Messages in this thread | | | Date | Fri, 4 Mar 2022 07:01:18 -0800 | Subject | Re: [PATCH v3 2/2] lkdtm: Add Shadow Call Stack tests | From | Dan Li <> |
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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; > }
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