mm: kmsan: remove runtime checks from kmsan_unpoison_memory()

Similarly to what's been done in commit 85716a80 ("kmsan: allow using
__msan_instrument_asm_store() inside runtime"), it should be safe to call
kmsan_unpoison_memory() from within the runtime, as it does not allocate
memory or take locks.  Remove the redundant runtime checks.

This should fix false positives seen with CONFIG_DEBUG_LIST=y when
the non-instrumented lib/stackdepot.c failed to unpoison the memory
chunks later checked by the instrumented lib/list_debug.c

Also replace the implementation of kmsan_unpoison_entry_regs() with
a call to kmsan_unpoison_memory().

Link: https://lkml.kernel.org/r/20240124173134.1165747-1-glider@google.com
Fixes: f80be457 ("kmsan: add KMSAN runtime core")
Signed-off-by: Alexander Potapenko <glider@google.com>
Tested-by: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Nicholas Miehlbradt <nicholas@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kmsan/hooks.c

@@ -359,6 +359,12 @@ void kmsan_handle_dma_sg(struct scatterlist *sg, int nents,
 }
 
 /* Functions from kmsan-checks.h follow. */
+
+/*
+ * To create an origin, kmsan_poison_memory() unwinds the stacks and stores it
+ * into the stack depot. This may cause deadlocks if done from within KMSAN
+ * runtime, therefore we bail out if kmsan_in_runtime().
+ */
 void kmsan_poison_memory(const void *address, size_t size, gfp_t flags)
 {
 	if (!kmsan_enabled || kmsan_in_runtime())
@@ -371,47 +377,31 @@ void kmsan_poison_memory(const void *address, size_t size, gfp_t flags)
 }
 EXPORT_SYMBOL(kmsan_poison_memory);
 
+/*
+ * Unlike kmsan_poison_memory(), this function can be used from within KMSAN
+ * runtime, because it does not trigger allocations or call instrumented code.
+ */
 void kmsan_unpoison_memory(const void *address, size_t size)
 {
 	unsigned long ua_flags;
 
-	if (!kmsan_enabled || kmsan_in_runtime())
+	if (!kmsan_enabled)
 		return;
 
 	ua_flags = user_access_save();
-	kmsan_enter_runtime();
 	/* The users may want to poison/unpoison random memory. */
 	kmsan_internal_unpoison_memory((void *)address, size,
 				       KMSAN_POISON_NOCHECK);
-	kmsan_leave_runtime();
 	user_access_restore(ua_flags);
 }
 EXPORT_SYMBOL(kmsan_unpoison_memory);
 
 /*
- * Version of kmsan_unpoison_memory() that can be called from within the KMSAN
- * runtime.
- *
- * Non-instrumented IRQ entry functions receive struct pt_regs from assembly
- * code. Those regs need to be unpoisoned, otherwise using them will result in
- * false positives.
- * Using kmsan_unpoison_memory() is not an option in entry code, because the
- * return value of in_task() is inconsistent - as a result, certain calls to
- * kmsan_unpoison_memory() are ignored. kmsan_unpoison_entry_regs() ensures that
- * the registers are unpoisoned even if kmsan_in_runtime() is true in the early
- * entry code.
+ * Version of kmsan_unpoison_memory() called from IRQ entry functions.
  */
 void kmsan_unpoison_entry_regs(const struct pt_regs *regs)
 {
-	unsigned long ua_flags;
-
-	if (!kmsan_enabled)
-		return;
-
-	ua_flags = user_access_save();
-	kmsan_internal_unpoison_memory((void *)regs, sizeof(*regs),
-				       KMSAN_POISON_NOCHECK);
-	user_access_restore(ua_flags);
+	kmsan_unpoison_memory((void *)regs, sizeof(*regs));
 }
 
 void kmsan_check_memory(const void *addr, size_t size)