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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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