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

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Date: Wed, 15 May 2024 17:42:03 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "olsajiri@...il.com" <olsajiri@...il.com>,
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Subject: Re: [PATCHv5 bpf-next 6/8] x86/shstk: Add return uprobe support

On 05/15, Edgecombe, Rick P wrote:
>
> On Wed, 2024-05-15 at 13:35 +0200, Oleg Nesterov wrote:
> >
> > > I'm ok with not using optimized uretprobe when shadow stack is detected
> > > as enabled and we go with current uretprobe in that case
> >
> > But how can we detect it? Again, suppose userspace does
>
> the rdssp instruction returns the value of the shadow stack pointer. On non-
> shadow stack it is a nop. So you could check if the SSP is non-zero to find if
> shadow stack is enabled.

But again, the ret-probed function can enable it before it returns? And we
need to check if it is enabled on the function entry if we want to avoid
sys_uretprobe() in this case. Although I don't understand why we want to
avoid it.

> This would catch most cases, but I guess there is the
> possibility of it getting enabled in a signal that hit between checking and the
> rest of operation.

Or from signal handler.

> Is this uretprobe stuff signal safe in general?

In what sense?

I forgot everything about this code but I can't recall any problem with signals.

Except it doesn't support sigaltstack() + siglongjmp().

Oleg.