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Date:   Fri, 16 Sep 2022 12:57:23 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Yu Zhao <yuzhao@...gle.com>, dev@...-flo.net,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-perf-users@...r.kernel.org,
        linux-mm@...ck.org, linux-hardening@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH 0/3] x86/dumpstack: Inline copy_from_user_nmi()

On Fri, 16 Sep 2022 06:59:51 -0700 Kees Cook <keescook@...omium.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;
_

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