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Subject[tip:x86/urgent] x86/speculation: Prevent deadlock on ssb_state::lock
Commit-ID:  2f5fb19341883bb6e37da351bc3700489d8506a7
Gitweb: https://git.kernel.org/tip/2f5fb19341883bb6e37da351bc3700489d8506a7
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Sun, 14 Apr 2019 19:51:06 +0200
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sun, 14 Apr 2019 23:05:52 +0200

x86/speculation: Prevent deadlock on ssb_state::lock

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@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Cc: Thomas Lendacky <thomas.lendacky@amd.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/alpine.DEB.2.21.1904141948200.4917@nanos.tec.linutronix.de

---
arch/x86/kernel/process.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 58ac7be52c7a..957eae13b370 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -426,6 +426,8 @@ static __always_inline void __speculation_ctrl_update(unsigned long tifp,
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
@@ -477,10 +479,12 @@ static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk)

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 */
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