Mailing List Archive: [PATCH v2] x86: entry_32.S: change ESPFIX test to not touch PT

[PATCH v2] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

Mar 9, 2015, 9:44 AM

Post #1 of 4 (200 views)

Old code was trying to avoid having three branch insns,
but instead it has a chain of six insns where each insn
depends on previos one.

And it was touching PT_OLDSS(%esp) unconditionally, even when it may
contain bogus data. Elsewhere we have to jump thru hoops
just to make sure here PT_OLDSS(%esp) is at least in a valid page.

All this just to have one branch instead of three?

The new code simply checks each condition.
All three checks can run in parallel on an out-of-order CPU.
Most of the time, none of branches will be taken.

Comparison of object code:
Old:
1e6: 8b 44 24 38 mov 0x38(%esp),%eax
1ea: 8a 64 24 40 mov 0x40(%esp),%ah
1ee: 8a 44 24 34 mov 0x34(%esp),%al
1f2: 25 03 04 02 00 and $0x20403,%eax
1f7: 3d 03 04 00 00 cmp $0x403,%eax
1fc: 74 0f je 20d <ldt_ss>
New:
1e6: f6 44 24 3a 02 testb $0x2,0x3a(%esp)
1eb: 75 0e jne 1fb <restore_nocheck>
1ed: f6 44 24 34 03 testb $0x3,0x34(%esp)
1f2: 74 07 je 1fb <restore_nocheck>
1f4: f6 44 24 40 04 testb $0x4,0x40(%esp)
1f9: 75 0f jne 20a <ldt_ss>

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Borislav Petkov <bp@alien8.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Frederic Weisbecker <fweisbec@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
CC: Will Drewry <wad@chromium.org>
CC: Kees Cook <keescook@chromium.org>
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
---
Changes since v1:
shorter insn to test EFLAGS.VM (suggested by Linus)
check EFLAGS.VM only if CONFIG_VM86

arch/x86/kernel/entry_32.S | 19 ++++++++++---------
1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index e33ba51..d185b5e 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -516,16 +516,17 @@ restore_all:
TRACE_IRQS_IRET
restore_all_notrace:
#ifdef CONFIG_X86_ESPFIX32
- movl PT_EFLAGS(%esp), %eax # mix EFLAGS, SS and CS
- # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
- # are returning to the kernel.
- # See comments in process.c:copy_thread() for details.
- movb PT_OLDSS(%esp), %ah
- movb PT_CS(%esp), %al
- andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
- cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
CFI_REMEMBER_STATE
- je ldt_ss # returning to user-space with LDT SS
+#ifdef CONFIG_VM86
+ testb $2, (PT_EFLAGS+2)(%esp)
+ jnz restore_nocheck # EFLAGS.VM set, not it
+#endif
+ testb $3, PT_CS(%esp)
+ jz restore_nocheck # CPL0, not it
+ # Note: we access PT_OLDSS only when we know it exists.
+ # If PT_CS is from CPL0, it does not exist.
+ testb $SEGMENT_LDT, PT_OLDSS(%esp)
+ jnz ldt_ss # returning to user-space with LDT SS
#endif
restore_nocheck:
RESTORE_REGS 4 # skip orig_eax/error_code
--
1.8.1.4

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Re: [PATCH v2] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp) [ In reply to ]

mingo at kernel

Mar 10, 2015, 12:57 AM

Post #2 of 4 (193 views)

Permalink

* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> Old code was trying to avoid having three branch insns,
> but instead it has a chain of six insns where each insn
> depends on previos one.
>
> And it was touching PT_OLDSS(%esp) unconditionally, even when it may
> contain bogus data. Elsewhere we have to jump thru hoops
> just to make sure here PT_OLDSS(%esp) is at least in a valid page.
>
> All this just to have one branch instead of three?
>
> The new code simply checks each condition.
> All three checks can run in parallel on an out-of-order CPU.
> Most of the time, none of branches will be taken.
>
> Comparison of object code:
> Old:
> 1e6: 8b 44 24 38 mov 0x38(%esp),%eax
> 1ea: 8a 64 24 40 mov 0x40(%esp),%ah
> 1ee: 8a 44 24 34 mov 0x34(%esp),%al
> 1f2: 25 03 04 02 00 and $0x20403,%eax
> 1f7: 3d 03 04 00 00 cmp $0x403,%eax
> 1fc: 74 0f je 20d <ldt_ss>
> New:
> 1e6: f6 44 24 3a 02 testb $0x2,0x3a(%esp)
> 1eb: 75 0e jne 1fb <restore_nocheck>
> 1ed: f6 44 24 34 03 testb $0x3,0x34(%esp)
> 1f2: 74 07 je 1fb <restore_nocheck>
> 1f4: f6 44 24 40 04 testb $0x4,0x40(%esp)
> 1f9: 75 0f jne 20a <ldt_ss>

Please do some benchmarking of this: a tight loop of getpid or getppid
syscalls ought to be enough to be able to time this accurately.

Thanks,

Ingo
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Re: [PATCH v2] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp) [ In reply to ]

luto at amacapital

Mar 10, 2015, 8:35 PM

Post #3 of 4 (183 views)

Permalink

On Tue, Mar 10, 2015 at 12:57 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Denys Vlasenko <dvlasenk@redhat.com> wrote:
>
>> Old code was trying to avoid having three branch insns,
>> but instead it has a chain of six insns where each insn
>> depends on previos one.
>>
>> And it was touching PT_OLDSS(%esp) unconditionally, even when it may
>> contain bogus data. Elsewhere we have to jump thru hoops
>> just to make sure here PT_OLDSS(%esp) is at least in a valid page.
>>
>> All this just to have one branch instead of three?
>>
>> The new code simply checks each condition.
>> All three checks can run in parallel on an out-of-order CPU.
>> Most of the time, none of branches will be taken.
>>
>> Comparison of object code:
>> Old:
>> 1e6: 8b 44 24 38 mov 0x38(%esp),%eax
>> 1ea: 8a 64 24 40 mov 0x40(%esp),%ah
>> 1ee: 8a 44 24 34 mov 0x34(%esp),%al
>> 1f2: 25 03 04 02 00 and $0x20403,%eax
>> 1f7: 3d 03 04 00 00 cmp $0x403,%eax
>> 1fc: 74 0f je 20d <ldt_ss>
>> New:
>> 1e6: f6 44 24 3a 02 testb $0x2,0x3a(%esp)
>> 1eb: 75 0e jne 1fb <restore_nocheck>
>> 1ed: f6 44 24 34 03 testb $0x3,0x34(%esp)
>> 1f2: 74 07 je 1fb <restore_nocheck>
>> 1f4: f6 44 24 40 04 testb $0x4,0x40(%esp)
>> 1f9: 75 0f jne 20a <ldt_ss>
>
> Please do some benchmarking of this: a tight loop of getpid or getppid
> syscalls ought to be enough to be able to time this accurately.

Before you benchmark, I think you should reorder it: check CS, then
OLDSS, then EFLAGS. The case where CS & 3 == 0, OLDSS & 4 == 4, and
EFLAGS & VM == VM should be *extremely* rare.

I'm going to hold off on resending my sp0/sp1/ss cleanups until we
resolve this -- if it turns out that your code is the same or faster
and therefore gets merged, then I think that all the -8 crap can just
be deleted instead of being fixed.

--Andy

>
> Thanks,
>
> Ingo

--
Andy Lutomirski
AMA Capital Management, LLC
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Re: [PATCH v2] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp) [ In reply to ]

vda.linux at googlemail

Mar 11, 2015, 5:50 AM

Post #4 of 4 (189 views)

Permalink

On Wed, Mar 11, 2015 at 4:35 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>>> Comparison of object code:
>>> Old:
>>> 1e6: 8b 44 24 38 mov 0x38(%esp),%eax
>>> 1ea: 8a 64 24 40 mov 0x40(%esp),%ah
>>> 1ee: 8a 44 24 34 mov 0x34(%esp),%al
>>> 1f2: 25 03 04 02 00 and $0x20403,%eax
>>> 1f7: 3d 03 04 00 00 cmp $0x403,%eax
>>> 1fc: 74 0f je 20d <ldt_ss>
>>> New:
>>> 1e6: f6 44 24 3a 02 testb $0x2,0x3a(%esp)
>>> 1eb: 75 0e jne 1fb <restore_nocheck>
>>> 1ed: f6 44 24 34 03 testb $0x3,0x34(%esp)
>>> 1f2: 74 07 je 1fb <restore_nocheck>
>>> 1f4: f6 44 24 40 04 testb $0x4,0x40(%esp)
>>> 1f9: 75 0f jne 20a <ldt_ss>
>>
>> Please do some benchmarking of this: a tight loop of getpid or getppid
>> syscalls ought to be enough to be able to time this accurately.
>
> Before you benchmark, I think you should reorder it: check CS, then
> OLDSS, then EFLAGS. The case where CS & 3 == 0, OLDSS & 4 == 4, and
> EFLAGS & VM == VM should be *extremely* rare.
>
> I'm going to hold off on resending my sp0/sp1/ss cleanups until we
> resolve this -- if it turns out that your code is the same or faster
> and therefore gets merged, then I think that all the -8 crap can just
> be deleted instead of being fixed.

I thought that I got NAKed by Linus on this change?

Basically, he wants to retain that padding because it guards against
_future_ bugs_ where someone would touch pt_regs->ss
and it will fail, very rarely. I think it's a sound reason to retain
the padding.

Anyway, since you ask. I benchmarked current code against this patch:

restore_all_notrace:
#ifdef CONFIG_X86_ESPFIX32
- movl PT_EFLAGS(%esp), %eax # mix EFLAGS, SS and CS
- # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
- # are returning to the kernel.
- # See comments in process.c:copy_thread() for details.
- movb PT_OLDSS(%esp), %ah
- movb PT_CS(%esp), %al
- andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
- cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
CFI_REMEMBER_STATE
- je ldt_ss # returning to user-space with LDT SS
+ testb $3, PT_CS(%esp)
+ jz restore_nocheck # CPL0, not it
+#ifdef CONFIG_VM86
+ testb $2, (PT_EFLAGS+2)(%esp)
+ jnz restore_nocheck # EFLAGS.VM set, not it
+#endif
+ # Note: we access PT_OLDSS only when we know it exists.
+ # If PT_CS is from CPL0, it does not exist.
+ testb $SEGMENT_LDT, PT_OLDSS(%esp)
+ jnz ldt_ss # returning to user-space with LDT SS
#endif

This code only gets executed on int80 path, not on sysenter.
The test was to run 10 million getpids.

The numbers I've got are *the same* before and after the patch:
227.99 ns per getpid().

I double-checked my test setup by adding a few PAUSE insns
in this code - which, as expected, was seen easily in the test run.
I did not forget to enable ESPFIX32 and VM86 in the .config.
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