From mboxrd@z Thu Jan 1 00:00:00 1970 From: Kees Cook Date: Wed, 13 Oct 2021 07:05:26 +0000 Subject: Re: [PATCH v1 08/10] lkdtm: Really write into kernel text in WRITE_KERN Message-Id: <202110130004.880A6C841@keescook> List-Id: References: <624940395e5d81967246f911a65740b9a15b5a70.1633964380.git.christophe.leroy@csgroup.eu> In-Reply-To: <624940395e5d81967246f911a65740b9a15b5a70.1633964380.git.christophe.leroy@csgroup.eu> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Christophe Leroy Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Andrew Morton , "James E.J. Bottomley" , Helge Deller , Arnd Bergmann , Greg Kroah-Hartman , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-ia64@vger.kernel.org, linux-parisc@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org On Mon, Oct 11, 2021 at 05:25:35PM +0200, Christophe Leroy wrote: > WRITE_KERN is supposed to overwrite some kernel text, namely > do_overwritten() function. > > But at the time being it overwrites do_overwritten() function > descriptor, not function text. > > Fix it by dereferencing the function descriptor to obtain > function text pointer. > > And make do_overwritten() noinline so that it is really > do_overwritten() which is called by lkdtm_WRITE_KERN(). > > Signed-off-by: Christophe Leroy > --- > drivers/misc/lkdtm/perms.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c > index 60b3b2fe929d..442d60ed25ef 100644 > --- a/drivers/misc/lkdtm/perms.c > +++ b/drivers/misc/lkdtm/perms.c > @@ -5,6 +5,7 @@ > * even non-readable regions. > */ > #include "lkdtm.h" > +#include Why not #include instead here? > #include > #include > #include > @@ -37,7 +38,7 @@ static noinline void do_nothing(void) > } > > /* 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; > @@ -113,8 +114,9 @@ void lkdtm_WRITE_KERN(void) > size_t size; > volatile unsigned char *ptr; > > - size = (unsigned long)do_overwritten - (unsigned long)do_nothing; > - ptr = (unsigned char *)do_overwritten; > + size = (unsigned long)dereference_symbol_descriptor(do_overwritten) - > + (unsigned long)dereference_symbol_descriptor(do_nothing); > + ptr = dereference_symbol_descriptor(do_overwritten); But otherwise, yup, I expect there will be a bunch of things like this to clean up in LKDTM. :| Sorry about that! Acked-by: Kees Cook > > pr_info("attempting bad %zu byte write at %px\n", size, ptr); > memcpy((void *)ptr, (unsigned char *)do_nothing, size); > -- > 2.31.1 > -- Kees Cook