From: "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
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Dave Martin <Dave.Martin@arm.com>,
Weijiang Yang <weijiang.yang@intel.com>,
Pengfei Xu <pengfei.xu@intel.com>,
Haitao Huang <haitao.huang@intel.com>
Subject: Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack)
Date: Fri, 30 Apr 2021 11:32:45 -0700 [thread overview]
Message-ID: <a7c332c8-9368-40b1-e221-ec921f7db948@intel.com> (raw)
In-Reply-To: <CALCETrX9z-73wpy-SCy8NE1XfQgXAN0mCmjv0jXDDomMyS7TKg@mail.gmail.com>
On 4/30/2021 10:47 AM, Andy Lutomirski wrote:
> On Fri, Apr 30, 2021 at 10:00 AM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>>
>> On 4/28/2021 4:03 PM, Andy Lutomirski wrote:
>>> On Tue, Apr 27, 2021 at 1:44 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>>>
>>>> When shadow stack is enabled, a task's shadow stack states must be saved
>>>> along with the signal context and later restored in sigreturn. However,
>>>> currently there is no systematic facility for extending a signal context.
>>>> There is some space left in the ucontext, but changing ucontext is likely
>>>> to create compatibility issues and there is not enough space for further
>>>> extensions.
>>>>
>>>> Introduce a signal context extension struct 'sc_ext', which is used to save
>>>> shadow stack restore token address. The extension is located above the fpu
>>>> states, plus alignment. The struct can be extended (such as the ibt's
>>>> wait_endbr status to be introduced later), and sc_ext.total_size field
>>>> keeps track of total size.
>>>
>>> I still don't like this.
>>>
>>> Here's how the signal layout works, for better or for worse:
>>>
>>> The kernel has:
>>>
>>> struct rt_sigframe {
>>> char __user *pretcode;
>>> struct ucontext uc;
>>> struct siginfo info;
>>> /* fp state follows here */
>>> };
>>>
>>> This is roughly the actual signal frame. But userspace does not have
>>> this struct declared, and user code does not know the sizes of the
>>> fields. So it's accessed in a nonsensical way. The signal handler
>>> function is passed a pointer to the whole sigframe implicitly in RSP,
>>> a pointer to &frame->info in RSI, anda pointer to &frame->uc in RDX.
>>> User code can *find* the fp state by following a pointer from
>>> mcontext, which is, in turn, found via uc:
>>>
>>> struct ucontext {
>>> unsigned long uc_flags;
>>> struct ucontext *uc_link;
>>> stack_t uc_stack;
>>> struct sigcontext uc_mcontext; <-- fp pointer is in here
>>> sigset_t uc_sigmask; /* mask last for extensibility */
>>> };
>>>
>>> The kernel, in sigreturn, works a bit differently. The sigreturn
>>> variants know the base address of the frame but don't have the benefit
>>> of receiving pointers to the fields. So instead the kernel takes
>>> advantage of the fact that it knows the offset to uc and parses uc
>>> accordingly. And the kernel follows the pointer in mcontext to find
>>> the fp state. The latter bit is quite important later. The kernel
>>> does not parse info at all.
>>>
>>> The fp state is its own mess. When XSAVE happened, Intel kindly (?)
>>> gave us a software defined area between the "legacy" x87 region and
>>> the modern supposedly extensible part. Linux sticks the following
>>> structure in that hole:
>>>
>>> struct _fpx_sw_bytes {
>>> /*
>>> * If set to FP_XSTATE_MAGIC1 then this is an xstate context.
>>> * 0 if a legacy frame.
>>> */
>>> __u32 magic1;
>>>
>>> /*
>>> * Total size of the fpstate area:
>>> *
>>> * - if magic1 == 0 then it's sizeof(struct _fpstate)
>>> * - if magic1 == FP_XSTATE_MAGIC1 then it's sizeof(struct _xstate)
>>> * plus extensions (if any)
>>> */
>>> __u32 extended_size;
>>>
>>> /*
>>> * Feature bit mask (including FP/SSE/extended state) that is present
>>> * in the memory layout:
>>> */
>>> __u64 xfeatures;
>>>
>>> /*
>>> * Actual XSAVE state size, based on the xfeatures saved in the layout.
>>> * 'extended_size' is greater than 'xstate_size':
>>> */
>>> __u32 xstate_size;
>>>
>>> /* For future use: */
>>> __u32 padding[7];
>>> };
>>>
>>>
>>> That's where we are right now upstream. The kernel has a parser for
>>> the FPU state that is bugs piled upon bugs and is going to have to be
>>> rewritten sometime soon. On top of all this, we have two upcoming
>>> features, both of which require different kinds of extensions:
>>>
>>> 1. AVX-512. (Yeah, you thought this story was over a few years ago,
>>> but no. And AMX makes it worse.) To make a long story short, we
>>> promised user code many years ago that a signal frame fit in 2048
>>> bytes with some room to spare. With AVX-512 this is false. With AMX
>>> it's so wrong it's not even funny. The only way out of the mess
>>> anyone has come up with involves making the length of the FPU state
>>> vary depending on which features are INIT, i.e. making it more compact
>>> than "compact" mode is. This has a side effect: it's no longer
>>> possible to modify the state in place, because enabling a feature with
>>> no space allocated will make the structure bigger, and the stack won't
>>> have room. Fortunately, one can relocate the entire FPU state, update
>>> the pointer in mcontext, and the kernel will happily follow the
>>> pointer. So new code on a new kernel using a super-compact state
>>> could expand the state by allocating new memory (on the heap? very
>>> awkwardly on the stack?) and changing the pointer. For all we know,
>>> some code already fiddles with the pointer. This is great, except
>>> that your patch sticks more data at the end of the FPU block that no
>>> one is expecting, and your sigreturn code follows that pointer, and
>>> will read off into lala land.
>>>
>>
>> Then, what about we don't do that at all. Is it possible from now on we
>> don't stick more data at the end, and take the relocating-fpu approach?
>>
>>> 2. CET. CET wants us to find a few more bytes somewhere, and those
>>> bytes logically belong in ucontext, and here we are.
>>>
>>
>> Fortunately, we can spare CET the need of ucontext extension. When the
>> kernel handles sigreturn, the user-mode shadow stack pointer is right at
>> the restore token. There is no need to put that in ucontext.
>
> That seems entirely reasonable. This might also avoid needing to
> teach CRIU about CET at all.
>
>>
>> However, the WAIT_ENDBR status needs to be saved/restored for signals.
>> Since IBT is now dependent on shadow stack, we can use a spare bit of
>> the shadow stack restore token for that.
>
> That seems like unnecessary ABI coupling. We have plenty of bits in
> uc_flags, and we have an entire reserved word in sigcontext. How
> about just sticking this bit in one of those places?
Yes, I will make it UC_WAIT_ENDBR.
Thanks,
Yu-cheng
next prev parent reply other threads:[~2021-04-30 18:32 UTC|newest]
Thread overview: 83+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-27 20:42 [PATCH v26 00/30] Control-flow Enforcement: Shadow Stack Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 01/30] Documentation/x86: Add CET description Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 02/30] x86/cet/shstk: Add Kconfig option for Shadow Stack Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 03/30] x86/cpufeatures: Add CET CPU feature flags for Control-flow Enforcement Technology (CET) Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 04/30] x86/cpufeatures: Introduce CPU setup and option parsing for CET Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 05/30] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 06/30] x86/cet: Add control-protection fault handler Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 07/30] x86/mm: Remove _PAGE_DIRTY from kernel RO pages Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 08/30] x86/mm: Move pmd_write(), pud_write() up in the file Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 09/30] x86/mm: Introduce _PAGE_COW Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 10/30] drm/i915/gvt: Change _PAGE_DIRTY to _PAGE_DIRTY_BITS Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 11/30] x86/mm: Update pte_modify for _PAGE_COW Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 12/30] x86/mm: Update ptep_set_wrprotect() and pmdp_set_wrprotect() for transition from _PAGE_DIRTY to _PAGE_COW Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 13/30] mm: Introduce VM_SHADOW_STACK for shadow stack memory Yu-cheng Yu
2021-04-27 20:42 ` [PATCH v26 14/30] x86/mm: Shadow Stack page fault error checking Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 15/30] x86/mm: Update maybe_mkwrite() for shadow stack Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 16/30] mm: Fixup places that call pte_mkwrite() directly Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 17/30] mm: Add guard pages around a shadow stack Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 18/30] mm/mmap: Add shadow stack pages to memory accounting Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 19/30] mm: Update can_follow_write_pte() for shadow stack Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 20/30] mm/mprotect: Exclude shadow stack from preserve_write Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 21/30] mm: Re-introduce vm_flags to do_mmap() Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 22/30] x86/cet/shstk: Add user-mode shadow stack support Yu-cheng Yu
2021-04-28 17:52 ` Borislav Petkov
2021-04-28 18:39 ` Yu, Yu-cheng
2021-04-29 9:12 ` Borislav Petkov
2021-04-29 16:17 ` Yu, Yu-cheng
2021-04-29 16:45 ` Borislav Petkov
2021-04-27 20:43 ` [PATCH v26 23/30] x86/cet/shstk: Handle thread shadow stack Yu-cheng Yu
2021-05-10 14:15 ` Borislav Petkov
2021-05-10 22:57 ` Yu, Yu-cheng
2021-05-11 17:09 ` Borislav Petkov
2021-05-12 8:12 ` David Laight
2021-05-11 18:35 ` Yu, Yu-cheng
2021-05-12 15:56 ` Borislav Petkov
2021-04-27 20:43 ` [PATCH v26 24/30] x86/cet/shstk: Introduce shadow stack token setup/verify routines Yu-cheng Yu
2021-05-17 7:45 ` Borislav Petkov
2021-05-17 20:55 ` Yu, Yu-cheng
2021-05-18 0:14 ` Eugene Syromiatnikov
2021-05-18 17:58 ` Borislav Petkov
2021-05-18 19:45 ` Yu, Yu-cheng
2021-05-18 18:05 ` Yu, Yu-cheng
2021-05-18 5:56 ` Borislav Petkov
2021-05-21 16:17 ` Yu, Yu-cheng
2021-05-21 18:40 ` Borislav Petkov
2021-04-27 20:43 ` [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack Yu-cheng Yu
2021-04-28 23:03 ` extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack) Andy Lutomirski
2021-04-28 23:20 ` Yu, Yu-cheng
2021-04-29 7:28 ` Cyrill Gorcunov
2021-04-29 14:44 ` Andy Lutomirski
2021-04-29 15:35 ` Cyrill Gorcunov
2021-04-30 6:45 ` Florian Weimer
2021-04-30 17:00 ` Yu, Yu-cheng
2021-04-30 17:47 ` Andy Lutomirski
2021-04-30 18:32 ` Yu, Yu-cheng [this message]
2021-05-04 20:49 ` Yu, Yu-cheng
2021-05-06 22:05 ` Yu, Yu-cheng
2021-05-06 23:31 ` Andy Lutomirski
2021-05-02 23:23 ` Andy Lutomirski
2021-05-03 6:03 ` H. Peter Anvin
2021-05-03 15:13 ` Yu, Yu-cheng
2021-05-03 15:29 ` Andy Lutomirski
2021-05-03 20:25 ` Yu, Yu-cheng
2021-04-27 20:43 ` [PATCH v26 26/30] ELF: Introduce arch_setup_elf_property() Yu-cheng Yu
2021-05-19 18:10 ` Borislav Petkov
2021-05-19 22:14 ` Yu, Yu-cheng
2021-05-20 9:26 ` Borislav Petkov
2021-05-20 17:18 ` Yu, Yu-cheng
2021-05-20 17:35 ` Borislav Petkov
2021-05-20 17:51 ` Yu, Yu-cheng
2021-05-20 17:38 ` Matthew Wilcox
2021-05-20 17:52 ` Yu, Yu-cheng
2021-05-20 21:06 ` Yu, Yu-cheng
2021-04-27 20:43 ` [PATCH v26 27/30] x86/cet/shstk: Add arch_prctl functions for shadow stack Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 28/30] mm: Move arch_calc_vm_prot_bits() to arch/x86/include/asm/mman.h Yu-cheng Yu
2021-04-27 20:43 ` [PATCH v26 29/30] mm: Update arch_validate_flags() to test vma anonymous Yu-cheng Yu
2021-05-11 11:35 ` Kirill A. Shutemov
2021-04-27 20:43 ` [PATCH v26 30/30] mm: Introduce PROT_SHADOW_STACK for shadow stack Yu-cheng Yu
2021-05-11 11:48 ` Kirill A. Shutemov
2021-05-11 14:44 ` Yu, Yu-cheng
2021-04-29 17:13 ` [PATCH v26 00/30] Control-flow Enforcement: Shadow Stack Borislav Petkov
2021-04-29 17:32 ` Yu, Yu-cheng
2021-04-29 17:49 ` Borislav Petkov
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