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Date: Tue, 6 Oct 2020 12:09:37 -0700
From: "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To: Andy Lutomirski <luto@...nel.org>
Cc: "H.J. Lu" <hjl.tools@...il.com>, X86 ML <x86@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
linux-arch <linux-arch@...r.kernel.org>,
Linux API <linux-api@...r.kernel.org>,
Arnd Bergmann <arnd@...db.de>,
Balbir Singh <bsingharora@...il.com>,
Borislav Petkov <bp@...en8.de>,
Cyrill Gorcunov <gorcunov@...il.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Eugene Syromiatnikov <esyr@...hat.com>,
Florian Weimer <fweimer@...hat.com>,
Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>,
Mike Kravetz <mike.kravetz@...cle.com>,
Nadav Amit <nadav.amit@...il.com>,
Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
Peter Zijlstra <peterz@...radead.org>,
Randy Dunlap <rdunlap@...radead.org>,
"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
Dave Martin <Dave.Martin@....com>,
Weijiang Yang <weijiang.yang@...el.com>,
Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect
Branch Tracking for vsyscall emulation
On 10/1/2020 10:26 AM, Andy Lutomirski wrote:
> On Thu, Oct 1, 2020 at 9:51 AM Yu, Yu-cheng <yu-cheng.yu@...el.com> wrote:
>>
>> On 9/30/2020 6:10 PM, Andy Lutomirski wrote:
>>> On Wed, Sep 30, 2020 at 6:01 PM H.J. Lu <hjl.tools@...il.com> wrote:
>>>>
>>>> On Wed, Sep 30, 2020 at 4:44 PM Andy Lutomirski <luto@...nel.org> wrote:
>>
>> [...]
>>
>>>>>>>>> From 09803e66dca38d7784e32687d0693550948199ed Mon Sep 17 00:00:00 2001
>>>>>>>>> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
>>>>>>>>> Date: Thu, 29 Nov 2018 14:15:38 -0800
>>>>>>>>> Subject: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and
>>>>>>>>> Indirect Branch
>>>>>>>>> Tracking for vsyscall emulation
>>>>>>>>>
>>>>>>>>> Vsyscall entry points are effectively branch targets. Mark them with
>>>>>>>>> ENDBR64 opcodes. When emulating the RET instruction, unwind shadow stack
>>>>>>>>> and reset IBT state machine.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
[...]
>>>>>>>>
>>>>>>>
>>>>>>> For what it's worth, I think there is an alternative. If you all
>>>>>>> (userspace people, etc) can come up with a credible way for a user
>>>>>>> program to statically declare that it doesn't need vsyscalls, then we
>>>>>>> could make SHSTK depend on *that*, and we could avoid this mess. This
>>>>>>> breaks orthogonality, but it's probably a decent outcome.
>>>>>>>
>>>>>>
>>>>>> Would an arch_prctl(DISABLE_VSYSCALL) work? The kernel then sets a
>>>>>> thread flag, and in emulate_vsyscall(), checks the flag.
>>>>>>
>>>>>> When CET is enabled, ld-linux will do DISABLE_VSYSCALL.
>>>>>>
>>>>>> How is that?
>>>>>
>>>>> Backwards, no? Presumably vsyscall needs to be disabled before or
>>>>> concurrently with CET being enabled, not after.
>>>>>
>>>>> I think the solution of making vsyscall emulation work correctly with
>>>>> CET is going to be better and possibly more straightforward.
>>>>>
>>>>
>>>> We can do
>>>>
>>>> 1. Add ARCH_X86_DISABLE_VSYSCALL to disable the vsyscall page.
>>>> 2. If CPU supports CET and the program is CET enabled:
>>>> a. Disable the vsyscall page.
>>>> b. Pass control to user.
>>>> c. Enable the vsyscall page when ARCH_X86_CET_DISABLE is called.
>>>>
>>>> So when control is passed from kernel to user, the vsyscall page is
>>>> disabled if the program
>>>> is CET enabled.
>>>
>>> Let me say this one more time:
>>>
>>> If we have a per-process vsyscall disable control and a per-process
>>> CET control, we are going to keep those settings orthogonal. I'm
>>> willing to entertain an option in which enabling SHSTK without also
>>> disabling vsyscalls is disallowed, We are *not* going to have any CET
>>> flags magically disable vsyscalls, though, and we are not going to
>>> have a situation where disabling vsyscalls on process startup requires
>>> enabling SHSTK.
>>>
>>> Any possible static vsyscall controls (and CET controls, for that
>>> matter) also need to come with some explanation of whether they are
>>> properties set on the ELF loader, the ELF program being loaded, or
>>> both. And this explanation needs to cover what happens when old
>>> binaries link against new libc versions and vice versa. A new
>>> CET-enabled binary linked against old libc running on a new kernel
>>> that is expected to work on a non-CET CPU MUST work on a CET CPU, too.
>>>
>>> Right now, literally the only thing preventing vsyscall emulation from
>>> coexisting with SHSTK is that the implementation eeds work.
>>>
>>> So your proposal is rejected. Sorry.
>>>
>> I think, even with shadow stack/ibt enabled, we can still allow XONLY
>> without too much mess.
>>
>> What about this?
>>
>> Thanks,
>> Yu-cheng
>>
>> ======
>>
>> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c
>> b/arch/x86/entry/vsyscall/vsyscall_64.c
>> index 8b0b32ac7791..d39da0a15521 100644
>> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
>> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
>> @@ -48,16 +48,16 @@
>> static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
>> #ifdef CONFIG_LEGACY_VSYSCALL_NONE
>> NONE;
>> -#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
>> +#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY) || defined(CONFIG_X86_CET)
>> XONLY;
>> -#else
>> +#else
>> EMULATE;
>> #endif
>
> I don't get it.
>
> First, you can't do any of this based on config -- it must be runtime.
>
> Second, and more importantly, I don't see how XONLY helps at all. The
> (non-executable) text that's exposed to user code in EMULATE mode is
> trivial to get right with CET -- your code already handles it. It's
> the emulation code (that runs identically in EMULATE and XONLY mode)
> that's tricky.
>
Hi,
There has been some ambiguity in my previous proposals. To make things
clear, I created a patch for arch_prctl(VSYSCALL_CTL), which controls
the TIF_VSYSCALL_DISABLE flag. It is entirely orthogonal to shadow
stack or IBT. On top of the patch, we can do SET_PERSONALITY2() to
disable vsyscall, e.g.
======
diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 0e1be2a13359..c730ff00bc62 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -394,6 +394,19 @@ struct arch_elf_state {
.gnu_property = 0, \
}
+#define SET_PERSONALITY2(ex, state) \
+do { \
+ unsigned int has_cet; \
+ \
+ has_cet = GNU_PROPERTY_X86_FEATURE_1_SHSTK | \
+ GNU_PROPERTY_X86_FEATURE_1_IBT; \
+ \
+ if ((state)->gnu_property & has_cet) \
+ set_thread_flag(TIF_VSYSCALL_DISABLE); \
+ \
+ SET_PERSONALITY(ex); \
+} while (0)
+
#define arch_elf_pt_proc(ehdr, phdr, elf, interp, state) (0)
#define arch_check_elf(ehdr, interp, interp_ehdr, state) (0)
#endif
======
The is the patch.
From a124b81086122495d6837f26df99db619cd5402a Mon Sep 17 00:00:00 2001
From: Yu-cheng Yu <yu-cheng.yu@...el.com>
Date: Mon, 5 Oct 2020 12:10:26 -0700
Subject: [PATCH 34/45] x86/vsyscall/64: Introduce arch_prctl(VSYCALL_CTL)
Vsyscall emulation provides compatibility to older applications. Newer
applications use the vDSO interface and do not use vsyscalls, and it is
desirable to have a per-task control of vsyscall.
One use case of the interface is when shadow stack and/or indirect branch
tracking is enabled and vsyscall emulation needs to cancel out the control-
flow protection. The cancelling code, if implemented, could become a back
door for evading the protection. Disabling vsyscall eliminates the risk.
Introduce arch_prctl(VSYSCALL_CTL), which sets/clears TIF_VSYSCALL_DISABLE
flag. When the flag is set, vsyscall is disabled.
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
---
arch/x86/entry/vsyscall/vsyscall_64.c | 3 +++
arch/x86/include/asm/thread_info.h | 2 ++
arch/x86/include/uapi/asm/prctl.h | 1 +
arch/x86/kernel/process_64.c | 17 +++++++++++++++++
tools/arch/x86/include/uapi/asm/prctl.h | 1 +
5 files changed, 24 insertions(+)
diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c
b/arch/x86/entry/vsyscall/vsyscall_64.c
index 44c33103a955..fe8f3db6d21b 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -127,6 +127,9 @@ bool emulate_vsyscall(unsigned long error_code,
long ret;
unsigned long orig_dx;
+ if (test_thread_flag(TIF_VSYSCALL_DISABLE))
+ return false;
+
/* Write faults or kernel-privilege faults never get fixed up. */
if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
return false;
diff --git a/arch/x86/include/asm/thread_info.h
b/arch/x86/include/asm/thread_info.h
index 267701ae3d86..c0cce3401c0f 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -98,6 +98,7 @@ struct thread_info {
#define TIF_IO_BITMAP 22 /* uses I/O bitmap */
#define TIF_FORCED_TF 24 /* true if TF in eflags artificially */
#define TIF_BLOCKSTEP 25 /* set when we want DEBUGCTLMSR_BTF */
+#define TIF_VSYSCALL_DISABLE 26 /* set when vsyscall is disallowed */
#define TIF_LAZY_MMU_UPDATES 27 /* task is updating the mmu lazily */
#define TIF_SYSCALL_TRACEPOINT 28 /* syscall tracepoint instrumentation */
#define TIF_ADDR32 29 /* 32-bit address space on 64 bits */
@@ -127,6 +128,7 @@ struct thread_info {
#define _TIF_IO_BITMAP (1 << TIF_IO_BITMAP)
#define _TIF_FORCED_TF (1 << TIF_FORCED_TF)
#define _TIF_BLOCKSTEP (1 << TIF_BLOCKSTEP)
+#define _TIF_VSYSCALL_DISABLE (1 << TIF_VSYSCALL_DISABLE)
#define _TIF_LAZY_MMU_UPDATES (1 << TIF_LAZY_MMU_UPDATES)
#define _TIF_SYSCALL_TRACEPOINT (1 << TIF_SYSCALL_TRACEPOINT)
#define _TIF_ADDR32 (1 << TIF_ADDR32)
diff --git a/arch/x86/include/uapi/asm/prctl.h
b/arch/x86/include/uapi/asm/prctl.h
index 9245bf629120..223fa382a81e 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -13,6 +13,7 @@
#define ARCH_MAP_VDSO_X32 0x2001
#define ARCH_MAP_VDSO_32 0x2002
#define ARCH_MAP_VDSO_64 0x2003
+#define ARCH_VSYSCALL_CTRL 0x2004
#define ARCH_X86_CET_STATUS 0x3001
#define ARCH_X86_CET_DISABLE 0x3002
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 1147a1052a07..eba61791c9cf 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -719,6 +719,20 @@ static long prctl_map_vdso(const struct vdso_image
*image, unsigned long addr)
}
#endif
+static long prctl_vsyscall_ctrl(unsigned int disable)
+{
+ if (IS_ENABLED(CONFIG_X86_VSYSCALL_EMULATION)) {
+ if (disable)
+ set_thread_flag(TIF_VSYSCALL_DISABLE);
+ else
+ clear_thread_flag(TIF_VSYSCALL_DISABLE);
+
+ return 0;
+ } else {
+ return disable ? 0 : -EINVAL;
+ }
+}
+
long do_arch_prctl_64(struct task_struct *task, int option, unsigned
long arg2)
{
int ret = 0;
@@ -807,6 +821,9 @@ long do_arch_prctl_64(struct task_struct *task, int
option, unsigned long arg2)
return prctl_map_vdso(&vdso_image_64, arg2);
#endif
+ case ARCH_VSYSCALL_CTRL:
+ return prctl_vsyscall_ctrl(arg2);
+
default:
ret = -EINVAL;
break;
diff --git a/tools/arch/x86/include/uapi/asm/prctl.h
b/tools/arch/x86/include/uapi/asm/prctl.h
index 9245bf629120..2476f46fa51f 100644
--- a/tools/arch/x86/include/uapi/asm/prctl.h
+++ b/tools/arch/x86/include/uapi/asm/prctl.h
@@ -13,6 +13,7 @@
#define ARCH_MAP_VDSO_X32 0x2001
#define ARCH_MAP_VDSO_32 0x2002
#define ARCH_MAP_VDSO_64 0x2003
+#define ARCH_VSYSCALL_CTL 0x2004
#define ARCH_X86_CET_STATUS 0x3001
#define ARCH_X86_CET_DISABLE 0x3002
--
2.21.0
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