[PATCH v6 41/44] entry: kmsan: introduce kmsan_unpoison_entry_regs()

[Alexander Potapenko <glider@google.com>]

struct pt_regs passed into IRQ entry code is set up by uninstrumented
asm functions, therefore KMSAN may not notice the registers are
initialized.

kmsan_unpoison_entry_regs() unpoisons the contents of struct pt_regs,
preventing potential false positives. Unlike kmsan_unpoison_memory(),
it can be called under kmsan_in_runtime(), which is often the case in
IRQ entry code.

Signed-off-by: Alexander Potapenko <glider@google.com>

---
Link: https://linux-review.googlesource.com/id/Ibfd7018ac847fd8e5491681f508ba5d14e4669cf
---
 include/linux/kmsan.h | 15 +++++++++++++++
 kernel/entry/common.c |  5 +++++
 mm/kmsan/hooks.c      | 26 ++++++++++++++++++++++++++
 3 files changed, 46 insertions(+)

diff --git a/include/linux/kmsan.h b/include/linux/kmsan.h
index c473e0e21683c..e38ae3c346184 100644
--- a/include/linux/kmsan.h
+++ b/include/linux/kmsan.h
@@ -214,6 +214,17 @@ void kmsan_handle_dma_sg(struct scatterlist *sg, int nents,
  */
 void kmsan_handle_urb(const struct urb *urb, bool is_out);
 
+/**
+ * kmsan_unpoison_entry_regs() - Handle pt_regs in low-level entry code.
+ * @regs:	struct pt_regs pointer received from assembly code.
+ *
+ * KMSAN unpoisons the contents of the passed pt_regs, preventing potential
+ * false positive reports. Unlike kmsan_unpoison_memory(),
+ * kmsan_unpoison_entry_regs() can be called from the regions where
+ * kmsan_in_runtime() returns true, which is the case in early entry code.
+ */
+void kmsan_unpoison_entry_regs(const struct pt_regs *regs);
+
 #else
 
 static inline void kmsan_init_shadow(void)
@@ -310,6 +321,10 @@ static inline void kmsan_handle_urb(const struct urb *urb, bool is_out)
 {
 }
 
+static inline void kmsan_unpoison_entry_regs(const struct pt_regs *regs)
+{
+}
+
 #endif
 
 #endif /* _LINUX_KMSAN_H */
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 063068a9ea9b3..846add8394c41 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -5,6 +5,7 @@
 #include <linux/resume_user_mode.h>
 #include <linux/highmem.h>
 #include <linux/jump_label.h>
+#include <linux/kmsan.h>
 #include <linux/livepatch.h>
 #include <linux/audit.h>
 #include <linux/tick.h>
@@ -24,6 +25,7 @@ static __always_inline void __enter_from_user_mode(struct pt_regs *regs)
 	user_exit_irqoff();
 
 	instrumentation_begin();
+	kmsan_unpoison_entry_regs(regs);
 	trace_hardirqs_off_finish();
 	instrumentation_end();
 }
@@ -352,6 +354,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 		lockdep_hardirqs_off(CALLER_ADDR0);
 		ct_irq_enter();
 		instrumentation_begin();
+		kmsan_unpoison_entry_regs(regs);
 		trace_hardirqs_off_finish();
 		instrumentation_end();
 
@@ -367,6 +370,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
 	 */
 	lockdep_hardirqs_off(CALLER_ADDR0);
 	instrumentation_begin();
+	kmsan_unpoison_entry_regs(regs);
 	rcu_irq_enter_check_tick();
 	trace_hardirqs_off_finish();
 	instrumentation_end();
@@ -452,6 +456,7 @@ irqentry_state_t noinstr irqentry_nmi_enter(struct pt_regs *regs)
 	ct_nmi_enter();
 
 	instrumentation_begin();
+	kmsan_unpoison_entry_regs(regs);
 	trace_hardirqs_off_finish();
 	ftrace_nmi_enter();
 	instrumentation_end();
diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
index 79d7e73e2cfd8..35f6b6e6a908c 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -348,6 +348,32 @@ void kmsan_unpoison_memory(const void *address, size_t size)
 }
 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.
+ */
+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);
+}
+
 void kmsan_check_memory(const void *addr, size_t size)
 {
 	if (!kmsan_enabled)
-- 
2.37.2.789.g6183377224-goog