[PATCH v4 40/45] x86: kmsan: don't instrument stack walking functions

[Alexander Potapenko <glider@google.com>]

Upon function exit, KMSAN marks local variables as uninitialized.
Further function calls may result in the compiler creating the stack
frame where these local variables resided. This results in frame
pointers being marked as uninitialized data, which is normally correct,
because they are not stack-allocated.

However stack unwinding functions are supposed to read and dereference
the frame pointers, in which case KMSAN might be reporting uses of
uninitialized values.

To work around that, we mark update_stack_state(), unwind_next_frame()
and show_trace_log_lvl() with __no_kmsan_checks, preventing all KMSAN
reports inside those functions and making them return initialized
values.

Signed-off-by: Alexander Potapenko <glider@google.com>
---
Link: https://linux-review.googlesource.com/id/I7001eaed630277e8d2ddaff1d6f223d54e997a6f
---
 arch/x86/kernel/dumpstack.c    |  6 ++++++
 arch/x86/kernel/unwind_frame.c | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index afae4dd774951..476eb504084e4 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -177,6 +177,12 @@ static void show_regs_if_on_stack(struct stack_info *info, struct pt_regs *regs,
 	}
 }
 
+/*
+ * This function reads pointers from the stack and dereferences them. The
+ * pointers may not have their KMSAN shadow set up properly, which may result
+ * in false positive reports. Disable instrumentation to avoid those.
+ */
+__no_kmsan_checks
 static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 			unsigned long *stack, const char *log_lvl)
 {
diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index 8e1c50c86e5db..d8ba93778ae32 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -183,6 +183,16 @@ static struct pt_regs *decode_frame_pointer(unsigned long *bp)
 }
 #endif
 
+/*
+ * While walking the stack, KMSAN may stomp on stale locals from other
+ * functions that were marked as uninitialized upon function exit, and
+ * now hold the call frame information for the current function (e.g. the frame
+ * pointer). Because KMSAN does not specifically mark call frames as
+ * initialized, false positive reports are possible. To prevent such reports,
+ * we mark the functions scanning the stack (here and below) with
+ * __no_kmsan_checks.
+ */
+__no_kmsan_checks
 static bool update_stack_state(struct unwind_state *state,
 			       unsigned long *next_bp)
 {
@@ -250,6 +260,7 @@ static bool update_stack_state(struct unwind_state *state,
 	return true;
 }
 
+__no_kmsan_checks
 bool unwind_next_frame(struct unwind_state *state)
 {
 	struct pt_regs *regs;
-- 
2.37.0.rc0.161.g10f37bed90-goog