From: "H.J. Lu" <hjl.tools@gmail.com>
To: "szabolcs.nagy@arm.com" <szabolcs.nagy@arm.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>,
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Subject: Re: [PATCH v7 01/41] Documentation/x86: Add CET shadow stack description
Date: Fri, 3 Mar 2023 08:57:27 -0800 [thread overview]
Message-ID: <CAMe9rOrM=HXBY25rYrjLnHzSvHFuui06qRpc4xufxeaaGW-Fmw@mail.gmail.com> (raw)
In-Reply-To: <ZAIgrXQ4670gxlE4@arm.com>
On Fri, Mar 3, 2023 at 8:31 AM szabolcs.nagy@arm.com
<szabolcs.nagy@arm.com> wrote:
>
> The 03/02/2023 21:17, Edgecombe, Rick P wrote:
> > Is the idea that shadow stack would be forced on regardless of if the
> > linked libraries support it? In which case it could be allowed to crash
> > if they do not?
>
> execute a binary
> - with shstk enabled and locked (only if marked?).
> - with shstk disabled and locked.
> could be managed in userspace, but it is libc dependent then.
>
> > > > > - I think it's better to have a new limit specifically for shadow
> > > > > stack size (which by default can be RLIMIT_STACK) so userspace
> > > > > can adjust it if needed (another reason is that stack size is
> > > > > not always a good indicator of max call depth).
> >
> > Looking at this again, I'm not sure why a new rlimit is needed. It
> > seems many of those points were just formulations of that the clone3
> > stack size was not used, but it actually is and just not documented. If
> > you disagree perhaps you could elaborate on what the requirements are
> > and we can see if it seems tricky to do in a follow up.
>
> - tiny thread stack and deep signal stack.
> (note that this does not really work with glibc because it has
> implementation internal signals that don't run on alt stack,
> cannot be masked and don't fit on a tiny thread stack, but
> with other runtimes this can be a valid use-case, e.g. musl
> allows tiny thread stacks, < pagesize.)
>
> - thread runtimes with clone (glibc uses clone3 but some dont).
>
> - huge stacks but small call depth (problem if some va limit
> is hit or memory overcommit is disabled).
>
> > > "sigaltshstk() is separate from sigaltstack(). You can have one
> > > without the other, neither or both together. Because the shadow
> > > stack specific state is pushed to the shadow stack, the two
> > > features don’t need to know about each other."
> ...
> > > i don't see why automatic alt shadow stack allocation would
> > > not work (kernel manages it transparently when an alt stack
> > > is installed or disabled).
> >
> > Ah, I think I see where maybe I can fill you in. Andy Luto had
> > discounted this idea out of hand originally, but I didn't see it at
> > first. sigaltstack lets you set, retrieve, or disable the shadow stack,
> > right... But this doesn't allocate anything, it just sets where the
> > next signal will be handled. This is different than things like threads
> > where there is a new resources being allocated and it makes coming up
> > with logic to guess when to de-allocate the alt shadow stack difficult.
> > You probably already know...
> >
> > But because of this there can be some modes where the shadow stack is
> > changed while on it. For one example, SS_AUTODISARM will disable the
> > alt shadow stack while switching to it and restore when sigreturning.
> > At which point a new altstack can be set. In the non-shadow stack case
> > this is nice because future signals won't clobber the alt stack if you
> > switch away from it (swapcontext(), etc). But it also means you can
> > "change" the alt stack while on it ("change" sort of, the auto disarm
> > results in the kernel forgetting it temporarily).
>
> the problem with swapcontext is that it may unmask signals
> that run on the alt stack, which means the code cannot jump
> back after another signal clobbered the alt stack.
>
> the non-standard SS_AUTODISARM aims to solve this by disabling
> alt stack settings on signal entry until the handler returns.
>
> so this use case is not about supporting swapcontext out, but
> about jumping back. however that does not work reliably with
> this patchset: if swapcontext goes to the thread stack (and
> not to another stack e.g. used by makecontext), then jump back
> fails. (and if there is a sigaltshstk installed then even jump
> out fails.)
>
> assuming
> - jump out from alt shadow stack can be made to work.
> - alt shadow stack management can be automatic.
> then this can be improved so jump back works reliably.
>
> > I hear where you are coming from with the desire to have it "just work"
> > with existing code, but I think the resulting ABI around the alt shadow
> > stack allocation lifecycle would be way too complicated even if it
> > could be made to work. Hence making a new interface. But also, the idea
> > was that the x86 signal ABI should support handling alt shadow stacks,
> > which is what we have done with this series. If a different interface
> > for configuring it is better than the one from the POC, I'm not seeing
> > a problem jump out. Is there any specific concern about backwards
> > compatibility here?
>
> sigaltstack syscall behaviour may be hard to change later
> and currently
> - shadow stack overflow cannot be recovered from.
> - longjmp out of signal handler fails (with sigaltshstk).
> - SS_AUTODISARM does not work (jump back can fail).
>
> > > "Since shadow alt stacks are a new feature, longjmp()ing from an
> > > alt shadow stack will simply not be supported. If a libc want’s
> > > to support this it will need to enable WRSS and write it’s own
> > > restore token."
> > >
> > > i think longjmp should work without enabling writes to the shadow
> > > stack in the libc. this can also affect unwinding across signal
> > > handlers (not for c++ but e.g. glibc thread cancellation).
> >
> > glibc today does not support longjmp()ing from a different stack (for
> > example even today after a swapcontext()) when shadow stack is used. If
> > glibc used wrss it could be supported maybe, but otherwise I don't see
> > how the HW can support it.
> >
> > HJ and I were actually just discussing this the other day. Are you
> > looking at this series with respect to the arm shadow stack feature by
> > any chance? I would love if glibc/tools would document what the shadow
> > stack limitations are. If the all the arch's have the same or similar
> > limitations perhaps this could be one developer guide. For the most
> > part though, the limitations I've encountered are in glibc and the
> > kernel is more the building blocks.
>
> well we hope that shadow stack behaviour and limitations can
> be similar across targets.
>
> longjmp to different stack should work: it can do the same as
> setcontext/swapcontext: scan for the pivot token. then only
> longjmp out of alt shadow stack fails. (this is non-conforming
> longjmp use, but e.g. qemu relies on it.)
Restore token may not be used with longjmp. Unlike setcontext/swapcontext,
longjmp is optional. If longjmp isn't called, there will be an extra
token on shadow
stack and RET will fail.
> for longjmp out of alt shadow stack, the target shadow stack
> needs a pivot token, which implies the kernel needs to push that
> on signal entry, which can overflow. but i suspect that can be
> handled the same way as stackoverflow on signal entry is handled.
>
> > A general comment. Not sure if you are aware, but this shadow stack
> > enabling effort is quite old at this point and there have been many
> > discussions on these topics stretching back years. The latest
> > conversation was around getting this series into linux-next soon to get
> > some testing on the MM pieces. I really appreciate getting this ABI
> > feedback as it is always tricky to get right, but at this stage I would
> > hope to be focusing mostly on concrete problems.
> >
> > I also expect to have some amount of ABI growth going forward with all
> > the normal things that entails. Shadow stack is not special in that it
> > can come fully finalized without the need for the real world usage
> > iterative feedback process. At some point we need to move forward with
> > something, and we have quite a bit of initial changes at this point.
> >
> > So I would like to minimize the initial implementation unless anyone
> > sees any likely problems with future growth. Can you be clear if you
> > see any concrete problems at this point or are more looking to evaluate
> > the design reasoning? I'm under the assumption there is nothing that
> > would prohibit linux-next testing while any ABI shakedown happens
> > concurrently at least?
>
> understood.
>
> the points that i think are worth raising:
>
> - shadow stack size logic may need to change later.
> (it can be too big, or too small in practice.)
> - shadow stack overflow is not recoverable and the
> possible fix for that (sigaltshstk) breaks longjmp
> out of signal handlers.
> - jump back after SS_AUTODISARM swapcontext cannot be
> reliable if alt signal uses thread shadow stack.
> - the above two concerns may be mitigated by different
> sigaltstack behaviour which may be hard to add later.
> - end token for backtrace may be useful, if added
> later it can be hard to check.
>
> thanks.
--
H.J.
next prev parent reply other threads:[~2023-03-03 16:58 UTC|newest]
Thread overview: 159+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-27 22:29 [PATCH v7 00/41] Shadow stacks for userspace Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 01/41] Documentation/x86: Add CET shadow stack description Rick Edgecombe
2023-03-01 14:21 ` Szabolcs Nagy
2023-03-01 14:38 ` Szabolcs Nagy
2023-03-01 18:07 ` Edgecombe, Rick P
2023-03-01 18:32 ` Edgecombe, Rick P
2023-03-02 16:34 ` szabolcs.nagy
2023-03-03 22:35 ` Edgecombe, Rick P
2023-03-06 16:20 ` szabolcs.nagy
2023-03-06 16:31 ` Florian Weimer
2023-03-06 18:08 ` Edgecombe, Rick P
2023-03-07 13:03 ` szabolcs.nagy
2023-03-07 14:00 ` Florian Weimer
2023-03-07 16:14 ` Szabolcs Nagy
2023-03-06 18:05 ` Edgecombe, Rick P
2023-03-06 20:31 ` Liang, Kan
2023-03-02 16:14 ` szabolcs.nagy
2023-03-02 21:17 ` Edgecombe, Rick P
2023-03-03 16:30 ` szabolcs.nagy
2023-03-03 16:57 ` H.J. Lu [this message]
2023-03-03 17:39 ` szabolcs.nagy
2023-03-03 17:50 ` H.J. Lu
2023-03-03 17:41 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 02/41] x86/shstk: Add Kconfig option for shadow stack Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 03/41] x86/cpufeatures: Add CPU feature flags for shadow stacks Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 04/41] x86/cpufeatures: Enable CET CR4 bit for shadow stack Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 05/41] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 06/41] x86/fpu: Add helper for modifying xstate Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 07/41] x86: Move control protection handler to separate file Rick Edgecombe
2023-03-01 15:38 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 08/41] x86/shstk: Add user control-protection fault handler Rick Edgecombe
2023-03-01 18:06 ` Borislav Petkov
2023-03-01 18:14 ` Edgecombe, Rick P
2023-03-01 18:37 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 09/41] x86/mm: Remove _PAGE_DIRTY from kernel RO pages Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 10/41] x86/mm: Move pmd_write(), pud_write() up in the file Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 11/41] mm: Introduce pte_mkwrite_kernel() Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 12/41] s390/mm: Introduce pmd_mkwrite_kernel() Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 13/41] mm: Make pte_mkwrite() take a VMA Rick Edgecombe
2023-03-01 7:03 ` Christophe Leroy
2023-03-01 8:16 ` David Hildenbrand
2023-03-02 12:19 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 14/41] x86/mm: Introduce _PAGE_SAVED_DIRTY Rick Edgecombe
2023-03-02 12:48 ` Borislav Petkov
2023-03-02 17:01 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 15/41] x86/mm: Update ptep/pmdp_set_wrprotect() for _PAGE_SAVED_DIRTY Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 16/41] x86/mm: Start actually marking _PAGE_SAVED_DIRTY Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 17/41] mm: Move VM_UFFD_MINOR_BIT from 37 to 38 Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 18/41] mm: Introduce VM_SHADOW_STACK for shadow stack memory Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 19/41] x86/mm: Check shadow stack page fault errors Rick Edgecombe
2023-03-03 14:00 ` Borislav Petkov
2023-03-03 14:39 ` Dave Hansen
2023-02-27 22:29 ` [PATCH v7 20/41] x86/mm: Teach pte_mkwrite() about stack memory Rick Edgecombe
2023-03-03 15:37 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 21/41] mm: Add guard pages around a shadow stack Rick Edgecombe
2023-03-06 8:08 ` Borislav Petkov
2023-03-07 1:29 ` Edgecombe, Rick P
2023-03-07 10:32 ` Borislav Petkov
2023-03-07 10:44 ` David Hildenbrand
2023-03-08 22:48 ` Edgecombe, Rick P
2023-03-17 17:09 ` Deepak Gupta
2023-02-27 22:29 ` [PATCH v7 22/41] mm/mmap: Add shadow stack pages to memory accounting Rick Edgecombe
2023-03-06 13:01 ` Borislav Petkov
2023-03-06 18:11 ` Edgecombe, Rick P
2023-03-06 18:16 ` Borislav Petkov
2023-03-07 10:42 ` David Hildenbrand
2023-03-17 17:12 ` Deepak Gupta
2023-03-17 17:16 ` Dave Hansen
2023-03-17 17:28 ` Deepak Gupta
2023-03-17 17:42 ` Edgecombe, Rick P
2023-03-17 19:26 ` Deepak Gupta
2023-02-27 22:29 ` [PATCH v7 23/41] mm: Re-introduce vm_flags to do_mmap() Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 24/41] mm: Don't allow write GUPs to shadow stack memory Rick Edgecombe
2023-03-06 13:10 ` Borislav Petkov
2023-03-06 18:15 ` Andy Lutomirski
2023-03-06 18:33 ` Edgecombe, Rick P
2023-03-06 18:57 ` Andy Lutomirski
2023-03-07 1:47 ` Edgecombe, Rick P
2023-03-17 17:05 ` Deepak Gupta
2023-02-27 22:29 ` [PATCH v7 25/41] x86/mm: Introduce MAP_ABOVE4G Rick Edgecombe
2023-03-06 18:09 ` Borislav Petkov
2023-03-07 1:10 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 26/41] mm: Warn on shadow stack memory in wrong vma Rick Edgecombe
2023-03-08 8:53 ` Borislav Petkov
2023-03-08 23:36 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot Rick Edgecombe
2023-02-27 22:54 ` Kees Cook
2023-03-08 9:23 ` Borislav Petkov
2023-03-08 23:35 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 28/41] x86: Introduce userspace API for shadow stack Rick Edgecombe
2023-03-08 10:27 ` Borislav Petkov
2023-03-08 23:32 ` Edgecombe, Rick P
2023-03-09 12:57 ` Borislav Petkov
2023-03-09 16:56 ` Edgecombe, Rick P
2023-03-09 23:51 ` Borislav Petkov
2023-03-10 1:13 ` Edgecombe, Rick P
2023-03-10 2:03 ` H.J. Lu
2023-03-10 20:00 ` H.J. Lu
2023-03-10 20:27 ` Edgecombe, Rick P
2023-03-10 20:43 ` H.J. Lu
2023-03-10 21:01 ` Edgecombe, Rick P
2023-03-10 11:40 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 29/41] x86/shstk: Add user-mode shadow stack support Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 30/41] x86/shstk: Handle thread shadow stack Rick Edgecombe
2023-03-02 17:34 ` Szabolcs Nagy
2023-03-02 21:48 ` Edgecombe, Rick P
2023-03-08 15:26 ` Borislav Petkov
2023-03-08 20:03 ` Edgecombe, Rick P
2023-03-09 14:12 ` Borislav Petkov
2023-03-09 16:59 ` Edgecombe, Rick P
2023-03-09 17:04 ` Borislav Petkov
2023-03-09 20:29 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 31/41] x86/shstk: Introduce routines modifying shstk Rick Edgecombe
2023-03-09 16:48 ` Borislav Petkov
2023-03-09 17:03 ` Edgecombe, Rick P
2023-03-09 17:22 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 32/41] x86/shstk: Handle signals for shadow stack Rick Edgecombe
2023-03-09 17:02 ` Borislav Petkov
2023-03-09 17:16 ` Edgecombe, Rick P
2023-03-09 23:35 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 33/41] x86/shstk: Introduce map_shadow_stack syscall Rick Edgecombe
2023-03-02 17:22 ` Szabolcs Nagy
2023-03-02 21:21 ` Edgecombe, Rick P
2023-03-09 18:55 ` Deepak Gupta
2023-03-09 19:39 ` Edgecombe, Rick P
2023-03-09 21:08 ` Deepak Gupta
2023-03-10 0:14 ` Edgecombe, Rick P
2023-03-10 21:00 ` Deepak Gupta
2023-03-10 21:43 ` Edgecombe, Rick P
2023-03-16 20:07 ` Deepak Gupta
2023-03-14 7:19 ` Mike Rapoport
2023-03-16 19:30 ` Deepak Gupta
2023-03-20 11:35 ` Szabolcs Nagy
2023-03-10 16:11 ` Borislav Petkov
2023-03-10 17:12 ` Edgecombe, Rick P
2023-03-10 20:05 ` Borislav Petkov
2023-03-10 20:19 ` Edgecombe, Rick P
2023-03-10 20:26 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 34/41] x86/shstk: Support WRSS for userspace Rick Edgecombe
2023-03-10 16:44 ` Borislav Petkov
2023-03-10 17:16 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 35/41] x86: Expose thread features in /proc/$PID/status Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 36/41] x86/shstk: Wire in shadow stack interface Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 37/41] selftests/x86: Add shadow stack test Rick Edgecombe
2023-02-27 22:29 ` [PATCH v7 38/41] x86/fpu: Add helper for initing features Rick Edgecombe
2023-03-11 12:54 ` Borislav Petkov
2023-03-13 2:45 ` Edgecombe, Rick P
2023-03-13 11:03 ` Borislav Petkov
2023-03-13 16:10 ` Edgecombe, Rick P
2023-03-13 17:10 ` Borislav Petkov
2023-03-13 23:31 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 39/41] x86: Add PTRACE interface for shadow stack Rick Edgecombe
2023-03-11 15:06 ` Borislav Petkov
2023-03-13 2:53 ` Edgecombe, Rick P
2023-02-27 22:29 ` [PATCH v7 40/41] x86/shstk: Add ARCH_SHSTK_UNLOCK Rick Edgecombe
2023-03-11 15:11 ` Borislav Petkov
2023-03-13 3:04 ` Edgecombe, Rick P
2023-03-13 11:05 ` Borislav Petkov
2023-02-27 22:29 ` [PATCH v7 41/41] x86/shstk: Add ARCH_SHSTK_STATUS Rick Edgecombe
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