[PATCH v3 1/5] x86/entry/64: Fix irqflag tracing wrt context tracking

[Andy Lutomirski]

Paolo pointed out that enter_from_user_mode could be called while
irqflags were traced as though IRQs were on.

In principle, this could confuse lockdep.  It doesn't cause any
problems that I've seen in any configuration, but if I build with
CONFIG_DEBUG_LOCKDEP=y, enable a nohz_full CPU, and add code like:

	if (irqs_disabled()) {
		spin_lock(&something);
		spin_unlock(&something);
	}

to the top of enter_from_user_mode, then lockdep will complain
without this fix.  It seems that lockdep's irqflags sanity checks
are too weak to detect this bug without forcing the issue.

This patch adds one byte to normal kernels, and it's IMO a bit ugly.
I haven't spotted a better way to do this yet, though.  The issue is
that we can't do TRACE_IRQS_OFF until after SWAPGS (if needed), but
we're also supposed to do it before calling C code.

An alternative approach would be to call trace_hardirqs_off in
enter_from_user_mode.  That would be less code and would not bloat
normal kernels at all, but it would be harder to see how the code
worked.

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/entry/entry_64.S | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 53616ca03244..a55697d19824 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -509,6 +509,17 @@ END(irq_entries_start)
 	 * tracking that we're in kernel mode.
 	 */
 	SWAPGS
+
+	/*
+	 * We need to tell lockdep that IRQs are off.  We can't do this until
+	 * we fix gsbase, and we should do it before enter_from_user_mode
+	 * (which can take locks).  Since TRACE_IRQS_OFF idempotent,
+	 * the simplest way to handle it is to just call it twice if
+	 * we enter from user mode.  There's no reason to optimize this since
+	 * TRACE_IRQS_OFF is a no-op if lockdep is off.
+	 */
+	TRACE_IRQS_OFF
+
 #ifdef CONFIG_CONTEXT_TRACKING
 	call enter_from_user_mode
 #endif
@@ -1049,12 +1060,18 @@ ENTRY(error_entry)
 	SWAPGS
 
 .Lerror_entry_from_usermode_after_swapgs:
+	/*
+	 * We need to tell lockdep that IRQs are off.  We can't do this until
+	 * we fix gsbase, and we should do it before enter_from_user_mode
+	 * (which can take locks).
+	 */
+	TRACE_IRQS_OFF
 #ifdef CONFIG_CONTEXT_TRACKING
 	call enter_from_user_mode
 #endif
+	ret
 
 .Lerror_entry_done:
-
 	TRACE_IRQS_OFF
 	ret
 
-- 
2.5.0

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