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Date:   Tue,  5 Oct 2021 12:58:45 +0200
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Subject: [PATCH -rcu/kcsan 03/23] kcsan: Avoid checking scoped accesses from
 nested contexts
From:   Marco Elver <elver@google.com>
To:     elver@google.com, "Paul E . McKenney" <paulmck@kernel.org>
Cc:     Alexander Potapenko <glider@google.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Dmitry Vyukov <dvyukov@google.com>,
        Ingo Molnar <mingo@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Waiman Long <longman@redhat.com>,
        Will Deacon <will@kernel.org>, kasan-dev@googlegroups.com,
        linux-arch@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, x86@kernel.org
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Precedence: bulk

Avoid checking scoped accesses from nested contexts (such as nested
interrupts or in scheduler code) which share the same kcsan_ctx.

This is to avoid detecting false positive races of accesses in the same
thread with currently scoped accesses: consider setting up a watchpoint
for a non-scoped (normal) access that also "conflicts" with a current
scoped access. In a nested interrupt (or in the scheduler), which shares
the same kcsan_ctx, we cannot check scoped accesses set up in the parent
context -- simply ignore them in this case.

With the introduction of kcsan_ctx::disable_scoped, we can also clean up
kcsan_check_scoped_accesses()'s recursion guard, and do not need to
modify the list's prev pointer.

Signed-off-by: Marco Elver <elver@google.com>
---
 include/linux/kcsan.h |  1 +
 kernel/kcsan/core.c   | 18 +++++++++++++++---
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
index fc266ecb2a4d..13cef3458fed 100644
--- a/include/linux/kcsan.h
+++ b/include/linux/kcsan.h
@@ -21,6 +21,7 @@
  */
 struct kcsan_ctx {
 	int disable_count; /* disable counter */
+	int disable_scoped; /* disable scoped access counter */
 	int atomic_next; /* number of following atomic ops */
 
 	/*
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index e34a1710b7bc..bd359f8ee63a 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -204,15 +204,17 @@ check_access(const volatile void *ptr, size_t size, int type, unsigned long ip);
 static noinline void kcsan_check_scoped_accesses(void)
 {
 	struct kcsan_ctx *ctx = get_ctx();
-	struct list_head *prev_save = ctx->scoped_accesses.prev;
 	struct kcsan_scoped_access *scoped_access;
 
-	ctx->scoped_accesses.prev = NULL;  /* Avoid recursion. */
+	if (ctx->disable_scoped)
+		return;
+
+	ctx->disable_scoped++;
 	list_for_each_entry(scoped_access, &ctx->scoped_accesses, list) {
 		check_access(scoped_access->ptr, scoped_access->size,
 			     scoped_access->type, scoped_access->ip);
 	}
-	ctx->scoped_accesses.prev = prev_save;
+	ctx->disable_scoped--;
 }
 
 /* Rules for generic atomic accesses. Called from fast-path. */
@@ -465,6 +467,15 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
 		goto out;
 	}
 
+	/*
+	 * Avoid races of scoped accesses from nested interrupts (or scheduler).
+	 * Assume setting up a watchpoint for a non-scoped (normal) access that
+	 * also conflicts with a current scoped access. In a nested interrupt,
+	 * which shares the context, it would check a conflicting scoped access.
+	 * To avoid, disable scoped access checking.
+	 */
+	ctx->disable_scoped++;
+
 	/*
 	 * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
 	 * runtime is entered for every memory access, and potentially useful
@@ -578,6 +589,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned
 	if (!kcsan_interrupt_watcher)
 		local_irq_restore(irq_flags);
 	kcsan_restore_irqtrace(current);
+	ctx->disable_scoped--;
 out:
 	user_access_restore(ua_flags);
 }
-- 
2.33.0.800.g4c38ced690-goog