From: Deepak Gupta <debug@rivosinc.com>
To: "Szabolcs.Nagy@arm.com" <Szabolcs.Nagy@arm.com>
Cc: Mark Brown <broonie@kernel.org>,
"Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
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nd@arm.com
Subject: Re: [PATCH RFC RFT 2/5] fork: Add shadow stack support to clone3()
Date: Mon, 30 Oct 2023 11:20:45 -0700 [thread overview]
Message-ID: <ZT/z/b9P8KLuZEFh@debug.ba.rivosinc.com> (raw)
In-Reply-To: <ZT+V5VlXg/PsIfpM@arm.com>
On Mon, Oct 30, 2023 at 11:39:17AM +0000, Szabolcs.Nagy@arm.com wrote:
>The 10/27/2023 16:24, Deepak Gupta wrote:
>> On Fri, Oct 27, 2023 at 12:49:59PM +0100, Szabolcs.Nagy@arm.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.
next prev parent reply other threads:[~2023-10-30 18:21 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-23 13:20 [PATCH RFC RFT 0/5] fork: Support shadow stacks in clone3() Mark Brown
2023-10-23 13:20 ` [PATCH RFC RFT 1/5] mm: Introduce ARCH_HAS_USER_SHADOW_STACK Mark Brown
2023-10-23 13:20 ` [PATCH RFC RFT 2/5] fork: Add shadow stack support to clone3() Mark Brown
2023-10-23 16:32 ` Edgecombe, Rick P
2023-10-23 18:32 ` Mark Brown
2023-10-26 17:10 ` Edgecombe, Rick P
2023-10-26 17:53 ` Mark Brown
2023-10-26 20:40 ` Deepak Gupta
2023-10-26 23:32 ` Edgecombe, Rick P
2023-10-27 11:49 ` Szabolcs.Nagy
2023-10-27 23:24 ` Deepak Gupta
2023-10-30 11:39 ` Szabolcs.Nagy
2023-10-30 18:20 ` Deepak Gupta [this message]
2023-10-26 23:31 ` Edgecombe, Rick P
2023-10-23 13:20 ` [PATCH RFC RFT 3/5] selftests/clone3: Factor more of main loop into test_clone3() Mark Brown
2023-10-23 13:20 ` [PATCH RFC RFT 4/5] selftests/clone3: Allow tests to flag if -E2BIG is a valid error code Mark Brown
2023-10-23 13:20 ` [PATCH RFC RFT 5/5] kselftest/clone3: Test shadow stack support Mark Brown
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