On Mon, Feb 28, 2022 at 02:55:30PM -0800, Andy Lutomirski wrote: > > > On Mon, Feb 28, 2022, at 1:30 PM, Mike Rapoport wrote: > > On Mon, Feb 28, 2022 at 12:30:41PM -0800, Andy Lutomirski wrote: > >> > >> > >> On Mon, Feb 28, 2022, at 12:27 PM, Mike Rapoport wrote: > >> > On Wed, Feb 09, 2022 at 06:37:53PM -0800, Andy Lutomirski wrote: > >> >> On 2/8/22 18:18, Edgecombe, Rick P wrote: > >> >> > On Tue, 2022-02-08 at 20:02 +0300, Cyrill Gorcunov wrote: > >> >> > > >> > > >> > Even with the current shadow stack interface Rick proposed, CRIU can restore > >> > the victim using ptrace without any additional knobs, but we loose an > >> > important ability to "self-cure" the victim from the parasite in case > >> > anything goes wrong with criu control process. > >> > > >> > Moreover, the issue with backward compatibility is not with ptrace but with > >> > sigreturn and it seems that criu is not its only user. > >> > >> So we need an ability for a tracer to cause the tracee to call a function > >> and to return successfully. Apparently a gdb branch can already do this > >> with shstk, and my PTRACE_CALL_FUNCTION_SIGFRAME should also do the > >> trick. I don't see why we need a sigretur-but-dont-verify -- we just > >> need this mechanism to create a frame such that sigreturn actually works. > > > > If I understand correctly, PTRACE_CALL_FUNCTION_SIGFRAME() injects a frame > > into the tracee and makes the tracee call sigreturn. > > I.e. the tracee is stopped and this is used pretty much as PTRACE_CONT or > > PTRACE_SYSCALL. > > > > In such case this defeats the purpose of sigreturn in CRIU because it is > > called asynchronously by the tracee when the tracer is about to detach or > > even already detached. > > The intent of PTRACE_CALL_FUNCTION_SIGFRAME is push a signal frame onto > the stack and call a function. That function should then be able to call > sigreturn just like any normal signal handler. Ok, let me reiterate. We have a seized and stopped tracee, use PTRACE_CALL_FUNCTION_SIGFRAME to push a signal frame onto the tracee's stack so that sigreturn could use that frame, then set the tracee %rip to the function we'd like to call and then we PTRACE_CONT the tracee. Tracee continues to execute the parasite code that calls sigreturn to clean up and restore the tracee process. PTRACE_CALL_FUNCTION_SIGFRAME also pushes a restore token to the shadow stack, just like setup_rt_frame() does, so that sys_rt_sigreturn() won't bail out at restore_signal_shadow_stack(). The only thing that CRIU actually needs is to push a restore token to the shadow stack, so for us a ptrace call that does that would be ideal. -- Sincerely yours, Mike.
