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Date:   Wed, 22 Nov 2017 20:39:16 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     X86 ML <x86@...nel.org>
Cc:     Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org
Subject: [PATCH] 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.

Cc: stable@...r.kernel.org
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---

Hi Ingo-

You asked me to look for an irqflag tracing bug, so I found one :)

 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 a2b30ec69497..3c288f260fdf 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
-- 
2.13.6

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