[PATCH] x86-64/Xen: fix stack switching

[PATCH] x86-64/Xen: fix stack switching

[Jan Beulich]

While on native entry into the kernel happens on the trampoline stack,
PV Xen kernels are being entered with the current thread stack right
away. Hence source and destination stacks are identical in that case,
and special care is needed.

Other than in sync_regs() the copying done on the INT80 path as well as
on the NMI path itself isn't NMI / #MC safe, as either of these events
occurring in the middle of the stack copying would clobber data on the
(source) stack. (Of course, in the NMI case only #MC could break
things.)

I'm not altering the similar code in interrupt_entry(), as that code
path is unreachable when running an PV Xen guest afaict.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@kernel.org 
---
There would certainly have been the option of using alternatives
patching, but afaict the patching code isn't NMI / #MC safe, so I'd
rather stay away from patching the NMI path. And I thought it would be
better to use similar code in both cases.

Another option would be to make the Xen case match the native one, by
going through the trampoline stack, but to me this would look like extra
overhead for no gain.
---
 arch/x86/entry/entry_64.S        |    8 ++++++++
 arch/x86/entry/entry_64_compat.S |    8 +++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

--- 4.17-rc4/arch/x86/entry/entry_64.S
+++ 4.17-rc4-x86_64-stack-switch-Xen/arch/x86/entry/entry_64.S
@@ -1399,6 +1399,12 @@ ENTRY(nmi)
 	swapgs
 	cld
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
+
+	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rdx
+	subq	$8, %rdx
+	xorq	%rsp, %rdx
+	shrq	$PAGE_SHIFT, %rdx
+	jz	.Lnmi_keep_stack
 	movq	%rsp, %rdx
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 	UNWIND_HINT_IRET_REGS base=%rdx offset=8
@@ -1408,6 +1414,8 @@ ENTRY(nmi)
 	pushq	2*8(%rdx)	/* pt_regs->cs */
 	pushq	1*8(%rdx)	/* pt_regs->rip */
 	UNWIND_HINT_IRET_REGS
+.Lnmi_keep_stack:
+
 	pushq   $-1		/* pt_regs->orig_ax */
 	PUSH_AND_CLEAR_REGS rdx=(%rdx)
 	ENCODE_FRAME_POINTER
--- 4.17-rc4/arch/x86/entry/entry_64_compat.S
+++ 4.17-rc4-x86_64-stack-switch-Xen/arch/x86/entry/entry_64_compat.S
@@ -356,15 +356,21 @@ ENTRY(entry_INT80_compat)
 
 	/* Need to switch before accessing the thread stack. */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
+
+	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rdi
+	subq	$8, %rdi
+	xorq	%rsp, %rdi
+	shrq	$PAGE_SHIFT, %rdi
+	jz	.Lint80_keep_stack
 	movq	%rsp, %rdi
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
-
 	pushq	6*8(%rdi)		/* regs->ss */
 	pushq	5*8(%rdi)		/* regs->rsp */
 	pushq	4*8(%rdi)		/* regs->eflags */
 	pushq	3*8(%rdi)		/* regs->cs */
 	pushq	2*8(%rdi)		/* regs->ip */
 	pushq	1*8(%rdi)		/* regs->orig_ax */
+.Lint80_keep_stack:
 
 	pushq	(%rdi)			/* pt_regs->di */
 	pushq	%rsi			/* pt_regs->si */

Re: [PATCH] x86-64/Xen: fix stack switching

[Andy Lutomirski]

On Mon, May 7, 2018 at 5:16 AM Jan Beulich <JBeulich@suse.com> wrote:

> While on native entry into the kernel happens on the trampoline stack,
> PV Xen kernels are being entered with the current thread stack right
> away. Hence source and destination stacks are identical in that case,
> and special care is needed.

> Other than in sync_regs() the copying done on the INT80 path as well as
> on the NMI path itself isn't NMI / #MC safe, as either of these events
> occurring in the middle of the stack copying would clobber data on the
> (source) stack. (Of course, in the NMI case only #MC could break
> things.)

I think I'd rather fix this by changing the stack switch code or
alternativing around it on non-stack-switching kernels.  Or make Xen use a
trampoline stack just like native.


> I'm not altering the similar code in interrupt_entry(), as that code
> path is unreachable when running an PV Xen guest afaict.

> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Cc: stable@kernel.org
> ---
> There would certainly have been the option of using alternatives
> patching, but afaict the patching code isn't NMI / #MC safe, so I'd
> rather stay away from patching the NMI path. And I thought it would be
> better to use similar code in both cases.

I would hope we do the patching before we enable any NMIs.


> Another option would be to make the Xen case match the native one, by
> going through the trampoline stack, but to me this would look like extra
> overhead for no gain.

Avoiding even more complexity in the nmi code seems like a big gain to me.

Re: [PATCH] x86-64/Xen: fix stack switching

[Jan Beulich]

>>> On 08.05.18 at 04:38, <luto@kernel.org> wrote:
> On Mon, May 7, 2018 at 5:16 AM Jan Beulich <JBeulich@suse.com> wrote:
> 
>> While on native entry into the kernel happens on the trampoline stack,
>> PV Xen kernels are being entered with the current thread stack right
>> away. Hence source and destination stacks are identical in that case,
>> and special care is needed.
> 
>> Other than in sync_regs() the copying done on the INT80 path as well as
>> on the NMI path itself isn't NMI / #MC safe, as either of these events
>> occurring in the middle of the stack copying would clobber data on the
>> (source) stack. (Of course, in the NMI case only #MC could break
>> things.)
> 
> I think I'd rather fix this by changing the stack switch code or

Well, isn't that what I'm doing in the patch?

> alternativing around it on non-stack-switching kernels.

Fine with me if that's considered better than adding the conditionals.

>  Or make Xen use a trampoline stack just like native.

Well, as said I'd rather not, unless x86 and Xen maintainers agree
that's the way to go. But see below for NMI.

>> I'm not altering the similar code in interrupt_entry(), as that code
>> path is unreachable when running an PV Xen guest afaict.
> 
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>> Cc: stable@kernel.org 
>> ---
>> There would certainly have been the option of using alternatives
>> patching, but afaict the patching code isn't NMI / #MC safe, so I'd
>> rather stay away from patching the NMI path. And I thought it would be
>> better to use similar code in both cases.
> 
> I would hope we do the patching before we enable any NMIs.

"Enable NMIs"? I don't think they're getting disabled anywhere in the
kernel. Perhaps you merely mean ones the kernel sends itself (which
I agree would hopefully only be enabled after alternatives patching?

>> Another option would be to make the Xen case match the native one, by
>> going through the trampoline stack, but to me this would look like extra
>> overhead for no gain.
> 
> Avoiding even more complexity in the nmi code seems like a big gain to me.

I'm not sure the added conditional is more complexity than making Xen
switch to the trampoline stack just to switch back almost immediately.
But yes, I could see complexity of the NMI code to be a reason to use
different solutions on the NMI and INT80 paths. It's just that I'd like
you, the x86 maintainters, and the Xen ones to agree on which solution
to use where before I'd send a v2.

Jan

Re: [PATCH] x86-64/Xen: fix stack switching

[Juergen Gross]

On 14/05/18 12:28, Jan Beulich wrote:
>>>> On 08.05.18 at 04:38, <luto@kernel.org> wrote:
>> On Mon, May 7, 2018 at 5:16 AM Jan Beulich <JBeulich@suse.com> wrote:
>>
>>> While on native entry into the kernel happens on the trampoline stack,
>>> PV Xen kernels are being entered with the current thread stack right
>>> away. Hence source and destination stacks are identical in that case,
>>> and special care is needed.
>>
>>> Other than in sync_regs() the copying done on the INT80 path as well as
>>> on the NMI path itself isn't NMI / #MC safe, as either of these events
>>> occurring in the middle of the stack copying would clobber data on the
>>> (source) stack. (Of course, in the NMI case only #MC could break
>>> things.)
>>
>> I think I'd rather fix this by changing the stack switch code or
> 
> Well, isn't that what I'm doing in the patch?
> 
>> alternativing around it on non-stack-switching kernels.
> 
> Fine with me if that's considered better than adding the conditionals.
> 
>>  Or make Xen use a trampoline stack just like native.
> 
> Well, as said I'd rather not, unless x86 and Xen maintainers agree
> that's the way to go. But see below for NMI.

I'd prefer not using a trampoline stack, too.

> 
>>> I'm not altering the similar code in interrupt_entry(), as that code
>>> path is unreachable when running an PV Xen guest afaict.
>>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>>> Cc: stable@kernel.org 
>>> ---
>>> There would certainly have been the option of using alternatives
>>> patching, but afaict the patching code isn't NMI / #MC safe, so I'd
>>> rather stay away from patching the NMI path. And I thought it would be
>>> better to use similar code in both cases.
>>
>> I would hope we do the patching before we enable any NMIs.
> 
> "Enable NMIs"? I don't think they're getting disabled anywhere in the
> kernel. Perhaps you merely mean ones the kernel sends itself (which
> I agree would hopefully only be enabled after alternatives patching?
> 
>>> Another option would be to make the Xen case match the native one, by
>>> going through the trampoline stack, but to me this would look like extra
>>> overhead for no gain.
>>
>> Avoiding even more complexity in the nmi code seems like a big gain to me.
> 
> I'm not sure the added conditional is more complexity than making Xen
> switch to the trampoline stack just to switch back almost immediately.

I agree.

> But yes, I could see complexity of the NMI code to be a reason to use
> different solutions on the NMI and INT80 paths. It's just that I'd like
> you, the x86 maintainters, and the Xen ones to agree on which solution
> to use where before I'd send a v2.

With my Xen maintainer hat on I'd prefer Jan's current solution.


Juergen

[PATCH v2] x86-64/Xen: fix stack switching

[Jan Beulich]

While in the native case entry into the kernel happens on the trampoline
stack, PV Xen kernels get entered with the current thread stack right
away. Hence source and destination stacks are identical in that case,
and special care is needed.

Other than in sync_regs() the copying done on the INT80 path as well as
on the NMI path itself isn't NMI / #MC safe, as either of these events
occurring in the middle of the stack copying would clobber data on the
(source) stack. (Of course, in the NMI case only #MC could break
things.)

I'm not altering the similar code in interrupt_entry(), as that code
path is unreachable afaict when running PV Xen guests.

Fixes: 7f2590a110b837af5679d08fc25c6227c5a8c497
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@kernel.org 
---
v2: Correct placement of .Lint80_keep_stack label.
---
 arch/x86/entry/entry_64.S        |    8 ++++++++
 arch/x86/entry/entry_64_compat.S |   10 ++++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

--- 4.20-rc3/arch/x86/entry/entry_64.S
+++ 4.20-rc3-x86_64-stack-switch-Xen/arch/x86/entry/entry_64.S
@@ -1380,6 +1380,12 @@ ENTRY(nmi)
 	swapgs
 	cld
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
+
+	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rdx
+	subq	$8, %rdx
+	xorq	%rsp, %rdx
+	shrq	$PAGE_SHIFT, %rdx
+	jz	.Lnmi_keep_stack
 	movq	%rsp, %rdx
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 	UNWIND_HINT_IRET_REGS base=%rdx offset=8
@@ -1389,6 +1395,8 @@ ENTRY(nmi)
 	pushq	2*8(%rdx)	/* pt_regs->cs */
 	pushq	1*8(%rdx)	/* pt_regs->rip */
 	UNWIND_HINT_IRET_REGS
+.Lnmi_keep_stack:
+
 	pushq   $-1		/* pt_regs->orig_ax */
 	PUSH_AND_CLEAR_REGS rdx=(%rdx)
 	ENCODE_FRAME_POINTER
--- 4.20-rc3/arch/x86/entry/entry_64_compat.S
+++ 4.20-rc3-x86_64-stack-switch-Xen/arch/x86/entry/entry_64_compat.S
@@ -361,17 +361,23 @@ ENTRY(entry_INT80_compat)
 
 	/* Need to switch before accessing the thread stack. */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
+
+	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rdi
+	subq	$8, %rdi
+	xorq	%rsp, %rdi
+	shrq	$PAGE_SHIFT, %rdi
+	jz	.Lint80_keep_stack
 	movq	%rsp, %rdi
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
-
 	pushq	6*8(%rdi)		/* regs->ss */
 	pushq	5*8(%rdi)		/* regs->rsp */
 	pushq	4*8(%rdi)		/* regs->eflags */
 	pushq	3*8(%rdi)		/* regs->cs */
 	pushq	2*8(%rdi)		/* regs->ip */
 	pushq	1*8(%rdi)		/* regs->orig_ax */
-
 	pushq	(%rdi)			/* pt_regs->di */
+.Lint80_keep_stack:
+
 	pushq	%rsi			/* pt_regs->si */
 	xorl	%esi, %esi		/* nospec   si */
 	pushq	%rdx			/* pt_regs->dx */

Re: [PATCH v2] x86-64/Xen: fix stack switching

[Andy Lutomirski]

On Wed, Nov 21, 2018 at 2:10 AM Jan Beulich <JBeulich@suse.com> wrote:
>
> While in the native case entry into the kernel happens on the trampoline
> stack, PV Xen kernels get entered with the current thread stack right
> away. Hence source and destination stacks are identical in that case,
> and special care is needed.
>
> Other than in sync_regs() the copying done on the INT80 path as well as
> on the NMI path itself isn't NMI / #MC safe, as either of these events
> occurring in the middle of the stack copying would clobber data on the
> (source) stack. (Of course, in the NMI case only #MC could break
> things.)
>
> I'm not altering the similar code in interrupt_entry(), as that code
> path is unreachable afaict when running PV Xen guests.
>
> Fixes: 7f2590a110b837af5679d08fc25c6227c5a8c497
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> Cc: stable@kernel.org
> ---
> v2: Correct placement of .Lint80_keep_stack label.
> ---
>  arch/x86/entry/entry_64.S        |    8 ++++++++
>  arch/x86/entry/entry_64_compat.S |   10 ++++++++--
>  2 files changed, 16 insertions(+), 2 deletions(-)
>
> --- 4.20-rc3/arch/x86/entry/entry_64.S
> +++ 4.20-rc3-x86_64-stack-switch-Xen/arch/x86/entry/entry_64.S
> @@ -1380,6 +1380,12 @@ ENTRY(nmi)
>         swapgs
>         cld
>         SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
> +
> +       movq    PER_CPU_VAR(cpu_current_top_of_stack), %rdx
> +       subq    $8, %rdx
> +       xorq    %rsp, %rdx
> +       shrq    $PAGE_SHIFT, %rdx
> +       jz      .Lnmi_keep_stack

This code shouldn't even be reachable on Xen:

commit 43e4111086a70c78bedb6ad990bee97f17b27a6e
Author: Juergen Gross <jgross@suse.com>
Date:   Thu Nov 2 00:59:07 2017 -0700

    xen, x86/entry/64: Add xen NMI trap entry

    Instead of trying to execute any NMI via the bare metal's NMI trap
    handler use a Xen specific one for PV domains, like we do for e.g.
    debug traps. As in a PV domain the NMI is handled via the normal
    kernel stack this is the correct thing to do.

    This will enable us to get rid of the very fragile and questionable
    dependencies between the bare metal NMI handler and Xen assumptions
    believed to be broken anyway.


>         movq    %rsp, %rdx
>         movq    PER_CPU_VAR(cpu_current_top_of_stack), %rsp
>         UNWIND_HINT_IRET_REGS base=%rdx offset=8
> @@ -1389,6 +1395,8 @@ ENTRY(nmi)
>         pushq   2*8(%rdx)       /* pt_regs->cs */
>         pushq   1*8(%rdx)       /* pt_regs->rip */
>         UNWIND_HINT_IRET_REGS
> +.Lnmi_keep_stack:
> +
>         pushq   $-1             /* pt_regs->orig_ax */
>         PUSH_AND_CLEAR_REGS rdx=(%rdx)
>         ENCODE_FRAME_POINTER
> --- 4.20-rc3/arch/x86/entry/entry_64_compat.S
> +++ 4.20-rc3-x86_64-stack-switch-Xen/arch/x86/entry/entry_64_compat.S
> @@ -361,17 +361,23 @@ ENTRY(entry_INT80_compat)
>
>         /* Need to switch before accessing the thread stack. */
>         SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
> +
> +       movq    PER_CPU_VAR(cpu_current_top_of_stack), %rdi
> +       subq    $8, %rdi
> +       xorq    %rsp, %rdi
> +       shrq    $PAGE_SHIFT, %rdi
> +       jz      .Lint80_keep_stack

This comparison is IMO the wrong test entirely.  How about something like:

/* On Xen PV, entry_INT80_compat is called on the thread stack, so
rewinding to the top of the thread stack would allow an NMI to
overwrite the hardware frame before we copy it. */
ALTERNATIVE "", "jmp .Lint80_keep_stack", X86_FEATURE_XENPV

Re: [PATCH v2] x86-64/Xen: fix stack switching

[Jan Beulich]

>>> On 21.11.18 at 16:24, <luto@kernel.org> wrote:
> On Wed, Nov 21, 2018 at 2:10 AM Jan Beulich <JBeulich@suse.com> wrote:
>> --- 4.20-rc3/arch/x86/entry/entry_64.S
>> +++ 4.20-rc3-x86_64-stack-switch-Xen/arch/x86/entry/entry_64.S
>> @@ -1380,6 +1380,12 @@ ENTRY(nmi)
>>         swapgs
>>         cld
>>         SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
>> +
>> +       movq    PER_CPU_VAR(cpu_current_top_of_stack), %rdx
>> +       subq    $8, %rdx
>> +       xorq    %rsp, %rdx
>> +       shrq    $PAGE_SHIFT, %rdx
>> +       jz      .Lnmi_keep_stack
> 
> This code shouldn't even be reachable on Xen:
> 
> commit 43e4111086a70c78bedb6ad990bee97f17b27a6e
> Author: Juergen Gross <jgross@suse.com>
> Date:   Thu Nov 2 00:59:07 2017 -0700
> 
>     xen, x86/entry/64: Add xen NMI trap entry
> 
>     Instead of trying to execute any NMI via the bare metal's NMI trap
>     handler use a Xen specific one for PV domains, like we do for e.g.
>     debug traps. As in a PV domain the NMI is handled via the normal
>     kernel stack this is the correct thing to do.
> 
>     This will enable us to get rid of the very fragile and questionable
>     dependencies between the bare metal NMI handler and Xen assumptions
>     believed to be broken anyway.

Oh, I didn't notice this. The beginnings of the patch here pre-date
this, though, and then I didn't notice the addition. Thanks for
pointing this out.

>> --- 4.20-rc3/arch/x86/entry/entry_64_compat.S
>> +++ 4.20-rc3-x86_64-stack-switch-Xen/arch/x86/entry/entry_64_compat.S
>> @@ -361,17 +361,23 @@ ENTRY(entry_INT80_compat)
>>
>>         /* Need to switch before accessing the thread stack. */
>>         SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
>> +
>> +       movq    PER_CPU_VAR(cpu_current_top_of_stack), %rdi
>> +       subq    $8, %rdi
>> +       xorq    %rsp, %rdi
>> +       shrq    $PAGE_SHIFT, %rdi
>> +       jz      .Lint80_keep_stack
> 
> This comparison is IMO the wrong test entirely.  How about something like:
> 
> /* On Xen PV, entry_INT80_compat is called on the thread stack, so
> rewinding to the top of the thread stack would allow an NMI to
> overwrite the hardware frame before we copy it. */
> ALTERNATIVE "", "jmp .Lint80_keep_stack", X86_FEATURE_XENPV

Indeed I had noted this as an alternative option in v1, but
didn't get respective feedback. If that's the preferred route, I'll
certainly switch.

Jan

[PATCH v3] x86-64/Xen: fix stack switching

[Jan Beulich]

While in the native case entry into the kernel happens on the trampoline
stack, PV Xen kernels get entered with the current thread stack right
away. Hence source and destination stacks are identical in that case,
and special care is needed.

Other than in sync_regs() the copying done on the INT80 path isn't
NMI / #MC safe, as either of these events occurring in the middle of the
stack copying would clobber data on the (source) stack.

I'm not altering the similar code in interrupt_entry() and nmi(), as
those code paths are unreachable afaict when running PV Xen guests.

Fixes: 7f2590a110b837af5679d08fc25c6227c5a8c497
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@kernel.org 
---
v3: Drop NMI path change. Use ALTERNATIVE.
v2: Correct placement of .Lint80_keep_stack label.
---
 arch/x86/entry/entry_64_compat.S |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- 5.0-rc2/arch/x86/entry/entry_64_compat.S
+++ 5.0-rc2-x86_64-stack-switch-Xen/arch/x86/entry/entry_64_compat.S
@@ -361,7 +361,8 @@ ENTRY(entry_INT80_compat)
 
 	/* Need to switch before accessing the thread stack. */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
-	movq	%rsp, %rdi
+	/* In the Xen PV case we already run on the thread stack. */
+	ALTERNATIVE "movq %rsp, %rdi", "jmp .Lint80_keep_stack", X86_FEATURE_XENPV
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
 	pushq	6*8(%rdi)		/* regs->ss */
@@ -370,8 +371,9 @@ ENTRY(entry_INT80_compat)
 	pushq	3*8(%rdi)		/* regs->cs */
 	pushq	2*8(%rdi)		/* regs->ip */
 	pushq	1*8(%rdi)		/* regs->orig_ax */
-
 	pushq	(%rdi)			/* pt_regs->di */
+.Lint80_keep_stack:
+
 	pushq	%rsi			/* pt_regs->si */
 	xorl	%esi, %esi		/* nospec   si */
 	pushq	%rdx			/* pt_regs->dx */

Re: [PATCH v3] x86-64/Xen: fix stack switching

[Andy Lutomirski]

On Tue, Jan 15, 2019 at 8:58 AM Jan Beulich <JBeulich@suse.com> wrote:
>
> While in the native case entry into the kernel happens on the trampoline
> stack, PV Xen kernels get entered with the current thread stack right
> away. Hence source and destination stacks are identical in that case,
> and special care is needed.
>
> Other than in sync_regs() the copying done on the INT80 path isn't
> NMI / #MC safe, as either of these events occurring in the middle of the
> stack copying would clobber data on the (source) stack.
>
> I'm not altering the similar code in interrupt_entry() and nmi(), as
> those code paths are unreachable afaict when running PV Xen guests.


Acked-by: Andy Lutomirski <luto@kernel.org>

[tip:x86/urgent] x86/entry/64/compat: Fix stack switching for XEN PV

[tip-bot for Jan Beulich]

Commit-ID:  fc24d75a7f91837d7918e40719575951820b2b8f
Gitweb:     https://git.kernel.org/tip/fc24d75a7f91837d7918e40719575951820b2b8f
Author:     Jan Beulich <JBeulich@suse.com>
AuthorDate: Tue, 15 Jan 2019 09:58:16 -0700
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Fri, 18 Jan 2019 00:39:33 +0100

x86/entry/64/compat: Fix stack switching for XEN PV

While in the native case entry into the kernel happens on the trampoline
stack, PV Xen kernels get entered with the current thread stack right
away. Hence source and destination stacks are identical in that case,
and special care is needed.

Other than in sync_regs() the copying done on the INT80 path isn't
NMI / #MC safe, as either of these events occurring in the middle of the
stack copying would clobber data on the (source) stack.

There is similar code in interrupt_entry() and nmi(), but there is no fixup
required because those code paths are unreachable in XEN PV guests.

[ tglx: Sanitized subject, changelog, Fixes tag and stable mail address. Sigh ]

Fixes: 7f2590a110b8 ("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Peter Anvin <hpa@zytor.com>
Cc: xen-devel@lists.xenproject.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/5C3E1128020000780020DFAD@prv1-mh.provo.novell.com

---
 arch/x86/entry/entry_64_compat.S | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 8eaf8952c408..39913770a44d 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -361,7 +361,8 @@ ENTRY(entry_INT80_compat)
 
 	/* Need to switch before accessing the thread stack. */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
-	movq	%rsp, %rdi
+	/* In the Xen PV case we already run on the thread stack. */
+	ALTERNATIVE "movq %rsp, %rdi", "jmp .Lint80_keep_stack", X86_FEATURE_XENPV
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
 	pushq	6*8(%rdi)		/* regs->ss */
@@ -370,8 +371,9 @@ ENTRY(entry_INT80_compat)
 	pushq	3*8(%rdi)		/* regs->cs */
 	pushq	2*8(%rdi)		/* regs->ip */
 	pushq	1*8(%rdi)		/* regs->orig_ax */
-
 	pushq	(%rdi)			/* pt_regs->di */
+.Lint80_keep_stack:
+
 	pushq	%rsi			/* pt_regs->si */
 	xorl	%esi, %esi		/* nospec   si */
 	pushq	%rdx			/* pt_regs->dx */