| Message ID | 20220303074339.86337-1-ashimida@linux.alibaba.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | AARCH64: Enable GCC-based Shadow Call Stack | expand |
On Wed, Mar 02, 2022 at 11:43:39PM -0800, Dan Li wrote: > Add tests for SCS (Shadow Call Stack) based > backward CFI (as implemented by Clang and GCC). Cool; thanks for writing these! > > Signed-off-by: Dan Li <ashimida@linux.alibaba.com> > --- > drivers/misc/lkdtm/Makefile | 1 + > drivers/misc/lkdtm/core.c | 2 + > drivers/misc/lkdtm/lkdtm.h | 4 ++ > drivers/misc/lkdtm/scs.c | 67 +++++++++++++++++++++++++ > tools/testing/selftests/lkdtm/tests.txt | 2 + > 5 files changed, 76 insertions(+) > create mode 100644 drivers/misc/lkdtm/scs.c > > diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile > index 2e0aa74ac185..e2fb17868af2 100644 > --- a/drivers/misc/lkdtm/Makefile > +++ b/drivers/misc/lkdtm/Makefile > @@ -10,6 +10,7 @@ lkdtm-$(CONFIG_LKDTM) += rodata_objcopy.o > lkdtm-$(CONFIG_LKDTM) += usercopy.o > lkdtm-$(CONFIG_LKDTM) += stackleak.o > lkdtm-$(CONFIG_LKDTM) += cfi.o > +lkdtm-$(CONFIG_LKDTM) += scs.o I'd expect these to be in cfi.c, rather than making a new source file. > lkdtm-$(CONFIG_LKDTM) += fortify.o > lkdtm-$(CONFIG_PPC_64S_HASH_MMU) += powerpc.o > > diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c > index f69b964b9952..d0ce0bec117c 100644 > --- a/drivers/misc/lkdtm/core.c > +++ b/drivers/misc/lkdtm/core.c > @@ -178,6 +178,8 @@ static const struct crashtype crashtypes[] = { > CRASHTYPE(USERCOPY_KERNEL), > CRASHTYPE(STACKLEAK_ERASING), > CRASHTYPE(CFI_FORWARD_PROTO), > + CRASHTYPE(CFI_BACKWARD_SHADOW), > + CRASHTYPE(CFI_BACKWARD_SHADOW_WITH_NOSCS), > CRASHTYPE(FORTIFIED_OBJECT), > CRASHTYPE(FORTIFIED_SUBOBJECT), > CRASHTYPE(FORTIFIED_STRSCPY), > diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h > index d6137c70ebbe..a23d32dfc10b 100644 > --- a/drivers/misc/lkdtm/lkdtm.h > +++ b/drivers/misc/lkdtm/lkdtm.h > @@ -158,6 +158,10 @@ void lkdtm_STACKLEAK_ERASING(void); > /* cfi.c */ > void lkdtm_CFI_FORWARD_PROTO(void); > > +/* scs.c */ > +void lkdtm_CFI_BACKWARD_SHADOW(void); > +void lkdtm_CFI_BACKWARD_SHADOW_WITH_NOSCS(void); > + > /* fortify.c */ > void lkdtm_FORTIFIED_OBJECT(void); > void lkdtm_FORTIFIED_SUBOBJECT(void); > diff --git a/drivers/misc/lkdtm/scs.c b/drivers/misc/lkdtm/scs.c > new file mode 100644 > index 000000000000..5922a55a8844 > --- /dev/null > +++ b/drivers/misc/lkdtm/scs.c > @@ -0,0 +1,67 @@ > +// SPDX-License-Identifier: GPL-2.0 > +/* > + * This is for all the tests relating directly to Shadow Call Stack. > + */ > +#include "lkdtm.h" > + > +#ifdef CONFIG_ARM64 > +/* Function clears its return address. */ > +static noinline void lkdtm_scs_clear_lr(void) > +{ > + unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1; > + > + asm volatile("str xzr, [%0]\n\t" : : "r"(lr) : "x30"); Is the asm needed here? Why not: unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1; *lr = 0; > +} > + > +/* Function with __noscs attribute clears its return address. */ > +static noinline void __noscs lkdtm_noscs_clear_lr(void) > +{ > + unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1; > + > + asm volatile("str xzr, [%0]\n\t" : : "r"(lr) : "x30"); > +} > +#endif > + > +/* > + * This tries to call a function protected by Shadow Call Stack, > + * which corrupts its own return address during execution. > + * Due to the protection, the corruption will not take effect > + * when the function returns. > + */ > +void lkdtm_CFI_BACKWARD_SHADOW(void) I think these two tests should be collapsed into a single one. > +{ > +#ifdef CONFIG_ARM64 > + if (!IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) { > + pr_err("FAIL: kernel not built with CONFIG_SHADOW_CALL_STACK\n"); > + return; > + } > + > + pr_info("Trying to corrupt lr in a function with scs protection ...\n"); > + lkdtm_scs_clear_lr(); > + > + pr_err("ok: scs takes effect.\n"); > +#else > + pr_err("XFAIL: this test is arm64-only\n"); > +#endif This is slightly surprising -- we have no detection when a function has its non-shadow-stack return address corrupted: it just _ignores_ the value stored there. That seems like a missed opportunity for warning about an unexpected state. > +} > + > +/* > + * This tries to call a function not protected by Shadow Call Stack, > + * which corrupts its own return address during execution. > + */ > +void lkdtm_CFI_BACKWARD_SHADOW_WITH_NOSCS(void) > +{ > +#ifdef CONFIG_ARM64 > + if (!IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) { > + pr_err("FAIL: kernel not built with CONFIG_SHADOW_CALL_STACK\n"); > + return; Other tests try to give some hints about failures, e.g.: pr_err("FAIL: cannot change for SCS\n"); pr_expected_config(CONFIG_SHADOW_CALL_STACK); Though, having the IS_ENABLED in there makes me wonder if this test should instead be made _survivable_ on failure. Something like this, completely untested: #ifdef CONFIG_ARM64 static noinline void lkdtm_scs_set_lr(unsigned long *addr) { unsigned long **lr = (unsigned long **)__builtin_frame_address(0) + 1; *lr = addr; } /* Function with __noscs attribute clears its return address. */ static noinline void __noscs lkdtm_noscs_set_lr(unsigned long *addr) { unsigned long **lr = (unsigned long **)__builtin_frame_address(0) + 1; *lr = addr; } #endif void lkdtm_CFI_BACKWARD_SHADOW(void) { #ifdef CONFIG_ARM64 /* Verify the "normal" condition of LR corruption working. */ do { /* Keep label in scope to avoid compiler warning. */ if ((volatile int)0) goto unexpected; pr_info("Trying to corrupt lr in a function without scs protection ...\n"); lkdtm_noscs_set_lr(&&expected); unexpected: pr_err("XPASS: Unexpectedly survived lr corruption without scs?!\n"); break; expected: pr_err("ok: lr corruption redirected without scs.\n"); } while (0); do { /* Keep labe in scope to avoid compiler warning. */ if ((volatile int)0) goto good_scs; pr_info("Trying to corrupt lr in a function with scs protection ...\n"); lkdtm_scs_set_lr(&&bad_scs); good_scs: pr_info("ok: scs takes effect.\n"); break; bad_scs: pr_err("FAIL: return address rewritten!\n"); pr_expected_config(CONFIG_SHADOW_CALL_STACK); } while (0); #else pr_err("XFAIL: this test is arm64-only\n"); #endif } And we should, actually, be able to make the "set_lr" functions be arch-specific, leaving the test itself arch-agnostic....
On Thu, Mar 03, 2022 at 10:42:45AM -0800, Kees Cook wrote: > Though, having the IS_ENABLED in there makes me wonder if this test > should instead be made _survivable_ on failure. Something like this, > completely untested: > > > #ifdef CONFIG_ARM64 > static noinline void lkdtm_scs_set_lr(unsigned long *addr) > { > unsigned long **lr = (unsigned long **)__builtin_frame_address(0) + 1; > *lr = addr; > } > > /* Function with __noscs attribute clears its return address. */ > static noinline void __noscs lkdtm_noscs_set_lr(unsigned long *addr) > { > unsigned long **lr = (unsigned long **)__builtin_frame_address(0) + 1; > *lr = addr; > } > #endif > > > void lkdtm_CFI_BACKWARD_SHADOW(void) > { > #ifdef CONFIG_ARM64 > > /* Verify the "normal" condition of LR corruption working. */ > do { > /* Keep label in scope to avoid compiler warning. */ > if ((volatile int)0) > goto unexpected; > > pr_info("Trying to corrupt lr in a function without scs protection ...\n"); > lkdtm_noscs_set_lr(&&expected); > > unexpected: > pr_err("XPASS: Unexpectedly survived lr corruption without scs?!\n"); > break; > > expected: > pr_err("ok: lr corruption redirected without scs.\n"); > } while (0); > > > do { > /* Keep labe in scope to avoid compiler warning. */ > if ((volatile int)0) > goto good_scs; > > pr_info("Trying to corrupt lr in a function with scs protection ...\n"); > lkdtm_scs_set_lr(&&bad_scs); > > good_scs: > pr_info("ok: scs takes effect.\n"); > break; > > bad_scs: > pr_err("FAIL: return address rewritten!\n"); > pr_expected_config(CONFIG_SHADOW_CALL_STACK); > } while (0); > #else > pr_err("XFAIL: this test is arm64-only\n"); > #endif > } > > And we should, actually, be able to make the "set_lr" functions be > arch-specific, leaving the test itself arch-agnostic.... Yeah, as a tested example, this works for x86_64, and based on what you had, I'd expect it to work on arm64 too: #include <stdio.h> static __attribute__((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 (*ret_addr == expected) *ret_addr = addr; } volatile int force_label; int main(void) { do { /* Keep labels in scope. */ if (force_label) goto normal; if (force_label) goto redirected; set_return_addr(&&normal, &&redirected); normal: printf("I should be skipped\n"); break; redirected: printf("Redirected\n"); } while (0); return 0; } It does _not_ work under Clang, though, which I'm still looking at.
On 3/3/22 10:42, Kees Cook wrote: > On Wed, Mar 02, 2022 at 11:43:39PM -0800, Dan Li wrote: >> Add tests for SCS (Shadow Call Stack) based >> backward CFI (as implemented by Clang and GCC). > > Cool; thanks for writing these! > >> +lkdtm-$(CONFIG_LKDTM) += scs.o > > I'd expect these to be in cfi.c, rather than making a new source file. > Got it. >> +static noinline void lkdtm_scs_clear_lr(void) >> +{ >> + unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1; >> + >> + asm volatile("str xzr, [%0]\n\t" : : "r"(lr) : "x30"); > > Is the asm needed here? Why not: > > unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1; > > *lr = 0; > Yeah, with "volatile", this one looks better. >> + >> +/* >> + * This tries to call a function protected by Shadow Call Stack, >> + * which corrupts its own return address during execution. >> + * Due to the protection, the corruption will not take effect >> + * when the function returns. >> + */ >> +void lkdtm_CFI_BACKWARD_SHADOW(void) > > I think these two tests should be collapsed into a single one. > It seems that there is currently no cross-line matching in selftests/lkdtm/run.sh, if we put these two into one function and assume we could make noscs_set_lr _survivable_ (like in your example). Then we could only match "CFI_BACKWARD_SHADOW ok: scs takes effect." in texts.txt But if the test result is: XPASS: Unexpectedly survived lr corruption without scs? ok: scs takes effect. It may not be a real pass, but the xxx_set_lr function doesn't work. >> +{ >> +#ifdef CONFIG_ARM64 >> + if (!IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) { >> + pr_err("FAIL: kernel not built with CONFIG_SHADOW_CALL_STACK\n"); >> + return; >> + } >> + >> + pr_info("Trying to corrupt lr in a function with scs protection ...\n"); >> + lkdtm_scs_clear_lr(); >> + >> + pr_err("ok: scs takes effect.\n"); >> +#else >> + pr_err("XFAIL: this test is arm64-only\n"); >> +#endif > > This is slightly surprising -- we have no detection when a function has > its non-shadow-stack return address corrupted: it just _ignores_ the > value stored there. That seems like a missed opportunity for warning > about an unexpected state. > Yes. Actually I used to try in the plugin to add a detection before the function returns, and call a callback when a mismatch is found. But since almost every function has to be instrumented, the performance penalty is improved from <3% to ~20% (rough calculation, should still be optimized). >> +} >> + >> +/* >> + * This tries to call a function not protected by Shadow Call Stack, >> + * which corrupts its own return address during execution. >> + */ >> +void lkdtm_CFI_BACKWARD_SHADOW_WITH_NOSCS(void) >> +{ >> +#ifdef CONFIG_ARM64 >> + if (!IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) { >> + pr_err("FAIL: kernel not built with CONFIG_SHADOW_CALL_STACK\n"); >> + return; > > Other tests try to give some hints about failures, e.g.: > > pr_err("FAIL: cannot change for SCS\n"); > pr_expected_config(CONFIG_SHADOW_CALL_STACK); > > Though, having the IS_ENABLED in there makes me wonder if this test > should instead be made _survivable_ on failure. Something like this, > completely untested: > > > #ifdef CONFIG_ARM64 > static noinline void lkdtm_scs_set_lr(unsigned long *addr) > { > unsigned long **lr = (unsigned long **)__builtin_frame_address(0) + 1; > *lr = addr; > } > > /* Function with __noscs attribute clears its return address. */ > static noinline void __noscs lkdtm_noscs_set_lr(unsigned long *addr) > { > unsigned long **lr = (unsigned long **)__builtin_frame_address(0) + 1; > *lr = addr; > } > #endif > > > void lkdtm_CFI_BACKWARD_SHADOW(void) > { > #ifdef CONFIG_ARM64 > > /* Verify the "normal" condition of LR corruption working. */ > do { > /* Keep label in scope to avoid compiler warning. */ > if ((volatile int)0) > goto unexpected; > > pr_info("Trying to corrupt lr in a function without scs protection ...\n"); > lkdtm_noscs_set_lr(&&expected); > > unexpected: > pr_err("XPASS: Unexpectedly survived lr corruption without scs?!\n"); > break; > > expected: > pr_err("ok: lr corruption redirected without scs.\n"); > } while (0); > > > do { > /* Keep labe in scope to avoid compiler warning. */ > if ((volatile int)0) > goto good_scs; > > pr_info("Trying to corrupt lr in a function with scs protection ...\n"); > lkdtm_scs_set_lr(&&bad_scs); > > good_scs: > pr_info("ok: scs takes effect.\n"); > break; > > bad_scs: > pr_err("FAIL: return address rewritten!\n"); > pr_expected_config(CONFIG_SHADOW_CALL_STACK); > } while (0); > #else > pr_err("XFAIL: this test is arm64-only\n"); > #endif > } > Thanks for the example, Kees :) This code (with a little modification) works correctly with clang 12, but to make sure it's always correct, I think we might need to add the __attribute__((optnone)) attribute to it, because under -O2 the result doesn't seem to be "very stable" (as in your example in the next email). > And we should, actually, be able to make the "set_lr" functions be > arch-specific, leaving the test itself arch-agnostic.... > I'm not sure if my understanding is correct, do it means we should remove the "#ifdef CONFIG_ARM64" in lkdtm_CFI_BACKWARD_SHADOW? Then we may not be able to distinguish between failures caused by platform unsupported (XFAIL) and features not enabled (or not working properly). Thanks, Dan.
On 3/3/22 11:09, Kees Cook wrote: > On Thu, Mar 03, 2022 at 10:42:45AM -0800, Kees Cook wrote: >> Though, having the IS_ENABLED in there makes me wonder if this test >> should instead be made _survivable_ on failure. Something like this, >> completely untested: >> >> >> And we should, actually, be able to make the "set_lr" functions be >> arch-specific, leaving the test itself arch-agnostic.... > > Yeah, as a tested example, this works for x86_64, and based on what you > had, I'd expect it to work on arm64 too: > > #include <stdio.h> > > static __attribute__((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 (*ret_addr == expected) > *ret_addr = addr; > } > > volatile int force_label; > int main(void) > { > do { > /* Keep labels in scope. */ > if (force_label) > goto normal; > if (force_label) > goto redirected; > > set_return_addr(&&normal, &&redirected); > normal: > printf("I should be skipped\n"); > break; From the assembly code, it seems that "&&normal" does't always equal to the address of label "normal" when we use clang with -O2. > redirected: > printf("Redirected\n"); > } while (0); > The address of "&&redirected" may appear in the middle of the assembly instructions of the printf. If we unconditionally jump to "&&normal", it may crash directly because x0 is not set correctly. > return 0; > } > > > It does _not_ work under Clang, though, which I'm still looking at. > AFAICT, maybe we could specify -O0 optimization to bypass this. BTW: Occasionally found, the following code works correctly, but i think it doesn't solve the issue :) #include <stdio.h> static __attribute__((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 (*ret_addr == expected) *ret_addr = addr; } volatile int force_label; int main(void) { do { /* Keep labels in scope. */ if (force_label) goto normal; if (force_label) goto redirected; set_return_addr(&&normal, &&redirected); normal: printf("I should be skipped\n"); break; redirected: printf("Redirected\n"); printf("\n"); //add a new printf } while (0); return 0; }
On 3/4/22 06:54, Dan Li wrote: > > > On 3/3/22 11:09, Kees Cook wrote: >> On Thu, Mar 03, 2022 at 10:42:45AM -0800, Kees Cook wrote: >>> Though, having the IS_ENABLED in there makes me wonder if this test >>> should instead be made _survivable_ on failure. Something like this, >>> completely untested: >>> >>> >>> And we should, actually, be able to make the "set_lr" functions be >>> arch-specific, leaving the test itself arch-agnostic.... >> >> Yeah, as a tested example, this works for x86_64, and based on what you >> had, I'd expect it to work on arm64 too: >> >> #include <stdio.h> >> >> static __attribute__((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 (*ret_addr == expected) >> *ret_addr = addr; >> } >> >> volatile int force_label; >> int main(void) >> { >> do { >> /* Keep labels in scope. */ >> if (force_label) >> goto normal; >> if (force_label) >> goto redirected; >> >> set_return_addr(&&normal, &&redirected); >> normal: >> printf("I should be skipped\n"); >> break; > > From the assembly code, it seems that "&&normal" does't always equal > to the address of label "normal" when we use clang with -O2. > >> redirected: >> printf("Redirected\n"); >> } while (0); >> > > The address of "&&redirected" may appear in the middle of the assembly > instructions of the printf. If we unconditionally jump to "&&normal",> it may crash directly because x0 is not set correctly. Sorry, it should be: The address of "&&redirected" may appear in the middle of the assembly instructions of the printf. If we unconditionally jump to "&&redirected", it may crash directly because x0 of printf is not set correctly. Thanks, Dan. > >> return 0; >> } >> >> >> It does _not_ work under Clang, though, which I'm still looking at. >> > > AFAICT, maybe we could specify -O0 optimization to bypass this. > > > BTW: > Occasionally found, the following code works correctly, but i think > it doesn't solve the issue :) > > #include <stdio.h> > > static __attribute__((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 (*ret_addr == expected) > *ret_addr = addr; > } > volatile int force_label; > int main(void) > { > do { > /* Keep labels in scope. */ > if (force_label) > goto normal; > if (force_label) > goto redirected; > > set_return_addr(&&normal, &&redirected); > normal: > printf("I should be skipped\n"); > break; > > redirected: > printf("Redirected\n"); > printf("\n"); //add a new printf > } while (0); > > return 0; > }
On 3/3/22 11:09, Kees Cook wrote: > On Thu, Mar 03, 2022 at 10:42:45AM -0800, Kees Cook wrote: >> And we should, actually, be able to make the "set_lr" functions be >> arch-specific, leaving the test itself arch-agnostic.... > > Yeah, as a tested example, this works for x86_64, and based on what you > had, I'd expect it to work on arm64 too: > > #include <stdio.h> > > static __attribute__((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 (*ret_addr == expected) > *ret_addr = addr; > } > > volatile int force_label; > int main(void) > { > do { > /* Keep labels in scope. */ > if (force_label) > goto normal; > if (force_label) > goto redirected; > > set_return_addr(&&normal, &&redirected); > normal: > printf("I should be skipped\n"); > break; > redirected: > printf("Redirected\n"); > } while (0); > > return 0; > } > > > It does _not_ work under Clang, though, which I'm still looking at. > The following code seems to work fine under clang/gcc, x86_64/aarch64 (also tested in lkdtm_CFI_BACKWARD_SHADOW): #include <stdio.h> static __attribute__((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(*ret_addr == expected) *ret_addr = (addr); } static volatile int force_label; int main(void) { void *array[] = {0, &&normal, &&redirected}; if (force_label) { /* Call it with a NULL to avoid parameters being treated as constants in -02. */ set_return_addr(NULL, NULL); goto * array[force_label]; } do { set_return_addr(&&normal, &&redirected); normal: printf("I should be skipped\n"); break; redirected: printf("Redirected\n"); } while (0); return 0; } But currently it still crashes when I try to enable "-mbranch-protection=pac-ret+leaf+bti". Because the address of "&&redirected" is not encrypted under pac, the autiasp check will fail when set_return_addr returns, and eventually cause the function to crash when it returns to "&&redirected" ("&&redirected" as a reserved label always seems to start with a bti j insn). For lkdtm, if we're going to handle both cases in one function, maybe it would be better to turn off the -mbranch-protection=pac-ret+leaf+bti and maybe also turn off -O2 options for the function :) Thanks, Dan.
On Mon, Mar 07, 2022 at 07:16:36AM -0800, Dan Li wrote: > The following code seems to work fine under clang/gcc, x86_64/aarch64 > (also tested in lkdtm_CFI_BACKWARD_SHADOW): > > #include <stdio.h> > > static __attribute__((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(*ret_addr == expected) > *ret_addr = (addr); > } > > static volatile int force_label; > > int main(void) > { > void *array[] = {0, &&normal, &&redirected}; > > if (force_label) { > /* Call it with a NULL to avoid parameters being treated as constants in -02. */ > set_return_addr(NULL, NULL); > goto * array[force_label]; > } Hah! I like that. :) I had a weird switch statement that was working for me; this is cleaner. > > do { > > set_return_addr(&&normal, &&redirected); > > normal: > printf("I should be skipped\n"); > break; > > redirected: > printf("Redirected\n"); > > } while (0); > > return 0; > } > > But currently it still crashes when I try to enable > "-mbranch-protection=pac-ret+leaf+bti". > > Because the address of "&&redirected" is not encrypted under pac, > the autiasp check will fail when set_return_addr returns, and > eventually cause the function to crash when it returns to "&&redirected" > ("&&redirected" as a reserved label always seems to start with a bti j > insn). Strictly speaking, this is entirely correct. :) > For lkdtm, if we're going to handle both cases in one function, maybe > it would be better to turn off the -mbranch-protection=pac-ret+leaf+bti > and maybe also turn off -O2 options for the function :) If we can apply a function attribute to turn off pac for the "does this work without protections", that should be sufficient.
On 3/9/22 12:16, Kees Cook wrote: > On Mon, Mar 07, 2022 at 07:16:36AM -0800, Dan Li wrote: >> But currently it still crashes when I try to enable >> "-mbranch-protection=pac-ret+leaf+bti". >> >> Because the address of "&&redirected" is not encrypted under pac, >> the autiasp check will fail when set_return_addr returns, and >> eventually cause the function to crash when it returns to "&&redirected" >> ("&&redirected" as a reserved label always seems to start with a bti j >> insn). > > Strictly speaking, this is entirely correct. :) > >> For lkdtm, if we're going to handle both cases in one function, maybe >> it would be better to turn off the -mbranch-protection=pac-ret+leaf+bti >> and maybe also turn off -O2 options for the function :) > > If we can apply a function attribute to turn off pac for the "does this > work without protections", that should be sufficient. > Got it, will do in the next version :) Thanks, Dan.
diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile index 2e0aa74ac185..e2fb17868af2 100644 --- a/drivers/misc/lkdtm/Makefile +++ b/drivers/misc/lkdtm/Makefile @@ -10,6 +10,7 @@ lkdtm-$(CONFIG_LKDTM) += rodata_objcopy.o lkdtm-$(CONFIG_LKDTM) += usercopy.o lkdtm-$(CONFIG_LKDTM) += stackleak.o lkdtm-$(CONFIG_LKDTM) += cfi.o +lkdtm-$(CONFIG_LKDTM) += scs.o lkdtm-$(CONFIG_LKDTM) += fortify.o lkdtm-$(CONFIG_PPC_64S_HASH_MMU) += powerpc.o diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c index f69b964b9952..d0ce0bec117c 100644 --- a/drivers/misc/lkdtm/core.c +++ b/drivers/misc/lkdtm/core.c @@ -178,6 +178,8 @@ static const struct crashtype crashtypes[] = { CRASHTYPE(USERCOPY_KERNEL), CRASHTYPE(STACKLEAK_ERASING), CRASHTYPE(CFI_FORWARD_PROTO), + CRASHTYPE(CFI_BACKWARD_SHADOW), + CRASHTYPE(CFI_BACKWARD_SHADOW_WITH_NOSCS), CRASHTYPE(FORTIFIED_OBJECT), CRASHTYPE(FORTIFIED_SUBOBJECT), CRASHTYPE(FORTIFIED_STRSCPY), diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h index d6137c70ebbe..a23d32dfc10b 100644 --- a/drivers/misc/lkdtm/lkdtm.h +++ b/drivers/misc/lkdtm/lkdtm.h @@ -158,6 +158,10 @@ void lkdtm_STACKLEAK_ERASING(void); /* cfi.c */ void lkdtm_CFI_FORWARD_PROTO(void); +/* scs.c */ +void lkdtm_CFI_BACKWARD_SHADOW(void); +void lkdtm_CFI_BACKWARD_SHADOW_WITH_NOSCS(void); + /* fortify.c */ void lkdtm_FORTIFIED_OBJECT(void); void lkdtm_FORTIFIED_SUBOBJECT(void); diff --git a/drivers/misc/lkdtm/scs.c b/drivers/misc/lkdtm/scs.c new file mode 100644 index 000000000000..5922a55a8844 --- /dev/null +++ b/drivers/misc/lkdtm/scs.c @@ -0,0 +1,67 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * This is for all the tests relating directly to Shadow Call Stack. + */ +#include "lkdtm.h" + +#ifdef CONFIG_ARM64 +/* Function clears its return address. */ +static noinline void lkdtm_scs_clear_lr(void) +{ + unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1; + + asm volatile("str xzr, [%0]\n\t" : : "r"(lr) : "x30"); +} + +/* Function with __noscs attribute clears its return address. */ +static noinline void __noscs lkdtm_noscs_clear_lr(void) +{ + unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1; + + asm volatile("str xzr, [%0]\n\t" : : "r"(lr) : "x30"); +} +#endif + +/* + * This tries to call a function protected by Shadow Call Stack, + * which corrupts its own return address during execution. + * Due to the protection, the corruption will not take effect + * when the function returns. + */ +void lkdtm_CFI_BACKWARD_SHADOW(void) +{ +#ifdef CONFIG_ARM64 + if (!IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) { + pr_err("FAIL: kernel not built with CONFIG_SHADOW_CALL_STACK\n"); + return; + } + + pr_info("Trying to corrupt lr in a function with scs protection ...\n"); + lkdtm_scs_clear_lr(); + + pr_err("ok: scs takes effect.\n"); +#else + pr_err("XFAIL: this test is arm64-only\n"); +#endif +} + +/* + * This tries to call a function not protected by Shadow Call Stack, + * which corrupts its own return address during execution. + */ +void lkdtm_CFI_BACKWARD_SHADOW_WITH_NOSCS(void) +{ +#ifdef CONFIG_ARM64 + if (!IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) { + pr_err("FAIL: kernel not built with CONFIG_SHADOW_CALL_STACK\n"); + return; + } + + pr_info("Trying to corrupt lr in a function with attribute __noscs ...\n"); + lkdtm_noscs_clear_lr(); + + pr_err("FAIL: __noscs attribute does not take effect!\n"); +#else + pr_err("XFAIL: this test is arm64-only\n"); +#endif +} diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt index 6b36b7f5dcf9..c849765c8dcc 100644 --- a/tools/testing/selftests/lkdtm/tests.txt +++ b/tools/testing/selftests/lkdtm/tests.txt @@ -73,6 +73,8 @@ USERCOPY_STACK_BEYOND USERCOPY_KERNEL STACKLEAK_ERASING OK: the rest of the thread stack is properly erased CFI_FORWARD_PROTO +CFI_BACKWARD_SHADOW ok: scs takes effect +CFI_BACKWARD_SHADOW_WITH_NOSCS FORTIFIED_STRSCPY FORTIFIED_OBJECT FORTIFIED_SUBOBJECT
Add tests for SCS (Shadow Call Stack) based backward CFI (as implemented by Clang and GCC). Signed-off-by: Dan Li <ashimida@linux.alibaba.com> --- drivers/misc/lkdtm/Makefile | 1 + drivers/misc/lkdtm/core.c | 2 + drivers/misc/lkdtm/lkdtm.h | 4 ++ drivers/misc/lkdtm/scs.c | 67 +++++++++++++++++++++++++ tools/testing/selftests/lkdtm/tests.txt | 2 + 5 files changed, 76 insertions(+) create mode 100644 drivers/misc/lkdtm/scs.c