[PATCH] rseq: Out-of-line rseq_exit_user_update() for KMSAN

KMSAN produces the following false positive on s390x:

    [    4.232136] Kernel panic - not syncing: kmsan.panic set ...
    [    4.232339] CPU: 7 UID: 0 PID: 1 Comm: virtme-init Tainted: G    B            N  7.2.0-rc6-virtme #27 PREEMPTLAZY
    [    4.232676] Tainted: [B]=BAD_PAGE, [N]=TEST
    [    4.232808] Hardware name: IBM 3931 A01 701 (KVM/Linux)
    [    4.232971] Call Trace:
    [    4.233065]  [<000002dcfb993140>] vpanic+0xb50/0x1080
    [    4.239698]  [<000002dcfb9925ec>] panic+0x22c/0x230
    [    4.239879]  [<000002dcfcec5d32>] kmsan_report+0x322/0x330
    [    4.240081]  [<000002dcfcec3592>] kmsan_internal_check_memory+0x222/0x280
    [    4.240304]  [<000002dcffe3444e>] irqentry_exit+0x50e/0x8f0
    [    4.240522]  [<000002dcffe33454>] __do_pgm_check+0x1b4/0x2b0
    [    4.240740]  [<000002dcffe75304>] pgm_check_handler+0x114/0x160

The reason is that the following call chain inlined into the noinstr
irqentry_exit() function:

    irqentry_exit()
      irqentry_exit_to_user_mode()
        irqentry_exit_to_user_mode_prepare()
          __exit_to_user_mode_prepare()
            exit_to_user_mode_loop()
              rseq_exit_to_user_mode_restart()
                __rseq_exit_to_user_mode_restart()
                  rseq_exit_user_update()

does not set KMSAN shadow on struct rseq_ids, which is then verified by

    rseq_exit_user_update()
      rseq_update_usr()
        rseq_set_ids_get_csaddr()
          unsafe_put_user()

regardless of presence of KMSAN instrumentation.

Fix by out-of-lining rseq_exit_user_update() and compiling the
out-of-lined version with KMSAN instrumentation.

This should affect other architectures as well, but whether one
actually sees it depends on the leftover shadow of struct rseq_ids
stack location: s390 is not special here.

Fixes: 05b44aef709c ("rseq: Implement fast path for exit to user")
Signed-off-by: Ilya Leoshkevich <[email protected]>
---
 include/linux/rseq_entry.h | 9 ++++++++-
 kernel/rseq.c              | 7 +++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/rseq_entry.h b/include/linux/rseq_entry.h
index ed9da6e41a2aa..a396050f771cc 100644
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -595,7 +595,8 @@ static rseq_inline bool rseq_update_usr(struct task_struct *t, struct pt_regs *r
  * tells the caller to loop back into exit_to_user_mode_loop(). The rseq
  * slow path there will handle the failure.
  */
-static __always_inline bool rseq_exit_user_update(struct pt_regs *regs, struct task_struct *t)
+static __always_inline bool rseq_exit_user_update_inline(struct pt_regs *regs,
+							 struct task_struct *t)
 {
 	/*
 	 * Page faults need to be disabled as this is called with
@@ -647,6 +648,12 @@ static __always_inline bool rseq_exit_user_update(struct pt_regs *regs, struct t
 	return false;
 }
 
+#ifdef CONFIG_KMSAN
+bool rseq_exit_user_update(struct pt_regs *regs, struct task_struct *t);
+#else
+#define rseq_exit_user_update rseq_exit_user_update_inline
+#endif
+
 static __always_inline bool __rseq_exit_to_user_mode_restart(struct pt_regs *regs)
 {
 	struct task_struct *t = current;
diff --git a/kernel/rseq.c b/kernel/rseq.c
index e75e3a5e312c8..be759d7d266b5 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -329,6 +329,13 @@ void __rseq_handle_slowpath(struct pt_regs *regs)
 	rseq_slowpath_update_usr(regs);
 }
 
+#if defined(CONFIG_KMSAN) && defined(CONFIG_GENERIC_ENTRY)
+bool rseq_exit_user_update(struct pt_regs *regs, struct task_struct *t)
+{
+	return rseq_exit_user_update_inline(regs, t);
+}
+#endif
+
 void __rseq_signal_deliver(int sig, struct pt_regs *regs)
 {
 	rseq_stat_inc(rseq_stats.signal);
-- 
2.55.0