linux-hardening - Re: [PATCH v2] x86/uaccess: Avoid check_object_size() in copy_from_user

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Message-ID: <YylmwJOyDCPP11fg@der-flo.net>
Date:   Tue, 20 Sep 2022 09:07:44 +0200
From:   Florian Lehner <dev@...-flo.net>
To:     Kees Cook <keescook@...omium.org>
Cc:     Matthew Wilcox <willy@...radead.org>, Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        stable@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] x86/uaccess: Avoid check_object_size() in
 copy_from_user_nmi()

On Mon, Sep 19, 2022 at 01:16:48PM -0700, Kees Cook wrote:
> 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().
> 
> Reported-by: Yu Zhao <yuzhao@...gle.com>
> Link: https://lore.kernel.org/all/CAOUHufaPshtKrTWOz7T7QFYUNVGFm0JBjvM700Nhf9qEL9b3EQ@mail.gmail.com
> Reported-by: dev@...-flo.net
> Suggested-by: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Josh Poimboeuf <jpoimboe@...nel.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: x86@...nel.org
> Fixes: 0aef499f3172 ("mm/usercopy: Detect vmalloc overruns")
> Cc: stable@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v2: drop the call explicitly instead of using inline to do it
> v1: https://lore.kernel.org/lkml/20220916135953.1320601-1-keescook@chromium.org
> ---
>  arch/x86/lib/usercopy.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c
> index ad0139d25401..d2aff9b176cf 100644
> --- a/arch/x86/lib/usercopy.c
> +++ b/arch/x86/lib/usercopy.c
> @@ -44,7 +44,8 @@ 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);
> +	instrument_copy_from_user(to, from, n);
> +	ret = raw_copy_from_user(to, from, n);
>  	pagefault_enable();
>  
>  	return ret;
> -- 
> 2.34.1
> 

Thanks!

Tested-by: Florian Lehner <dev@...-flo.net>