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Date: Thu, 14 Apr 2022 03:19:02 -0700 From: Dan Li <ashimida@...ux.alibaba.com> To: Kees Cook <keescook@...omium.org> 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] lkdtm: Add CFI_BACKWARD to test ROP mitigations Hi, Kees, Thanks for the rewrite. I tested this patch, and it works fine for me except for a few minor comments below :) On 4/13/22 14:39, Kees Cook wrote: > +/* 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(*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(*ret_addr == expected) > + *ret_addr = (addr); When PAC is enabled, I get a mismatch as follows: /kselftest_install/lkdtm # ./CFI_BACKWARD.sh [ 182.120133] lkdtm: Performing direct entry CFI_BACKWARD [ 182.120665] lkdtm: Attempting unchecked stack return address redirection ... [ 182.122543] lkdtm: ok: redirected stack return address. [ 182.123521] lkdtm: Attempting checked stack return address redirection ... [ 182.123964] lkdtm: Eek: return address mismatch! bfff800008fa8014 != ffff800008fa8030 [ 182.124502] lkdtm: ok: control flow unchanged. CFI_BACKWARD: saw 'call trace:|ok: control flow unchanged': ok We may need to ignore the pac high bits of return address according to TCR.T1SZ (or simply remove the high 16 bits before comparing). > + else > + /* Check architecture, stack layout, or compiler behavior... */ > + pr_warn("Eek: return address mismatch! %px != %px\n", > + *ret_addr, addr); According to the context, it might be "expected" here? pr_warn("Eek: return address mismatch! %px != %px\n", *ret_addr, expected); I simply ignored the upper 16 bits, and tested it separately in gcc/llvm 12 with SCS/PAC and all the four cases worked fine for me. Thanks, Dan.
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