linux-kernel - Re: [PATCH v2 7/7] lkdtm: test copy_to_user() on bad kernel pointer under KERNEL

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Message-ID: <CAGXu5jLrmzowaoSq_a0KM2O604ri45V-JMF9un=dE4zSc7sLSA@mail.gmail.com>
Date:   Mon, 27 Aug 2018 15:08:36 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Tony Luck <tony.luck@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-edac <linux-edac@...r.kernel.org>
Subject: Re: [PATCH v2 7/7] lkdtm: test copy_to_user() on bad kernel pointer
 under KERNEL_DS

On Mon, Aug 27, 2018 at 11:56 AM, Jann Horn <jannh@...gle.com> wrote:
> Test whether the kernel WARN()s when, under KERNEL_DS, a bad kernel pointer
> is used as "userspace" pointer. Should normally be used in "DIRECT" mode.
>
> Signed-off-by: Jann Horn <jannh@...gle.com>

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  drivers/misc/lkdtm/core.c     |  1 +
>  drivers/misc/lkdtm/lkdtm.h    |  1 +
>  drivers/misc/lkdtm/usercopy.c | 13 +++++++++++++
>  3 files changed, 15 insertions(+)
>
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index 2154d1bfd18b..5a755590d3dc 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -183,6 +183,7 @@ static const struct crashtype crashtypes[] = {
>         CRASHTYPE(USERCOPY_STACK_FRAME_FROM),
>         CRASHTYPE(USERCOPY_STACK_BEYOND),
>         CRASHTYPE(USERCOPY_KERNEL),
> +       CRASHTYPE(USERCOPY_KERNEL_DS),
>  };
>
>
> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> index 9e513dcfd809..07db641d71d0 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -82,5 +82,6 @@ void lkdtm_USERCOPY_STACK_FRAME_TO(void);
>  void lkdtm_USERCOPY_STACK_FRAME_FROM(void);
>  void lkdtm_USERCOPY_STACK_BEYOND(void);
>  void lkdtm_USERCOPY_KERNEL(void);
> +void lkdtm_USERCOPY_KERNEL_DS(void);
>
>  #endif
> diff --git a/drivers/misc/lkdtm/usercopy.c b/drivers/misc/lkdtm/usercopy.c
> index 9725aed305bb..389475b25bb7 100644
> --- a/drivers/misc/lkdtm/usercopy.c
> +++ b/drivers/misc/lkdtm/usercopy.c
> @@ -322,6 +322,19 @@ void lkdtm_USERCOPY_KERNEL(void)
>         vm_munmap(user_addr, PAGE_SIZE);
>  }
>
> +void lkdtm_USERCOPY_KERNEL_DS(void)
> +{
> +       char __user *user_ptr = (char __user *)ERR_PTR(-EINVAL);
> +       mm_segment_t old_fs = get_fs();
> +       char buf[10] = {0};
> +
> +       pr_info("attempting copy_to_user on unmapped kernel address\n");
> +       set_fs(KERNEL_DS);
> +       if (copy_to_user(user_ptr, buf, sizeof(buf)))
> +               pr_info("copy_to_user un unmapped kernel address failed\n");
> +       set_fs(old_fs);
> +}
> +
>  void __init lkdtm_usercopy_init(void)
>  {
>         /* Prepare cache that lacks SLAB_USERCOPY flag. */
> --
> 2.19.0.rc0.228.g281dcd1b4d0-goog
>



-- 
Kees Cook
Pixel Security