[PATCH v4 41/45] entry: kmsan: introduce kmsan_unpoison_entry_regs()

Alexander Potapenko
2022-07-01T14:30:15Z

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 | 27 +++++++++++++++++++++++++++
3 files changed, 47 insertions(+)

diff --git a/include/linux/kmsan.h b/include/linux/kmsan.h
index e8b5c306c4aa1..c4412622b9a78 100644
--- a/include/linux/kmsan.h
+++ b/include/linux/kmsan.h
@@ -246,6 +246,17 @@ 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)
@@ -342,6 +353,10 @@ 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 032f164abe7ce..055d3bdb0442c 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);
rcu_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)
rcu_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 9aecbf2825837..c7528bcbb2f91 100644
--- a/mm/kmsan/hooks.c
+++ b/mm/kmsan/hooks.c
@@ -358,6 +358,33 @@ 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);
+}
+EXPORT_SYMBOL(kmsan_unpoison_entry_regs);
+
void kmsan_check_memory(const void *addr, size_t size)
{
if (!kmsan_enabled)
--
2.37.0.rc0.161.g10f37bed90-goog