linux-kernel - Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

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Date: Wed, 15 May 2024 08:36:03 -0600
From: Jiri Olsa <olsajiri@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Deepak Gupta <debug@...osinc.com>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"olsajiri@...il.com" <olsajiri@...il.com>,
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Subject: Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

On Wed, May 15, 2024 at 01:19:20PM +0200, Oleg Nesterov wrote:
> Sorry for the late reply, I was on PTO.
> 
> On 05/14, Deepak Gupta wrote:
> >
> > Question,
> >
> > Is it kernel who is maintaining all return probes, meaning original return addresses
> > are saved in kernel data structures on per task basis.
> 
> Yes. task_struct->utask->return_instances
> 
> See prepare_uretprobe() which inserts the new return_instance with
> ->orig_ret_vaddr = original return addresses
> when the tracee enters the ret-probed function.
> 
> > Once uretprobe did its job then
> > its kernel who is ensuring return to original return address ?
> 
> Yes. See instruction_pointer_set(regs, ri->orig_ret_vaddr) in
> handle_trampoline().
> 
> 
> 
> I know absolutely nothing about the shadow stacks, trying to read
> Documentation/arch/x86/shstk.rst but it doesn't tell me too much...
> Where can I find more documentation? I didn't try to google yet.
> 
> 	Upon function return, the processor pops the shadow stack copy
> 	and compares it to the normal stack copy. If the two differ, the
> 	processor raises a control-protection fault.
> 
> grep-grep-grep... exc_control_protection I guess.
> 
> Let me ask a couple of really stupid questions. What if the shadow stack
> is "shorter" than the normal stack? I mean,
> 
> 	enable_shstk()
> 	{
> 		prctl(ARCH_SHSTK_SHSTK);
> 	}
> 
> what happens when enable_shstk() returns?

I think it will crash, there's explanation in the comment in
tools/testing/selftests/x86/test_shadow_stack.c test

that's why ARCH_PRCTL is using syscall instruction directly and
not calling syscall function

jirka

> And what is the purpose of fpregs_lock_and_load() ? Why do we need to
> fpregs_restore_userregs() in shstk_setup() and other places?
> 
> Oleg.
>