linux-kernel - Re: [PATCH RFC RFT 2/5] fork: Add shadow stack support to clone3()

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Date:   Mon, 30 Oct 2023 11:20:45 -0700
From:   Deepak Gupta <debug@...osinc.com>
To:     "Szabolcs.Nagy@....com" <Szabolcs.Nagy@....com>
Cc:     Mark Brown <broonie@...nel.org>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
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Subject: Re: [PATCH RFC RFT 2/5] fork: Add shadow stack support to clone3()

On Mon, Oct 30, 2023 at 11:39:17AM +0000, Szabolcs.Nagy@....com wrote:
>The 10/27/2023 16:24, Deepak Gupta wrote:
>> On Fri, Oct 27, 2023 at 12:49:59PM +0100, Szabolcs.Nagy@....com wrote:
>> > no. the lifetime is the issue: a stack in principle can outlive
>> > a thread and resumed even after the original thread exited.
>> > for that to work the shadow stack has to outlive the thread too.
>>
>> I understand an application can pre-allocate a pool of stack and re-use
>> them whenever it's spawning new threads using clone3 system call.
>>
>> However, once a new thread has been spawned how can it resume?
>
>a thread can getcontext then exit. later another thread
>can setcontext and execute on the stack of the exited
>thread and return to a previous stack frame there.
>
>(unlikely to work on runtimes where tls or thread id is
>exposed and thus may be cached on the stack. so not for
>posix.. but e.g. a go runtime could do this)

Aah then as you mentioned, we basically need clear lifetime rules around
their creation and deletion.
Because `getcontext/swapcontext/setcontext` can be updated to save shadow
stack token on stack itself and use that to resume. It's just lifetime
that needs to be managed.

>
>> By resume I mean consume the callstack context from an earlier thread.
>> Or you meant something else by `resume` here?
>>
>> Can you give an example of such an application or runtime where a newly
>> created thread consumes callstack context created by going away thread?
>
>my claim was not that existing runtimes are doing this,
>but that the linux interface contract allows this and
>tieing the stack lifetime to the thread is a change of
>contract.
>
>> > (or the other way around: a stack can be freed before the thread
>> > exits, if the thread pivots away from that stack.)
>>
>> This is simply a thread saying that I am moving to a different stack.
>> Again, interested in learning why would a thread do that. If I've to
>> speculate on reasons, I could think of user runtime managing it's own
>> pool of worker items (some people call them green threads) or current
>> stack became too small.
>
>switching stack is common, freeing the original stack may not be,
>but there is nothing that prevents this and then the corresponding
>shadow stack is clearly leaked if the kernel manages it. the amount
>of leak is proportional to the number of live threads and the sum
>of their original stack size which can be big.
>
>but as i said i think this lifetime issue is minor compared
>to other shadow stack issues, so it is ok if the shadow stack
>is kernel managed.