg, pseg + para->sh_size);
-	}
-
 	/* make jump label nops */
 	jump_label_apply_nops(me);
 
-- 
2.33.1