lkml.org 
[lkml]   [2021]   [Oct]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v2 13/13] lkdtm: Add a test for function descriptors protection
    On Thu, Oct 14, 2021 at 07:50:02AM +0200, Christophe Leroy wrote:
    > Add WRITE_OPD to check that you can't modify function
    > descriptors.
    >
    > Gives the following result when function descriptors are
    > not protected:
    >
    > lkdtm: Performing direct entry WRITE_OPD
    > lkdtm: attempting bad 16 bytes write at c00000000269b358
    > lkdtm: FAIL: survived bad write
    > lkdtm: do_nothing was hijacked!
    >
    > Looks like a standard compiler barrier(); is not enough to force
    > GCC to use the modified function descriptor. Add to add a fake empty
    > inline assembly to force GCC to reload the function descriptor.
    >
    > Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
    > ---
    > drivers/misc/lkdtm/core.c | 1 +
    > drivers/misc/lkdtm/lkdtm.h | 1 +
    > drivers/misc/lkdtm/perms.c | 22 ++++++++++++++++++++++
    > 3 files changed, 24 insertions(+)
    >
    > diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
    > index fe6fd34b8caf..de092aa03b5d 100644
    > --- a/drivers/misc/lkdtm/core.c
    > +++ b/drivers/misc/lkdtm/core.c
    > @@ -148,6 +148,7 @@ static const struct crashtype crashtypes[] = {
    > CRASHTYPE(WRITE_RO),
    > CRASHTYPE(WRITE_RO_AFTER_INIT),
    > CRASHTYPE(WRITE_KERN),
    > + CRASHTYPE(WRITE_OPD),
    > CRASHTYPE(REFCOUNT_INC_OVERFLOW),
    > CRASHTYPE(REFCOUNT_ADD_OVERFLOW),
    > CRASHTYPE(REFCOUNT_INC_NOT_ZERO_OVERFLOW),
    > diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
    > index c212a253edde..188bd0fd6575 100644
    > --- a/drivers/misc/lkdtm/lkdtm.h
    > +++ b/drivers/misc/lkdtm/lkdtm.h
    > @@ -105,6 +105,7 @@ void __init lkdtm_perms_init(void);
    > void lkdtm_WRITE_RO(void);
    > void lkdtm_WRITE_RO_AFTER_INIT(void);
    > void lkdtm_WRITE_KERN(void);
    > +void lkdtm_WRITE_OPD(void);
    > void lkdtm_EXEC_DATA(void);
    > void lkdtm_EXEC_STACK(void);
    > void lkdtm_EXEC_KMALLOC(void);
    > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
    > index 96b3ebfcb8ed..3870bc82d40d 100644
    > --- a/drivers/misc/lkdtm/perms.c
    > +++ b/drivers/misc/lkdtm/perms.c
    > @@ -44,6 +44,11 @@ static noinline void do_overwritten(void)
    > return;
    > }
    >
    > +static noinline void do_almost_nothing(void)
    > +{
    > + pr_info("do_nothing was hijacked!\n");
    > +}
    > +
    > static void *setup_function_descriptor(func_desc_t *fdesc, void *dst)
    > {
    > memcpy(fdesc, do_nothing, sizeof(*fdesc));
    > @@ -143,6 +148,23 @@ void lkdtm_WRITE_KERN(void)
    > do_overwritten();
    > }
    >
    > +void lkdtm_WRITE_OPD(void)
    > +{
    > + size_t size = sizeof(func_desc_t);
    > + void (*func)(void) = do_nothing;
    > +
    > + if (!have_function_descriptors()) {
    > + pr_info("Platform doesn't have function descriptors.\n");

    This should be more explicit ('xfail'):

    pr_info("XFAIL: platform doesn't use function descriptors.\n");

    > + return;
    > + }
    > + pr_info("attempting bad %zu bytes write at %px\n", size, do_nothing);
    > + memcpy(do_nothing, do_almost_nothing, size);
    > + pr_err("FAIL: survived bad write\n");
    > +
    > + asm("" : "=m"(func));

    Since this is a descriptor, I assume no icache flush is needed. Are
    function descriptors strictly dcache? (Is anything besides just a
    barrier needed?)

    > + func();
    > +}
    > +
    > void lkdtm_EXEC_DATA(void)
    > {
    > execute_location(data_area, CODE_WRITE);
    > --
    > 2.31.1
    >

    --
    Kees Cook

    \
     
     \ /
      Last update: 2021-10-15 23:37    [W:4.233 / U:0.176 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site