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Date:   Wed, 22 Nov 2017 22:58:36 -0800
From:   tip-bot for Andy Lutomirski <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     hpa@...or.com, mingo@...nel.org, torvalds@...ux-foundation.org,
        peterz@...radead.org, jpoimboe@...hat.com, dave.hansen@...el.com,
        bpetkov@...e.de, tglx@...utronix.de, brgerst@...il.com,
        linux-kernel@...r.kernel.org, luto@...nel.org
Subject: [tip:x86/urgent] x86/entry/64: Add missing irqflags tracing to
 native_load_gs_index()

Commit-ID:  ca37e57bbe0cf1455ea3e84eb89ed04a132d59e1
Gitweb:     https://git.kernel.org/tip/ca37e57bbe0cf1455ea3e84eb89ed04a132d59e1
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Wed, 22 Nov 2017 20:39:16 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 23 Nov 2017 07:54:25 +0100

x86/entry/64: Add missing irqflags tracing to native_load_gs_index()

Running this code with IRQs enabled (where dummy_lock is a spinlock):

static void check_load_gs_index(void)
{
	/* This will fail. */
	load_gs_index(0xffff);

	spin_lock(&dummy_lock);
	spin_unlock(&dummy_lock);
}

Will generate a lockdep warning.  The issue is that the actual write
to %gs would cause an exception with IRQs disabled, and the exception
handler would, as an inadvertent side effect, update irqflag tracing
to reflect the IRQs-off status.  native_load_gs_index() would then
turn IRQs back on and return with irqflag tracing still thinking that
IRQs were off.  The dummy lock-and-unlock causes lockdep to notice the
error and warn.

Fix it by adding the missing tracing.

Apparently nothing did this in a context where it mattered.  I haven't
tried to find a code path that would actually exhibit the warning if
appropriately nasty user code were running.

I suspect that the security impact of this bug is very, very low --
production systems don't run with lockdep enabled, and the warning is
mostly harmless anyway.

Found during a quick audit of the entry code to try to track down an
unrelated bug that Ingo found in some still-in-development code.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
Cc: Borislav Petkov <bpetkov@...e.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
Link: http://lkml.kernel.org/r/e1aeb0e6ba8dd430ec36c8a35e63b429698b4132.1511411918.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/entry/entry_64.S | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 5063ed1..f81d50d 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -51,15 +51,19 @@ ENTRY(native_usergs_sysret64)
 END(native_usergs_sysret64)
 #endif /* CONFIG_PARAVIRT */
 
-.macro TRACE_IRQS_IRETQ
+.macro TRACE_IRQS_FLAGS flags:req
 #ifdef CONFIG_TRACE_IRQFLAGS
-	bt	$9, EFLAGS(%rsp)		/* interrupts off? */
+	bt	$9, \flags		/* interrupts off? */
 	jnc	1f
 	TRACE_IRQS_ON
 1:
 #endif
 .endm
 
+.macro TRACE_IRQS_IRETQ
+	TRACE_IRQS_FLAGS EFLAGS(%rsp)
+.endm
+
 /*
  * When dynamic function tracer is enabled it will add a breakpoint
  * to all locations that it is about to modify, sync CPUs, update
@@ -943,11 +947,13 @@ ENTRY(native_load_gs_index)
 	FRAME_BEGIN
 	pushfq
 	DISABLE_INTERRUPTS(CLBR_ANY & ~CLBR_RDI)
+	TRACE_IRQS_OFF
 	SWAPGS
 .Lgs_change:
 	movl	%edi, %gs
 2:	ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
 	SWAPGS
+	TRACE_IRQS_FLAGS (%rsp)
 	popfq
 	FRAME_END
 	ret

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