[tip: x86/entry] x86/entry: Mark enter_from_user_mode() noinstr

[tip-bot2 for Thomas Gleixner]

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

Commit-ID:     8bd73999307ba1bba5152efd327b020ad38f8c13
Gitweb:        https://git.kernel.org/tip/8bd73999307ba1bba5152efd327b020ad38f8c13
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Sat, 29 Feb 2020 15:12:33 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 19 May 2020 16:03:50 +02:00

x86/entry: Mark enter_from_user_mode() noinstr

Both the callers in the low level ASM code and __context_tracking_exit()
which is invoked from enter_from_user_mode() via user_exit_irqoff() are
marked NOKPROBE. Allowing enter_from_user_mode() to be probed is
inconsistent at best.

Aside of that while function tracing per se is safe the function trace
entry/exit points can be used via BPF as well which is not safe to use
before context tracking has reached CONTEXT_KERNEL and adjusted RCU.

Mark it noinstr which moves it into the instrumentation protected text
section and includes notrace.

Note, this needs further fixups in context tracking to ensure that the
full call chain is protected. Will be addressed in follow up changes.

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/20200505134340.429059405@xxxxxxxxxxxxx


---
 arch/x86/entry/common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 76735ec..d862add 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -41,7 +41,7 @@
 
 #ifdef CONFIG_CONTEXT_TRACKING
 /* Called on entry from user mode with IRQs off. */
-__visible inline void enter_from_user_mode(void)
+__visible inline noinstr void enter_from_user_mode(void)
 {
 	CT_WARN_ON(ct_state() != CONTEXT_USER);
 	user_exit_irqoff();