[tip: x86/entry] context_tracking: Ensure that the critical path cannot be instrumented

From: tip-bot2 for Thomas Gleixner
Date: Tue May 19 2020 - 16:00:41 EST

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The following commit has been merged into the x86/entry branch of tip:

Commit-ID: fcb10ef4544407ecdb536f9563fe63afcad44209
Gitweb: https://git.kernel.org/tip/fcb10ef4544407ecdb536f9563fe63afcad44209
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Wed, 04 Mar 2020 11:05:22 +01:00
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 19 May 2020 15:47:22 +02:00

context_tracking: Ensure that the critical path cannot be instrumented

context tracking lacks a few protection mechanisms against instrumentation:

- While the core functions are marked NOKPROBE they lack protection
against function tracing which is required as the function entry/exit
points can be utilized by BPF.

- static functions invoked from the protected functions need to be marked
as well as they can be instrumented otherwise.

- using plain inline allows the compiler to emit traceable and probable
functions.

Fix this by marking the functions noinstr and converting the plain inlines
to __always_inline.

The NOKPROBE_SYMBOL() annotations are removed as the .noinstr.text section
is already excluded from being probed.

Cures the following objtool warnings:

vmlinux.o: warning: objtool: enter_from_user_mode()+0x34: call to __context_tracking_exit() leaves .noinstr.text section
vmlinux.o: warning: objtool: prepare_exit_to_usermode()+0x29: call to __context_tracking_enter() leaves .noinstr.text section
vmlinux.o: warning: objtool: syscall_return_slowpath()+0x29: call to __context_tracking_enter() leaves .noinstr.text section
vmlinux.o: warning: objtool: do_syscall_64()+0x7f: call to __context_tracking_enter() leaves .noinstr.text section
vmlinux.o: warning: objtool: do_int80_syscall_32()+0x3d: call to __context_tracking_enter() leaves .noinstr.text section
vmlinux.o: warning: objtool: do_fast_syscall_32()+0x9c: call to __context_tracking_enter() leaves .noinstr.text section

and generates new ones...

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Reviewed-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200505134340.811520478@xxxxxxxxxxxxx

---
include/linux/context_tracking.h | 6 +++---
include/linux/context_tracking_state.h | 6 +++---
kernel/context_tracking.c | 14 ++++++++------
3 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index 8cac62e..981b880 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -33,13 +33,13 @@ static inline void user_exit(void)
}

/* Called with interrupts disabled. */
-static inline void user_enter_irqoff(void)
+static __always_inline void user_enter_irqoff(void)
{
if (context_tracking_enabled())
__context_tracking_enter(CONTEXT_USER);

}
-static inline void user_exit_irqoff(void)
+static __always_inline void user_exit_irqoff(void)
{
if (context_tracking_enabled())
__context_tracking_exit(CONTEXT_USER);
@@ -75,7 +75,7 @@ static inline void exception_exit(enum ctx_state prev_ctx)
* is enabled. If context tracking is disabled, returns
* CONTEXT_DISABLED. This should be used primarily for debugging.
*/
-static inline enum ctx_state ct_state(void)
+static __always_inline enum ctx_state ct_state(void)
{
return context_tracking_enabled() ?
this_cpu_read(context_tracking.state) : CONTEXT_DISABLED;
diff --git a/include/linux/context_tracking_state.h b/include/linux/context_tracking_state.h
index e7fe667..65a60d3 100644
--- a/include/linux/context_tracking_state.h
+++ b/include/linux/context_tracking_state.h
@@ -26,12 +26,12 @@ struct context_tracking {
extern struct static_key_false context_tracking_key;
DECLARE_PER_CPU(struct context_tracking, context_tracking);

-static inline bool context_tracking_enabled(void)
+static __always_inline bool context_tracking_enabled(void)
{
return static_branch_unlikely(&context_tracking_key);
}

-static inline bool context_tracking_enabled_cpu(int cpu)
+static __always_inline bool context_tracking_enabled_cpu(int cpu)
{
return context_tracking_enabled() && per_cpu(context_tracking.active, cpu);
}
@@ -41,7 +41,7 @@ static inline bool context_tracking_enabled_this_cpu(void)
return context_tracking_enabled() && __this_cpu_read(context_tracking.active);
}

-static inline bool context_tracking_in_user(void)
+static __always_inline bool context_tracking_in_user(void)
{
return __this_cpu_read(context_tracking.state) == CONTEXT_USER;
}
diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index ce43088..36a98c4 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -31,7 +31,7 @@ EXPORT_SYMBOL_GPL(context_tracking_key);
DEFINE_PER_CPU(struct context_tracking, context_tracking);
EXPORT_SYMBOL_GPL(context_tracking);

-static bool context_tracking_recursion_enter(void)
+static noinstr bool context_tracking_recursion_enter(void)
{
int recursion;

@@ -45,7 +45,7 @@ static bool context_tracking_recursion_enter(void)
return false;
}

-static void context_tracking_recursion_exit(void)
+static __always_inline void context_tracking_recursion_exit(void)
{
__this_cpu_dec(context_tracking.recursion);
}
@@ -59,7 +59,7 @@ static void context_tracking_recursion_exit(void)
* instructions to execute won't use any RCU read side critical section
* because this function sets RCU in extended quiescent state.
*/
-void __context_tracking_enter(enum ctx_state state)
+void noinstr __context_tracking_enter(enum ctx_state state)
{
/* Kernel threads aren't supposed to go to userspace */
WARN_ON_ONCE(!current->mm);
@@ -77,8 +77,10 @@ void __context_tracking_enter(enum ctx_state state)
* on the tick.
*/
if (state == CONTEXT_USER) {
+ instrumentation_begin();
trace_user_enter(0);
vtime_user_enter(current);
+ instrumentation_end();
}
rcu_user_enter();
}
@@ -99,7 +101,6 @@ void __context_tracking_enter(enum ctx_state state)
}
context_tracking_recursion_exit();
}
-NOKPROBE_SYMBOL(__context_tracking_enter);
EXPORT_SYMBOL_GPL(__context_tracking_enter);

void context_tracking_enter(enum ctx_state state)
@@ -142,7 +143,7 @@ NOKPROBE_SYMBOL(context_tracking_user_enter);
* This call supports re-entrancy. This way it can be called from any exception
* handler without needing to know if we came from userspace or not.
*/
-void __context_tracking_exit(enum ctx_state state)
+void noinstr __context_tracking_exit(enum ctx_state state)
{
if (!context_tracking_recursion_enter())
return;
@@ -155,15 +156,16 @@ void __context_tracking_exit(enum ctx_state state)
*/
rcu_user_exit();
if (state == CONTEXT_USER) {
+ instrumentation_begin();
vtime_user_exit(current);
trace_user_exit(0);
+ instrumentation_end();
}
}
__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
}
context_tracking_recursion_exit();
}
-NOKPROBE_SYMBOL(__context_tracking_exit);
EXPORT_SYMBOL_GPL(__context_tracking_exit);

void context_tracking_exit(enum ctx_state state)

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