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Subject[PATCH 1/4] context_tracking: Protect against recursion
Date
Context tracking recursion can happen when an exception triggers in the
middle of a call to a context tracking probe.

This special case can be caused by vmalloc faults. If an access to a
memory area allocated by vmalloc happens in the middle of
context_tracking_enter(), we may run into an endless fault loop because
the exception in turn calls context_tracking_enter() which faults on
the same vmalloc'ed memory, triggering an exception again, etc...

Some rare crashes have been reported so lets protect against this with
a recursion counter.

Reported-by: Dave Jones <davej@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Dave Jones <davej@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
---
include/linux/context_tracking_state.h | 1 +
kernel/context_tracking.c | 30 ++++++++++++++++++++++++++++++
2 files changed, 31 insertions(+)

diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
index 6b7b96a..678ecdf 100644
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -12,6 +12,7 @@ struct context_tracking {
* may be further optimized using static keys.
*/
bool active;
+ int recursion;
enum ctx_state {
CONTEXT_KERNEL = 0,
CONTEXT_USER,
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index 72d59a1..b9e0b4f 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -38,6 +38,25 @@ void context_tracking_cpu_set(int cpu)
}
}

+static bool context_tracking_recursion_enter(void)
+{
+ int recursion;
+
+ recursion = __this_cpu_inc_return(context_tracking.recursion);
+ if (recursion == 1)
+ return true;
+
+ WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion);
+ __this_cpu_dec(context_tracking.recursion);
+
+ return false;
+}
+
+static void context_tracking_recursion_exit(void)
+{
+ __this_cpu_dec(context_tracking.recursion);
+}
+
/**
* context_tracking_enter - Inform the context tracking that the CPU is going
* enter user or guest space mode.
@@ -75,6 +94,11 @@ void context_tracking_enter(enum ctx_state state)
WARN_ON_ONCE(!current->mm);

local_irq_save(flags);
+ if (!context_tracking_recursion_enter()) {
+ local_irq_restore(flags);
+ return;
+ }
+
if ( __this_cpu_read(context_tracking.state) != state) {
if (__this_cpu_read(context_tracking.active)) {
/*
@@ -105,6 +129,7 @@ void context_tracking_enter(enum ctx_state state)
*/
__this_cpu_write(context_tracking.state, state);
}
+ context_tracking_recursion_exit();
local_irq_restore(flags);
}
NOKPROBE_SYMBOL(context_tracking_enter);
@@ -139,6 +164,10 @@ void context_tracking_exit(enum ctx_state state)
return;

local_irq_save(flags);
+ if (!context_tracking_recursion_enter()) {
+ local_irq_restore(flags);
+ return;
+ }
if (__this_cpu_read(context_tracking.state) == state) {
if (__this_cpu_read(context_tracking.active)) {
/*
@@ -153,6 +182,7 @@ void context_tracking_exit(enum ctx_state state)
}
__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
}
+ context_tracking_recursion_exit();
local_irq_restore(flags);
}
NOKPROBE_SYMBOL(context_tracking_exit);
--
2.1.4


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