From: "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
To: Andy Lutomirski <luto@kernel.org>,
linux-arch <linux-arch@vger.kernel.org>
Cc: X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
Linux API <linux-api@vger.kernel.org>,
Arnd Bergmann <arnd@arndb.de>,
Balbir Singh <bsingharora@gmail.com>,
Borislav Petkov <bp@alien8.de>,
Cyrill Gorcunov <gorcunov@gmail.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Eugene Syromiatnikov <esyr@redhat.com>,
Florian Weimer <fweimer@redhat.com>,
"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
Jonathan Corbet <corbet@lwn.net>,
Kees Cook <keescook@chromium.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Nadav Amit <nadav.amit@gmail.com>,
Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
Peter Zijlstra <peterz@infradead.org>,
Randy Dunlap <rdunlap@infradead.org>,
"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
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: Wed, 28 Apr 2021 16:20:55 -0700 [thread overview]
Message-ID: <f6e61dae-9805-c855-8873-7481ceb7ea79@intel.com> (raw)
In-Reply-To: <CALCETrVTeYfzO-XWh+VwTuKCyPyp-oOMGH=QR_msG9tPQ4xPmA@mail.gmail.com>
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.
>
> 2. CET. CET wants us to find a few more bytes somewhere, and those
> bytes logically belong in ucontext, and here we are.
>
> This is *almost*, but not quite, easy: struct ucontext is already
> variable length! Unfortunately, the whole variable length portion is
> used up by uc_sigmask. So I propose that we introduce a brand new
> bona fide extension mechanism. It works like this:
>
> First, we add a struct ucontext_extension at the end. It looks like:
>
> struct ucontext_extension {
> u64 length; /* sizeof(struct ucontext_extension) */
> u64 flags; /* we will want this some day */
> [CET stuff here]
> [future stuff here]
> };
>
> And we locate it by scrounging a word somewhere in ucontext to give
> the offset from the beginning of struct ucontext to
> ucontext_extension. We indicate the presence of this feature using a
> new uc_flags bit. I can think of a couple of vaguely reasonable
> places:
>
> a) the reserved word in sigcontext. This is fine for x86 but not so
> great if other architectures want to do this.
>
> b) uc_link. Fine everywhere but powerpc. Oops.
>
> c) use the high bits of uc_flags. After all, once we add extensions,
> we don't need new flags, so we can steal 16 high bits of uc_flags for
> this.
>
> I think I'm in favor of (c). We do:
>
> (uc_flags & 0xffff0000) == 0: extension not present
>
> Otherwise the extension region is at ucontext + (uc_flags >> 16).
>
> And sigreturn finds the extension the same way, because CRIU can
> already migrate a signal frame from one kernel to another, your patch
> breaks this, and having sigreturn hardcode the offset would also break
> it.
>
> What do you think?
>
There are a lot of things in here. I think I could create some patches
for ucontext_extension and send out for discussion. Thanks for
explaining this!
Yu-cheng
next prev parent reply other threads:[~2021-04-28 23:21 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 [this message]
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
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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