linux-kernel - [RFC PATCH 10/10] x86: snapshot thread flags

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Date:   Wed,  9 Jun 2021 13:20:01 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     linux-kernel@...r.kernel.org
Cc:     benh@...nel.crashing.org, boqun.feng@...il.com, bp@...en8.de,
        catalin.marinas@....com, dvyukov@...gle.com, elver@...gle.com,
        ink@...assic.park.msu.ru, jonas@...thpole.se,
        juri.lelli@...hat.com, linux@...linux.org.uk, luto@...nel.org,
        mark.rutland@....com, mattst88@...il.com, mingo@...hat.com,
        monstr@...str.eu, mpe@...erman.id.au, paulmck@...nel.org,
        paulus@...ba.org, peterz@...radead.org, rth@...ddle.net,
        shorne@...il.com, stefan.kristiansson@...nalahti.fi,
        tglx@...utronix.de, vincent.guittot@...aro.org, will@...nel.org
Subject: [RFC PATCH 10/10] x86: snapshot thread flags

Some thread flags can be set remotely, and so even when IRQs are
disabled, the flags can change under our feet. Generally this is
unlikely to cause a problem in practice, but it is somewhat unsound, and
KCSAN will legitimately warn that there is a data race.

To avoid such issues, we should snapshot the flags prior to using them.
Let's use the new helpers to do so on x86.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/kernel/process.c | 8 ++++----
 arch/x86/kernel/process.h | 6 +++---
 arch/x86/mm/tlb.c         | 2 +-
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 5e1f38179f49..d1c4a93c6e26 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -332,7 +332,7 @@ void arch_setup_new_exec(void)
 		clear_thread_flag(TIF_SSBD);
 		task_clear_spec_ssb_disable(current);
 		task_clear_spec_ssb_noexec(current);
-		speculation_ctrl_update(task_thread_info(current)->flags);
+		speculation_ctrl_update(read_thread_flags());
 	}
 }
 
@@ -584,7 +584,7 @@ static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk)
 			clear_tsk_thread_flag(tsk, TIF_SPEC_IB);
 	}
 	/* Return the updated threadinfo flags*/
-	return task_thread_info(tsk)->flags;
+	return read_task_thread_flags(tsk);
 }
 
 void speculation_ctrl_update(unsigned long tif)
@@ -620,8 +620,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
 {
 	unsigned long tifp, tifn;
 
-	tifn = READ_ONCE(task_thread_info(next_p)->flags);
-	tifp = READ_ONCE(task_thread_info(prev_p)->flags);
+	tifn = read_task_thread_flags(next_p);
+	tifp = read_task_thread_flags(prev_p);
 
 	switch_to_bitmap(tifp);
 
diff --git a/arch/x86/kernel/process.h b/arch/x86/kernel/process.h
index 1d0797b2338a..0b1be8685b49 100644
--- a/arch/x86/kernel/process.h
+++ b/arch/x86/kernel/process.h
@@ -13,9 +13,9 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p);
 static inline void switch_to_extra(struct task_struct *prev,
 				   struct task_struct *next)
 {
-	unsigned long next_tif = task_thread_info(next)->flags;
-	unsigned long prev_tif = task_thread_info(prev)->flags;
-
+	unsigned long next_tif = read_task_thread_flags(next);
+	unsigned long prev_tif = read_task_thread_flags(prev);
+	
 	if (IS_ENABLED(CONFIG_SMP)) {
 		/*
 		 * Avoid __switch_to_xtra() invocation when conditional
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 78804680e923..fcc5e22050d4 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -318,7 +318,7 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 
 static unsigned long mm_mangle_tif_spec_ib(struct task_struct *next)
 {
-	unsigned long next_tif = task_thread_info(next)->flags;
+	unsigned long next_tif = read_task_thread_flags(next);
 	unsigned long ibpb = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_IBPB;
 
 	return (unsigned long)next->mm | ibpb;
-- 
2.11.0