Re: [PATCH 0/3] x86/dumpstack: Inline copy_from_user_nmi()

Andrew Morton
2022-09-16T19:57:29Z

On Fri, 16 Sep 2022 06:59:51 -0700 Kees Cook <keescook@chromium.org> wrote:

> Hi,
>
> This fixes a find_vmap_area() deadlock. The main fix is patch 2, repeated here:
>
> 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. In order for check_object_size() to see that the length is
> a fixed size, inline copy_from_user_nmi(), as already done with all the
> other uaccess helpers.
>

Why is this so complicated.

There's virtually zero value in running all those debug checks from within
copy_from_user_nmi().

--- a/arch/x86/lib/usercopy.c~a
+++ a/arch/x86/lib/usercopy.c
@@ -44,7 +44,7 @@ copy_from_user_nmi(void *to, const void
* 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;
_