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lkdtm: Really write into kernel text in WRITE_KERN

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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. Export dereference_function_descriptor()
for when LKDTM is built as a module.

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>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/31e58eaffb5bc51c07d8d4891d1982100ade8cfc.1644928018.git.christophe.leroy@csgroup.eu
This commit is contained in:
Christophe Leroy 2022-02-15 13:41:06 +01:00 committed by Michael Ellerman
parent 69b420ed8f
commit b64913394f
2 changed files with 6 additions and 3 deletions
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8
drivers/misc/lkdtm/perms.c
View file

@ -10,6 +10,7 @@
#include <linux/mman.h>
#include <linux/uaccess.h>
#include <asm/cacheflush.h>
#include <asm/sections.h>
/* Whether or not to fill the target memory area with do_nothing(). */
#define CODE_WRITE true
@ -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_function_descriptor(do_overwritten) -
(unsigned long)dereference_function_descriptor(do_nothing);
ptr = dereference_function_descriptor(do_overwritten);
pr_info("attempting bad %zu byte write at %px\n", size, ptr);
memcpy((void *)ptr, (unsigned char *)do_nothing, size);

1
kernel/extable.c
View file

@ -149,6 +149,7 @@ void *dereference_function_descriptor(void *ptr)
ptr = p;
return ptr;
}
EXPORT_SYMBOL_GPL(dereference_function_descriptor);
void *dereference_kernel_function_descriptor(void *ptr)
{