x86/uaccess: avoid check_object_size() in copy_from_user_nmi()

The check_object_size() helper under CONFIG_HARDENED_USERCOPY is designed
to skip any checks where the length is known at compile time as a
reasonable heuristic to avoid "likely known-good" cases.  However, it can
only do this when the copy_*_user() helpers are, themselves, inline too.

Using find_vmap_area() requires taking a spinlock.  The
check_object_size() helper can call find_vmap_area() when the destination
is in vmap memory.  If show_regs() is called in interrupt context, it will
attempt a call to copy_from_user_nmi(), which may call check_object_size()
and then find_vmap_area().  If something in normal context happens to be
in the middle of calling find_vmap_area() (with the spinlock held), the
interrupt handler will hang forever.

The copy_from_user_nmi() call is actually being called with a fixed-size
length, so check_object_size() should never have been called in the first
place.  Given the narrow constraints, just replace the
__copy_from_user_inatomic() call with an open-coded version that calls
only into the sanitizers and not check_object_size(), followed by a call
to raw_copy_from_user().

[akpm@linux-foundation.org: no instrument_copy_from_user() in my tree...]
Link: https://lkml.kernel.org/r/20220919201648.2250764-1-keescook@chromium.org
Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@mail.gmail.com
Fixes: 0aef499f31 ("mm/usercopy: Detect vmalloc overruns")
Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Yu Zhao <yuzhao@google.com>
Reported-by: Florian Lehner <dev@der-flo.net>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Florian Lehner <dev@der-flo.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/x86/lib/usercopy.c

@@ -44,7 +44,7 @@ copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
 	 * called from other contexts.
 	 */
 	pagefault_disable();
-	ret = __copy_from_user_inatomic(to, from, n);
+	ret = raw_copy_from_user(to, from, n);
 	pagefault_enable();
 
 	return ret;