[PATCH 3.16 084/157] x86/speculation: Prevent deadlock on ssb_state::lock

[Ben Hutchings]

3.16.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit 2f5fb19341883bb6e37da351bc3700489d8506a7 upstream.

Mikhail reported a lockdep splat related to the AMD specific ssb_state
lock:

  CPU0                       CPU1
  lock(&st->lock);
                             local_irq_disable();
                             lock(&(&sighand->siglock)->rlock);
                             lock(&st->lock);
  <Interrupt>
     lock(&(&sighand->siglock)->rlock);

  *** DEADLOCK ***

The connection between sighand->siglock and st->lock comes through seccomp,
which takes st->lock while holding sighand->siglock.

Make sure interrupts are disabled when __speculation_ctrl_update() is
invoked via prctl() -> speculation_ctrl_update(). Add a lockdep assert to
catch future offenders.

Fixes: 1f50ddb4f418 ("x86/speculation: Handle HT correctly on AMD")
Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
Cc: Thomas Lendacky <thomas.lendacky@xxxxxxx>
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1904141948200.4917@xxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kernel/process.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -351,6 +351,8 @@ static __always_inline void __speculatio
 	u64 msr = x86_spec_ctrl_base;
 	bool updmsr = false;
 
+	lockdep_assert_irqs_disabled();
+
 	/*
 	 * If TIF_SSBD is different, select the proper mitigation
 	 * method. Note that if SSBD mitigation is disabled or permanentely
@@ -402,10 +404,12 @@ static unsigned long speculation_ctrl_up
 
 void speculation_ctrl_update(unsigned long tif)
 {
+	unsigned long flags;
+
 	/* Forced update. Make sure all relevant TIF flags are different */
-	preempt_disable();
+	local_irq_save(flags);
 	__speculation_ctrl_update(~tif, tif);
-	preempt_enable();
+	local_irq_restore(flags);
 }
 
 /* Called from seccomp/prctl update */