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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 <[email protected]> wrote:
>>
>> On 9/30/2020 6:10 PM, Andy Lutomirski wrote:
>>> On Wed, Sep 30, 2020 at 6:01 PM H.J. Lu <[email protected]> wrote:
>>>>
>>>> On Wed, Sep 30, 2020 at 4:44 PM Andy Lutomirski <[email protected]> wrote:
>>
>> [...]
>>
>>>>>>>>> From 09803e66dca38d7784e32687d0693550948199ed Mon Sep 17 00:00:00 2001
>>>>>>>>> From: Yu-cheng Yu <[email protected]>
>>>>>>>>> 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 <[email protected]>

[...]

>>>>>>>>
>>>>>>>
>>>>>>> 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 <[email protected]>
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 <[email protected]>
---
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

2020-10-09 19:56:26

by Andy Lutomirski

[permalink] [raw]

Subject: Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation

On Tue, Oct 6, 2020 at 12:09 PM Yu, Yu-cheng <[email protected]> wrote:
>
> On 10/1/2020 10:26 AM, Andy Lutomirski wrote:
> > On Thu, Oct 1, 2020 at 9:51 AM Yu, Yu-cheng <[email protected]> wrote:
> >>
> >> On 9/30/2020 6:10 PM, Andy Lutomirski wrote:
> >>> On Wed, Sep 30, 2020 at 6:01 PM H.J. Lu <[email protected]> wrote:
> >>>>
> >>>> On Wed, Sep 30, 2020 at 4:44 PM Andy Lutomirski <[email protected]> wrote:
> >>
> >> [...]
> >>
> >>>>>>>>> From 09803e66dca38d7784e32687d0693550948199ed Mon Sep 17 00:00:00 2001
> >>>>>>>>> From: Yu-cheng Yu <[email protected]>
> >>>>>>>>> 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 <[email protected]>
>
> [...]
>
> >>>>>>>>
> >>>>>>>
> >>>>>>> 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.

NAK. Let me try explaining again.

>
> ======
> 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)
> +

This is not what "orthogonal" means. If the bits were orthogonal, the
logic would be:

if (gnu_property & DISABLE_VSYSCALL)
disable vsyscall;
if (gnu_property & SHSTK)
enable SHSTK;
if (gnu_property & IBT);
enable IBT;

and, if necessarily (although I still think it would be preferable not
to do this):

if ((gnu_property & (DISABLE_VSYSCALL | SHSTK)) == SHSTK)
return -EINVAL;

As far as I'm concerned, you have two choices:

a) Make SHSTK work *correctly* with vsyscall emulation.

b) Add a high quality mechanism to disable vsyscall emulation and make
SHSTK depend on that.

As far as I'm concerned, (a) is preferable. Ideally we'd get (a)
*and* a high quality vsyscall emulation disable mechanism with no
dependencies.

> 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;
> +

This needs to be per-mm, not per-thread. There's a patch floating
around that gets us about a quarter of the way there. I'm not
convinced that CET should wait for this to finish.