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

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Date: Wed, 15 May 2024 13:19:20 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Deepak Gupta <debug@...osinc.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
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Subject: Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

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?


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.