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Message-ID: <202204181451.AC3DB70EE@keescook>
Date: Mon, 18 Apr 2022 14:51:14 -0700
From: Kees Cook <keescook@...omium.org>
To: Dan Li <ashimida@...ux.alibaba.com>
Cc: Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] lkdtm: Add CFI_BACKWARD to test ROP mitigations
On Sun, Apr 17, 2022 at 02:15:43AM -0700, Dan Li wrote:
>
>
> On 4/15/22 17:11, Kees Cook wrote:
> > In order to test various backward-edge control flow integrity methods,
> > add a test that manipulates the return address on the stack. Currently
> > only arm64 Pointer Authentication and Shadow Call Stack is supported.
> >
> > $ echo CFI_BACKWARD | cat >/sys/kernel/debug/provoke-crash/DIRECT
> >
> > Under SCS, successful test of the mitigation is reported as:
> >
> > lkdtm: Performing direct entry CFI_BACKWARD
> > lkdtm: Attempting unchecked stack return address redirection ...
> > lkdtm: ok: redirected stack return address.
> > lkdtm: Attempting checked stack return address redirection ...
> > lkdtm: ok: control flow unchanged.
> >
> > Under PAC, successful test of the mitigation is reported by the PAC
> > exception handler:
> >
> > lkdtm: Performing direct entry CFI_BACKWARD
> > lkdtm: Attempting unchecked stack return address redirection ...
> > lkdtm: ok: redirected stack return address.
> > lkdtm: Attempting checked stack return address redirection ...
> > Unable to handle kernel paging request at virtual address bfffffc0088d0514
> > Mem abort info:
> > ESR = 0x86000004
> > EC = 0x21: IABT (current EL), IL = 32 bits
> > SET = 0, FnV = 0
> > EA = 0, S1PTW = 0
> > FSC = 0x04: level 0 translation fault
> > [bfffffc0088d0514] address between user and kernel address ranges
> > ...
> >
> > If the CONFIGs are missing (or the mitigation isn't working), failure
> > is reported as:
> >
> > lkdtm: Performing direct entry CFI_BACKWARD
> > lkdtm: Attempting unchecked stack return address redirection ...
> > lkdtm: ok: redirected stack return address.
> > lkdtm: Attempting checked stack return address redirection ...
> > lkdtm: FAIL: stack return address was redirected!
> > lkdtm: This is probably expected, since this kernel was built *without* CONFIG_ARM64_PTR_AUTH_KERNEL=y nor CONFIG_SHADOW_CALL_STACK=y
> >
> > Co-developed-by: Dan Li <ashimida@...ux.alibaba.com>
> > Signed-off-by: Dan Li <ashimida@...ux.alibaba.com>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> > v1: https://lore.kernel.org/lkml/20220413213917.711770-1-keescook@chromium.org
> > v2:
> > - add PAGE_OFFSET setting for PAC bits (Dan Li)
> > ---
> > drivers/misc/lkdtm/cfi.c | 134 ++++++++++++++++++++++++
> > tools/testing/selftests/lkdtm/tests.txt | 1 +
> > 2 files changed, 135 insertions(+)
> >
> > diff --git a/drivers/misc/lkdtm/cfi.c b/drivers/misc/lkdtm/cfi.c
> > index e88f778be0d5..804965a480b7 100644
> > --- a/drivers/misc/lkdtm/cfi.c
> > +++ b/drivers/misc/lkdtm/cfi.c
> > @@ -3,6 +3,7 @@
> > * This is for all the tests relating directly to Control Flow Integrity.
> > */
> > #include "lkdtm.h"
> > +#include <asm/page.h>
> > static int called_count;
> > @@ -42,8 +43,141 @@ static void lkdtm_CFI_FORWARD_PROTO(void)
> > pr_expected_config(CONFIG_CFI_CLANG);
> > }
> > +/*
> > + * This can stay local to LKDTM, as there should not be a production reason
> > + * to disable PAC && SCS.
> > + */
> > +#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
> > +# ifdef CONFIG_ARM64_BTI_KERNEL
> > +# define __no_pac "branch-protection=bti"
> > +# else
> > +# define __no_pac "branch-protection=none"
> > +# endif
> > +# define __no_ret_protection __noscs __attribute__((__target__(__no_pac)))
> > +#else
> > +# define __no_ret_protection __noscs
> > +#endif
> > +
> > +#define no_pac_addr(addr) \
> > + ((__force __typeof__(addr))((__force u64)(addr) | PAGE_OFFSET))
> > +
> > +/* The ultimate ROP gadget. */
> > +static noinline __no_ret_protection
> > +void set_return_addr_unchecked(unsigned long *expected, unsigned long *addr)
> > +{
> > + /* Use of volatile is to make sure final write isn't seen as a dead store. */
> > + unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
> > +
> > + /* Make sure we've found the right place on the stack before writing it. */
> > + if (no_pac_addr(*ret_addr) == expected)
> > + *ret_addr = (addr);
> > + else
> > + /* Check architecture, stack layout, or compiler behavior... */
> > + pr_warn("Eek: return address mismatch! %px != %px\n",
> > + *ret_addr, addr);
> > +}
> > +
> > +static noinline
> > +void set_return_addr(unsigned long *expected, unsigned long *addr)
> > +{
> > + /* Use of volatile is to make sure final write isn't seen as a dead store. */
> > + unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
> > +
> > + /* Make sure we've found the right place on the stack before writing it. */
> > + if (no_pac_addr(*ret_addr) == expected)
> > + *ret_addr = (addr);
> > + else
> > + /* Check architecture, stack layout, or compiler behavior... */
> > + pr_warn("Eek: return address mismatch! %px != %px\n",
> > + *ret_addr, addr);
> > +}
> > +
> > +static volatile int force_check;
> > +
> > +static void lkdtm_CFI_BACKWARD(void)
> > +{
> > + /* Use calculated gotos to keep labels addressable. */
> > + void *labels[] = {0, &&normal, &&redirected, &&check_normal, &&check_redirected};
> > +
> > + pr_info("Attempting unchecked stack return address redirection ...\n");
> > +
> > + /* Always false */
> > + if (force_check) {
> > + /*
> > + * Prepare to call with NULLs to avoid parameters being treated as
> > + * constants in -02.
> > + */
> > + set_return_addr_unchecked(NULL, NULL);
> > + set_return_addr(NULL, NULL);
> > + if (force_check)
> > + goto *labels[1];
> > + if (force_check)
> > + goto *labels[2];
> > + if (force_check)
> > + goto *labels[3];
> > + if (force_check)
> > + goto *labels[4];
> > + return;
> > + }
> > +
> > + /*
> > + * Use fallthrough switch case to keep basic block ordering between
> > + * set_return_addr*() and the label after it.
> > + */
> > + switch (force_check) {
> > + case 0:
> > + set_return_addr_unchecked(&&normal, &&redirected);
> > + fallthrough;
> > + case 1:
> > +normal:
> > + /* Always true */
> > + if (!force_check) {
> > + pr_err("FAIL: stack return address manipulation failed!\n");
> > + /* If we can't redirect "normally", we can't test mitigations. */
> > + return;
> > + }
> > + break;
> > + default:
> > +redirected:
> > + pr_info("ok: redirected stack return address.\n");
> > + break;
> > + }
> > +
> > + pr_info("Attempting checked stack return address redirection ...\n");
> > +
> > + switch (force_check) {
> > + case 0:
> > + set_return_addr(&&check_normal, &&check_redirected);
> > + fallthrough;
> > + case 1:
> > +check_normal:
> > + /* Always true */
> > + if (!force_check) {
> > + pr_info("ok: control flow unchanged.\n");
> > + return;
> > + }
> > +
> > +check_redirected:
> > + pr_err("FAIL: stack return address was redirected!\n");
> > + break;
> > + }
> > +
> > + if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)) {
> > + pr_expected_config(CONFIG_ARM64_PTR_AUTH_KERNEL);
> > + return;
> > + }
> > + if (IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) {
> > + pr_expected_config(CONFIG_SHADOW_CALL_STACK);
> > + return;
> > + }
> > + pr_warn("This is probably expected, since this %s was built *without* %s=y nor %s=y\n",
> > + lkdtm_kernel_info,
> > + "CONFIG_ARM64_PTR_AUTH_KERNEL", "CONFIG_SHADOW_CALL_STACK");
> > +}
> > +
> > static struct crashtype crashtypes[] = {
> > CRASHTYPE(CFI_FORWARD_PROTO),
> > + CRASHTYPE(CFI_BACKWARD),
> > };
> > struct crashtype_category cfi_crashtypes = {
> > diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
> > index 243c781f0780..9dace01dbf15 100644
> > --- a/tools/testing/selftests/lkdtm/tests.txt
> > +++ b/tools/testing/selftests/lkdtm/tests.txt
> > @@ -74,6 +74,7 @@ USERCOPY_STACK_BEYOND
> > USERCOPY_KERNEL
> > STACKLEAK_ERASING OK: the rest of the thread stack is properly erased
> > CFI_FORWARD_PROTO
> > +CFI_BACKWARD call trace:|ok: control flow unchanged
> > FORTIFIED_STRSCPY
> > FORTIFIED_OBJECT
> > FORTIFIED_SUBOBJECT
>
>
> Compiling with gcc/llvm 12 on aarch64 platform with scs/pac enabled
> respectively, all four cases work fine for me :)
Great! Thanks for confirming it. :)
--
Kees Cook
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