x86: kmsan: don't instrument stack walking functions

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.

Link: https://lkml.kernel.org/r/20220915150417.722975-40-glider@google.com
Signed-off-by: Alexander Potapenko <glider@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Marco Elver <elver@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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)
 {

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;