From: Nick Desaulniers <ndesaulniers@google.com>
To: Candle Sun <candlesea@gmail.com>
Cc: Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
Greg KH <gregkh@linuxfoundation.org>,
Nathan Chancellor <natechancellor@gmail.com>,
candle.sun@unisoc.com, LKML <linux-kernel@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] lkdtm: fix memory copy size for WRITE_KERN
Date: Mon, 25 Jan 2021 13:16:01 -0800 [thread overview]
Message-ID: <CAKwvOd=b+ku7cd24KTYpNecYAUHxR5aBmrC1q+BL67+1YBHjzA@mail.gmail.com> (raw)
In-Reply-To: <20210125085622.2322-1-candlesea@gmail.com>
On Mon, Jan 25, 2021 at 12:56 AM Candle Sun <candlesea@gmail.com> wrote:
>
> From: Candle Sun <candle.sun@unisoc.com>
>
> Though do_overwritten() follows do_nothing() in source code, the final
> memory address order is determined by compiler. We can't always assume
> address of do_overwritten() is bigger than do_nothing(). At least the
> Clang we are using places do_overwritten() before do_nothing() in the
> object. This causes the copy size in lkdtm_WRITE_KERN() is *really*
> big and WRITE_KERN test on ARM32 arch will fail.
>
> Compare the address order before doing the subtraction.
>
> Signed-off-by: Candle Sun <candle.sun@unisoc.com>
> ---
> drivers/misc/lkdtm/perms.c | 19 +++++++++----------
> 1 file changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
> index 2dede2ef658f..fbfbdf89d668 100644
> --- a/drivers/misc/lkdtm/perms.c
> +++ b/drivers/misc/lkdtm/perms.c
> @@ -31,13 +31,13 @@ static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
> * This just returns to the caller. It is designed to be copied into
> * non-executable memory regions.
> */
> -static void do_nothing(void)
> +static noinline void do_nothing(void)
> {
> return;
> }
>
> /* Must immediately follow do_nothing for size calculuations to work out. */
> -static void do_overwritten(void)
> +static noinline void do_overwritten(void)
> {
> pr_info("do_overwritten wasn't overwritten!\n");
> return;
> @@ -110,15 +110,14 @@ void lkdtm_WRITE_RO_AFTER_INIT(void)
>
> void lkdtm_WRITE_KERN(void)
> {
> - size_t size;
> - volatile unsigned char *ptr;
> + unsigned long value_dow = (unsigned long)do_overwritten;
> + unsigned long value_do = (unsigned long)do_nothing;
> + size_t size = (size_t)(value_dow > value_do ?
> + value_dow - value_do : value_do - value_dow);
>
> - size = (unsigned long)do_overwritten - (unsigned long)do_nothing;
Thanks for the patch! Might it be simpler to do:
size = abs((uintptr_t)do_overwritten - (uintptr_t)do_nothing));
Then I don't think much of this function needs to be changed.
> - ptr = (unsigned char *)do_overwritten;
> -
> - pr_info("attempting bad %zu byte write at %px\n", size, ptr);
> - memcpy((void *)ptr, (unsigned char *)do_nothing, size);
> - flush_icache_range((unsigned long)ptr, (unsigned long)(ptr + size));
> + pr_info("attempting bad %zu byte write at %px\n", size, do_overwritten);
> + memcpy((void *)value_dow, (void *)value_do, size);
> + flush_icache_range(value_dow, value_dow + (unsigned long)size);
> pr_err("FAIL: survived bad write\n");
>
> do_overwritten();
> --
> 2.17.0
>
--
Thanks,
~Nick Desaulniers
next prev parent reply other threads:[~2021-01-25 21:19 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-25 8:56 [PATCH] lkdtm: fix memory copy size for WRITE_KERN Candle Sun
2021-01-25 10:37 ` David Laight
2021-01-26 14:13 ` Candle Sun
2021-01-26 18:39 ` Nick Desaulniers
2021-01-26 22:53 ` David Laight
2021-01-27 11:00 ` Candle Sun
2021-01-27 17:42 ` Nick Desaulniers
2021-01-25 21:16 ` Nick Desaulniers [this message]
2021-01-26 14:17 ` Candle Sun
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