linux-kernel - Re: [RFC PATCH] rseq: x86: implement abort-at-ip extension

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Date:   Fri, 7 Jan 2022 20:08:10 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Florian Weimer <fw@...eb.enyo.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        carlos <carlos@...hat.com>
Subject: Re: [RFC PATCH] rseq: x86: implement abort-at-ip extension

----- On Jan 7, 2022, at 5:27 PM, David Laight David.Laight@...LAB.COM wrote:

>> That being said, there might be an architecture agnostic alternative available.
>> On abort of a RSEQ_CS_FLAG_ABORT_AT_IP critical section, we could let the kernel
>> decrement/increment the stack pointer to make room for a pointer (depending if
>> the
>> stack grows down or up). It would then store the abort-at-ip value at the top of
>> stack.
> 
> Stack redzone in a leaf function?

Good point!

For x86-64 for instance, which has a redzone of 128 bytes, there are a few alternatives.
On abort:

Approach A) Move the stack pointer as little as possible

   1. On abort in kernel:
   1.1 Subtract 8 bytes from the stack pointer
   1.2 Store the abort-at-ip value at stack pointer - 128
   2. In abort handler (userspace)
   2.1 user-space knows that it can find the abort-at-ip value at stack pointer - 128
   2.2 after using that value, the abort handler needs to add 8 bytes to the stack pointer

Approach B) Move the stack pointer to skip the entire redzone

   1. On abort in kernel:
   1.1 Subtract 128 bytes from the stack pointer
   1.2 Store the abort-at-ip value at stack pointer - 8 (basically within a new red zone)
   2. In abort handler (userspace)
   2.1 user-space knows that it can find the abort-at-ip value at stack pointer - 8
   2.2 after using that value, the abort handler needs to add 128 bytes to the stack pointer

I suspect both approaches should work.

Any preference or other ideas ?

Thanks,

Mathieu

> 
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com