Re: [PATCH v1 08/10] lkdtm: Really write into kernel text in WRITE_KERN

Christophe Leroy
2021-10-13T07:29:53Z

Le 13/10/2021 à 09:05, Kees Cook a écrit :
> 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 <christophe.leroy@csgroup.eu>
>> ---
>> 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 <linux/kallsyms.h>
>
> Why not #include <asm/sections.h> instead here?

dereference_symbol_descriptor() is defined in linux/kallsyms.h

>
>> #include <linux/slab.h>
>> #include <linux/vmalloc.h>
>> #include <linux/mman.h>
>> @@ -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 <keescook@chromium.org>
>
>>
>> pr_info("attempting bad %zu byte write at %px\n", size, ptr);
>> memcpy((void *)ptr, (unsigned char *)do_nothing, size);
>> --
>> 2.31.1
>>
>