linux-kernel - [tip:x86/paravirt] x86/paravirt: Detect over-sized patching bugs in paravirt_patch

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Message-ID: <tip-11e86dc7f2746210f9c7dc10deaa7658f8dc8350@git.kernel.org>
Date:   Fri, 24 May 2019 00:59:23 -0700
From:   tip-bot for Ingo Molnar <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, brgerst@...il.com, jgross@...e.com,
        hpa@...or.com, peterz@...radead.org, bp@...en8.de,
        tglx@...utronix.de, dvlasenk@...hat.com,
        linux-kernel@...r.kernel.org, luto@...nel.org,
        torvalds@...ux-foundation.org
Subject: [tip:x86/paravirt] x86/paravirt: Detect over-sized patching bugs in
 paravirt_patch_call()

Commit-ID:  11e86dc7f2746210f9c7dc10deaa7658f8dc8350
Gitweb:     https://git.kernel.org/tip/11e86dc7f2746210f9c7dc10deaa7658f8dc8350
Author:     Ingo Molnar <mingo@...nel.org>
AuthorDate: Thu, 25 Apr 2019 11:50:39 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 25 Apr 2019 12:00:44 +0200

x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call()

paravirt_patch_call() currently handles patching failures inconsistently:
we generate a warning in the retpoline case, but don't in other cases where
we might end up with a non-working kernel as well.

So just convert it all to a BUG_ON(), these patching calls are *not* supposed
to fail, and if they do we want to know it immediately.

This also makes the kernel smaller and removes an #ifdef ugly.

I tried it with a richly paravirt-enabled kernel and no patching bugs
were detected.

Cc: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/20190425095039.GC115378@gmail.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/paravirt.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 7f9121f2fdac..544d386ded45 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -73,21 +73,21 @@ struct branch {
 static unsigned paravirt_patch_call(void *insnbuf, const void *target,
 				    unsigned long addr, unsigned len)
 {
+	const int call_len = 5;
 	struct branch *b = insnbuf;
-	unsigned long delta = (unsigned long)target - (addr+5);
+	unsigned long delta = (unsigned long)target - (addr+call_len);
 
-	if (len < 5) {
-#ifdef CONFIG_RETPOLINE
-		WARN_ONCE(1, "Failing to patch indirect CALL in %ps\n", (void *)addr);
-#endif
-		return len;	/* call too long for patch site */
+	if (len < call_len) {
+		pr_warn("paravirt: Failed to patch indirect CALL at %ps\n", (void *)addr);
+		/* Kernel might not be viable if patching fails, bail out: */
+		BUG_ON(1);
 	}
 
 	b->opcode = 0xe8; /* call */
 	b->delta = delta;
-	BUILD_BUG_ON(sizeof(*b) != 5);
+	BUILD_BUG_ON(sizeof(*b) != call_len);
 
-	return 5;
+	return call_len;
 }
 
 #ifdef CONFIG_PARAVIRT_XXL