Re: [PATCH] lkdtm: Add CFI_BACKWARD to test ROP mitigations

[Kees Cook <keescook@chromium.org>]

On Thu, Apr 14, 2022 at 03:19:02AM -0700, Dan Li wrote:
> 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).

Oh! Hah, yes, I totally forgot that. Thanks for testing -- getting PAC
emulation working in QEMU has eluded me. I think untagged_addr() will
work yes? i.e.:

	if((untagged_addr(*ret_addr) == expected)

> 
> > +	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.

Great! Do you have the PAC "Oops" text handy so I can include it in the
commit log as an example of what should be expected?

Thanks!

-- 
Kees Cook