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    SubjectRe: [PATCH v1 10/10] lkdtm: Fix execute_[user]_location()
    On Mon, Oct 11, 2021 at 05:25:37PM +0200, Christophe Leroy wrote:
    > execute_location() and execute_user_location() intent
    > to copy do_nothing() text and execute it at a new location.
    > However, at the time being it doesn't copy do_nothing() function
    > but do_nothing() function descriptor which still points to the
    > original text. So at the end it still executes do_nothing() at
    > its original location allthough using a copied function descriptor.
    >
    > So, fix that by really copying do_nothing() text and build a new
    > function descriptor by copying do_nothing() function descriptor and
    > updating the target address with the new location.
    >
    > Also fix the displayed addresses by dereferencing do_nothing()
    > function descriptor.
    >
    > Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
    > ---
    > drivers/misc/lkdtm/perms.c | 45 +++++++++++++++++++++++++++++++-------
    > 1 file changed, 37 insertions(+), 8 deletions(-)
    >
    > diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
    > index da16564e1ecd..1d03cd44c21d 100644
    > --- a/drivers/misc/lkdtm/perms.c
    > +++ b/drivers/misc/lkdtm/perms.c
    > @@ -44,19 +44,42 @@ static noinline void do_overwritten(void)
    > return;
    > }
    >
    > +static void *setup_function_descriptor(funct_descr_t *fdesc, void *dst)
    > +{
    > + int err;
    > +
    > + if (!__is_defined(HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR))
    > + return dst;
    > +
    > + err = copy_from_kernel_nofault(fdesc, do_nothing, sizeof(*fdesc));
    > + if (err < 0)
    > + return ERR_PTR(err);
    > +
    > + fdesc->addr = (unsigned long)dst;
    > + barrier();
    > +
    > + return fdesc;
    > +}
    > +
    > static noinline void execute_location(void *dst, bool write)
    > {
    > - void (*func)(void) = dst;
    > + void (*func)(void);
    > + funct_descr_t fdesc;
    > + void *do_nothing_text = dereference_symbol_descriptor(do_nothing);
    >
    > - pr_info("attempting ok execution at %px\n", do_nothing);
    > + pr_info("attempting ok execution at %px\n", do_nothing_text);
    > do_nothing();
    >
    > if (write == CODE_WRITE) {
    > - memcpy(dst, do_nothing, EXEC_SIZE);
    > + memcpy(dst, do_nothing_text, EXEC_SIZE);
    > flush_icache_range((unsigned long)dst,
    > (unsigned long)dst + EXEC_SIZE);
    > }
    > - pr_info("attempting bad execution at %px\n", func);
    > + func = setup_function_descriptor(&fdesc, dst);
    > + if (IS_ERR(func))
    > + return;

    I think this error case should complain with some details. :) Maybe:

    pr_err("FAIL: could not build function descriptor for %px\n", dst);

    > +
    > + pr_info("attempting bad execution at %px\n", dst);

    And then leave this pr_info() as it was, before the
    setup_function_descriptor() call.

    > func();
    > pr_err("FAIL: func returned\n");
    > }
    > @@ -66,16 +89,22 @@ static void execute_user_location(void *dst)
    > int copied;
    >
    > /* Intentionally crossing kernel/user memory boundary. */
    > - void (*func)(void) = dst;
    > + void (*func)(void);
    > + funct_descr_t fdesc;
    > + void *do_nothing_text = dereference_symbol_descriptor(do_nothing);
    >
    > - pr_info("attempting ok execution at %px\n", do_nothing);
    > + pr_info("attempting ok execution at %px\n", do_nothing_text);
    > do_nothing();
    >
    > - copied = access_process_vm(current, (unsigned long)dst, do_nothing,
    > + copied = access_process_vm(current, (unsigned long)dst, do_nothing_text,
    > EXEC_SIZE, FOLL_WRITE);
    > if (copied < EXEC_SIZE)
    > return;
    > - pr_info("attempting bad execution at %px\n", func);
    > + func = setup_function_descriptor(&fdesc, dst);
    > + if (IS_ERR(func))
    > + return;
    > +
    > + pr_info("attempting bad execution at %px\n", dst);

    Same here.

    > func();
    > pr_err("FAIL: func returned\n");
    > }
    > --
    > 2.31.1
    >

    --
    Kees Cook

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