that the first NMI will iret to. However, the copy of registers
modified is exactly the one that is the part of pt_regs in
the first NMI. This can change the behaviour of the first NMI.
In particular, Google's arch_trigger_all_cpu_backtrace handler
also prints regions of memory surrounding addresses appearing in
registers. This results in handled exceptions, after which nested NMIs
start coming in. These nested NMIs change the value of registers
in pt_regs. This can cause the original NMI handler to produce
incorrect output.
We solve this problem by interchanging the position of the preserved
copy of the iret registers ("saved") and the copy subject to being
trampled by nested NMI ("copied").
Signed-off-by: Salman Qazi <[email protected]>
---
arch/x86/kernel/entry_64.S | 41 +++++++++++++++++++++++++++--------------
1 files changed, 27 insertions(+), 14 deletions(-)
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 44531ac..b5d6e43 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1739,9 +1739,10 @@ nested_nmi:
1:
/* Set up the interrupted NMIs stack to jump to repeat_nmi */
- leaq -6*8(%rsp), %rdx
+ leaq -1*8(%rsp), %rdx
movq %rdx, %rsp
- CFI_ADJUST_CFA_OFFSET 6*8
+ CFI_ADJUST_CFA_OFFSET 1*8
+ leaq -10*8(%rsp), %rdx
pushq_cfi $__KERNEL_DS
pushq_cfi %rdx
pushfq_cfi
@@ -1749,8 +1750,8 @@ nested_nmi:
pushq_cfi $repeat_nmi
/* Put stack back */
- addq $(11*8), %rsp
- CFI_ADJUST_CFA_OFFSET -11*8
+ addq $(6*8), %rsp
+ CFI_ADJUST_CFA_OFFSET -6*8
nested_nmi_out:
popq_cfi %rdx
@@ -1776,18 +1777,18 @@ first_nmi:
* +-------------------------+
* | NMI executing variable |
* +-------------------------+
- * | Saved SS |
- * | Saved Return RSP |
- * | Saved RFLAGS |
- * | Saved CS |
- * | Saved RIP |
- * +-------------------------+
* | copied SS |
* | copied Return RSP |
* | copied RFLAGS |
* | copied CS |
* | copied RIP |
* +-------------------------+
+ * | Saved SS |
+ * | Saved Return RSP |
+ * | Saved RFLAGS |
+ * | Saved CS |
+ * | Saved RIP |
+ * +-------------------------+
* | pt_regs |
* +-------------------------+
*
@@ -1803,9 +1804,14 @@ first_nmi:
/* Set the NMI executing variable on the stack. */
pushq_cfi $1
+ /*
+ * Leave room for the "copied" frame
+ */
+ subq $(5*8), %rsp
+
/* Copy the stack frame to the Saved frame */
.rept 5
- pushq_cfi 6*8(%rsp)
+ pushq_cfi 11*8(%rsp)
.endr
CFI_DEF_CFA_OFFSET SS+8-RIP
@@ -1826,12 +1832,15 @@ repeat_nmi:
* is benign for the non-repeat case, where 1 was pushed just above
* to this very stack slot).
*/
- movq $1, 5*8(%rsp)
+ movq $1, 10*8(%rsp)
/* Make another copy, this one may be modified by nested NMIs */
+ addq $(10*8), %rsp
.rept 5
- pushq_cfi 4*8(%rsp)
+ pushq_cfi -6*8(%rsp)
.endr
+ subq $(5*8), %rsp
+
CFI_DEF_CFA_OFFSET SS+8-RIP
end_repeat_nmi:
@@ -1882,8 +1891,12 @@ nmi_swapgs:
SWAPGS_UNSAFE_STACK
nmi_restore:
RESTORE_ALL 8
+
+ /* Pop the extra iret frame */
+ addq $(5*8), %rsp
+
/* Clear the NMI executing stack variable */
- movq $0, 10*8(%rsp)
+ movq $0, 5*8(%rsp)
jmp irq_return
CFI_ENDPROC
END(nmi)
The nested NMI modifies the place (instruction, flags and stack)
that the first NMI will iret to. However, the copy of registers
modified is exactly the one that is the part of pt_regs in
the first NMI. This can change the behaviour of the first NMI.
In particular, Google's arch_trigger_all_cpu_backtrace handler
also prints regions of memory surrounding addresses appearing in
registers. This results in handled exceptions, after which nested NMIs
start coming in. These nested NMIs change the value of registers
in pt_regs. This can cause the original NMI handler to produce
incorrect output.
We solve this problem by interchanging the position of the preserved
copy of the iret registers ("saved") and the copy subject to being
trampled by nested NMI ("copied").
Signed-off-by: Salman Qazi <[email protected]>
---
arch/x86/kernel/entry_64.S | 41 +++++++++++++++++++++++++++--------------
1 files changed, 27 insertions(+), 14 deletions(-)
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 44531ac..b5d6e43 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1739,9 +1739,10 @@ nested_nmi:
1:
/* Set up the interrupted NMIs stack to jump to repeat_nmi */
- leaq -6*8(%rsp), %rdx
+ leaq -1*8(%rsp), %rdx
movq %rdx, %rsp
- CFI_ADJUST_CFA_OFFSET 6*8
+ CFI_ADJUST_CFA_OFFSET 1*8
+ leaq -10*8(%rsp), %rdx
pushq_cfi $__KERNEL_DS
pushq_cfi %rdx
pushfq_cfi
@@ -1749,8 +1750,8 @@ nested_nmi:
pushq_cfi $repeat_nmi
/* Put stack back */
- addq $(11*8), %rsp
- CFI_ADJUST_CFA_OFFSET -11*8
+ addq $(6*8), %rsp
+ CFI_ADJUST_CFA_OFFSET -6*8
nested_nmi_out:
popq_cfi %rdx
@@ -1776,18 +1777,18 @@ first_nmi:
* +-------------------------+
* | NMI executing variable |
* +-------------------------+
- * | Saved SS |
- * | Saved Return RSP |
- * | Saved RFLAGS |
- * | Saved CS |
- * | Saved RIP |
- * +-------------------------+
* | copied SS |
* | copied Return RSP |
* | copied RFLAGS |
* | copied CS |
* | copied RIP |
* +-------------------------+
+ * | Saved SS |
+ * | Saved Return RSP |
+ * | Saved RFLAGS |
+ * | Saved CS |
+ * | Saved RIP |
+ * +-------------------------+
* | pt_regs |
* +-------------------------+
*
@@ -1803,9 +1804,14 @@ first_nmi:
/* Set the NMI executing variable on the stack. */
pushq_cfi $1
+ /*
+ * Leave room for the "copied" frame
+ */
+ subq $(5*8), %rsp
+
/* Copy the stack frame to the Saved frame */
.rept 5
- pushq_cfi 6*8(%rsp)
+ pushq_cfi 11*8(%rsp)
.endr
CFI_DEF_CFA_OFFSET SS+8-RIP
@@ -1826,12 +1832,15 @@ repeat_nmi:
* is benign for the non-repeat case, where 1 was pushed just above
* to this very stack slot).
*/
- movq $1, 5*8(%rsp)
+ movq $1, 10*8(%rsp)
/* Make another copy, this one may be modified by nested NMIs */
+ addq $(10*8), %rsp
.rept 5
- pushq_cfi 4*8(%rsp)
+ pushq_cfi -6*8(%rsp)
.endr
+ subq $(5*8), %rsp
+
CFI_DEF_CFA_OFFSET SS+8-RIP
end_repeat_nmi:
@@ -1882,8 +1891,12 @@ nmi_swapgs:
SWAPGS_UNSAFE_STACK
nmi_restore:
RESTORE_ALL 8
+
+ /* Pop the extra iret frame */
+ addq $(5*8), %rsp
+
/* Clear the NMI executing stack variable */
- movq $0, 10*8(%rsp)
+ movq $0, 5*8(%rsp)
jmp irq_return
CFI_ENDPROC
END(nmi)
On Mon, 2012-10-01 at 17:29 -0700, Salman Qazi wrote:
> The nested NMI modifies the place (instruction, flags and stack)
> that the first NMI will iret to. However, the copy of registers
> modified is exactly the one that is the part of pt_regs in
> the first NMI. This can change the behaviour of the first NMI.
>
> In particular, Google's arch_trigger_all_cpu_backtrace handler
> also prints regions of memory surrounding addresses appearing in
> registers. This results in handled exceptions, after which nested NMIs
> start coming in. These nested NMIs change the value of registers
> in pt_regs. This can cause the original NMI handler to produce
> incorrect output.
>
> We solve this problem by interchanging the position of the preserved
> copy of the iret registers ("saved") and the copy subject to being
> trampled by nested NMI ("copied").
>
I was all ready to push this forward, but on my last final review I
found some nits that prevent me from doing so.
> Signed-off-by: Salman Qazi <[email protected]>
> ---
> arch/x86/kernel/entry_64.S | 41 +++++++++++++++++++++++++++--------------
> 1 files changed, 27 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 44531ac..b5d6e43 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -1739,9 +1739,10 @@ nested_nmi:
>
> 1:
> /* Set up the interrupted NMIs stack to jump to repeat_nmi */
> - leaq -6*8(%rsp), %rdx
> + leaq -1*8(%rsp), %rdx
> movq %rdx, %rsp
> - CFI_ADJUST_CFA_OFFSET 6*8
> + CFI_ADJUST_CFA_OFFSET 1*8
> + leaq -10*8(%rsp), %rdx
> pushq_cfi $__KERNEL_DS
> pushq_cfi %rdx
> pushfq_cfi
> @@ -1749,8 +1750,8 @@ nested_nmi:
> pushq_cfi $repeat_nmi
>
> /* Put stack back */
> - addq $(11*8), %rsp
> - CFI_ADJUST_CFA_OFFSET -11*8
> + addq $(6*8), %rsp
> + CFI_ADJUST_CFA_OFFSET -6*8
>
> nested_nmi_out:
> popq_cfi %rdx
> @@ -1776,18 +1777,18 @@ first_nmi:
> * +-------------------------+
> * | NMI executing variable |
> * +-------------------------+
> - * | Saved SS |
> - * | Saved Return RSP |
> - * | Saved RFLAGS |
> - * | Saved CS |
> - * | Saved RIP |
> - * +-------------------------+
> * | copied SS |
> * | copied Return RSP |
> * | copied RFLAGS |
> * | copied CS |
> * | copied RIP |
> * +-------------------------+
> + * | Saved SS |
> + * | Saved Return RSP |
> + * | Saved RFLAGS |
> + * | Saved CS |
> + * | Saved RIP |
> + * +-------------------------+
> * | pt_regs |
> * +-------------------------+
> *
> @@ -1803,9 +1804,14 @@ first_nmi:
> /* Set the NMI executing variable on the stack. */
> pushq_cfi $1
>
> + /*
> + * Leave room for the "copied" frame
> + */
> + subq $(5*8), %rsp
> +
> /* Copy the stack frame to the Saved frame */
> .rept 5
> - pushq_cfi 6*8(%rsp)
> + pushq_cfi 11*8(%rsp)
> .endr
> CFI_DEF_CFA_OFFSET SS+8-RIP
>
> @@ -1826,12 +1832,15 @@ repeat_nmi:
> * is benign for the non-repeat case, where 1 was pushed just above
> * to this very stack slot).
> */
> - movq $1, 5*8(%rsp)
> + movq $1, 10*8(%rsp)
>
> /* Make another copy, this one may be modified by nested NMIs */
> + addq $(10*8), %rsp
This breaks the CFI magic.
> .rept 5
> - pushq_cfi 4*8(%rsp)
> + pushq_cfi -6*8(%rsp)
> .endr
> + subq $(5*8), %rsp
So does this.
This needs to be annotated correctly before I can push it out. But the
good news is, I stressed tested this change, and it all works out.
Jan, can you help out here?
-- Steve
> +
> CFI_DEF_CFA_OFFSET SS+8-RIP
> end_repeat_nmi:
>
> @@ -1882,8 +1891,12 @@ nmi_swapgs:
> SWAPGS_UNSAFE_STACK
> nmi_restore:
> RESTORE_ALL 8
> +
> + /* Pop the extra iret frame */
> + addq $(5*8), %rsp
> +
> /* Clear the NMI executing stack variable */
> - movq $0, 10*8(%rsp)
> + movq $0, 5*8(%rsp)
> jmp irq_return
> CFI_ENDPROC
> END(nmi)
>>> Steven Rostedt <[email protected]> 11/01/12 2:04 AM >>>
>On Mon, 2012-10-01 at 17:29 -0700, Salman Qazi wrote:
>> @@ -1826,12 +1832,15 @@ repeat_nmi:
>> * is benign for the non-repeat case, where 1 was pushed just above
>> * to this very stack slot).
>> */
>> - movq $1, 5*8(%rsp)
>> + movq $1, 10*8(%rsp)
>>
>> /* Make another copy, this one may be modified by nested NMIs */
>> + addq $(10*8), %rsp
>
>This breaks the CFI magic.
>
>> .rept 5
>> - pushq_cfi 4*8(%rsp)
>> + pushq_cfi -6*8(%rsp)
>> .endr
>> + subq $(5*8), %rsp
>
>So does this.
>
>This needs to be annotated correctly before I can push it out. But the
>good news is, I stressed tested this change, and it all works out.
>
>Jan, can you help out here?
There doesn't appear to be anything special about these adjustments, so I
don't see what help would be required here - it ought to be the normal use
of CFI_ADJUST_CFA_OFFSET that needs adding.
Jan
On Thu, 2012-11-01 at 19:53 +0000, Jan Beulich wrote:
> >>> Steven Rostedt <[email protected]> 11/01/12 2:04 AM >>>
> >On Mon, 2012-10-01 at 17:29 -0700, Salman Qazi wrote:
> >> @@ -1826,12 +1832,15 @@ repeat_nmi:
> >> * is benign for the non-repeat case, where 1 was pushed just above
> >> * to this very stack slot).
> >> */
> >> - movq $1, 5*8(%rsp)
> >> + movq $1, 10*8(%rsp)
> >>
> >> /* Make another copy, this one may be modified by nested NMIs */
> >> + addq $(10*8), %rsp
> >
> >This breaks the CFI magic.
> >
> >> .rept 5
> >> - pushq_cfi 4*8(%rsp)
> >> + pushq_cfi -6*8(%rsp)
> >> .endr
> >> + subq $(5*8), %rsp
> >
> >So does this.
> >
> >This needs to be annotated correctly before I can push it out. But the
> >good news is, I stressed tested this change, and it all works out.
> >
> >Jan, can you help out here?
>
> There doesn't appear to be anything special about these adjustments, so I
> don't see what help would be required here - it ought to be the normal use
> of CFI_ADJUST_CFA_OFFSET that needs adding.
Even the simple CFI adjustments look like magic to me :-) OK, I'll
update the patch and send it out. I'll Cc you in case I screw up even
the most simple case ;-)
-- Steve
On Thu, 2012-11-01 at 19:53 +0000, Jan Beulich wrote:
> There doesn't appear to be anything special about these adjustments, so I
> don't see what help would be required here - it ought to be the normal use
> of CFI_ADJUST_CFA_OFFSET that needs adding.
This change look fine to you?
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 52edf92..7ba5342 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1796,10 +1796,12 @@ repeat_nmi:
/* Make another copy, this one may be modified by nested NMIs */
addq $(10*8), %rsp
+ CFI_ADJUST_CFA_OFFSET -10*8
.rept 5
pushq_cfi -6*8(%rsp)
.endr
subq $(5*8), %rsp
+ CFI_ADJUST_CFA_OFFSET 5*8
CFI_DEF_CFA_OFFSET SS+8-RIP
end_repeat_nmi:
-- Steve
On Fri, 2012-11-02 at 09:51 -0400, Steven Rostedt wrote:
> On Thu, 2012-11-01 at 19:53 +0000, Jan Beulich wrote:
>
> > There doesn't appear to be anything special about these adjustments, so I
> > don't see what help would be required here - it ought to be the normal use
> > of CFI_ADJUST_CFA_OFFSET that needs adding.
>
> This change look fine to you?
>
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 52edf92..7ba5342 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -1796,10 +1796,12 @@ repeat_nmi:
>
> /* Make another copy, this one may be modified by nested NMIs */
> addq $(10*8), %rsp
> + CFI_ADJUST_CFA_OFFSET -10*8
> .rept 5
> pushq_cfi -6*8(%rsp)
> .endr
> subq $(5*8), %rsp
> + CFI_ADJUST_CFA_OFFSET 5*8
>
> CFI_DEF_CFA_OFFSET SS+8-RIP
> end_repeat_nmi:
>
Is that second one even needed? Or will the CFI_DEF_CFA_OFFSET SS+8-RIP
fix it?
-- Steve
>>> On 02.11.12 at 14:53, Steven Rostedt <[email protected]> wrote:
> On Fri, 2012-11-02 at 09:51 -0400, Steven Rostedt wrote:
>> On Thu, 2012-11-01 at 19:53 +0000, Jan Beulich wrote:
>>
>> > There doesn't appear to be anything special about these adjustments, so I
>> > don't see what help would be required here - it ought to be the normal use
>> > of CFI_ADJUST_CFA_OFFSET that needs adding.
>>
>> This change look fine to you?
>>
>> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
>> index 52edf92..7ba5342 100644
>> --- a/arch/x86/kernel/entry_64.S
>> +++ b/arch/x86/kernel/entry_64.S
>> @@ -1796,10 +1796,12 @@ repeat_nmi:
>>
>> /* Make another copy, this one may be modified by nested NMIs */
>> addq $(10*8), %rsp
>> + CFI_ADJUST_CFA_OFFSET -10*8
>> .rept 5
>> pushq_cfi -6*8(%rsp)
>> .endr
>> subq $(5*8), %rsp
>> + CFI_ADJUST_CFA_OFFSET 5*8
>>
>> CFI_DEF_CFA_OFFSET SS+8-RIP
>> end_repeat_nmi:
>>
>
> Is that second one even needed? Or will the CFI_DEF_CFA_OFFSET SS+8-RIP
> fix it?
Yes it will (as long as no intervening instructions get added; that's
to say that I'd recommend removing the blank line to make clear
that instruction and annotation belong together).
Jan
On Fri, 2012-11-02 at 14:09 +0000, Jan Beulich wrote:
> >> subq $(5*8), %rsp
> >> + CFI_ADJUST_CFA_OFFSET 5*8
> >>
> >> CFI_DEF_CFA_OFFSET SS+8-RIP
> >> end_repeat_nmi:
> >>
> >
> > Is that second one even needed? Or will the CFI_DEF_CFA_OFFSET SS+8-RIP
> > fix it?
>
> Yes it will (as long as no intervening instructions get added; that's
> to say that I'd recommend removing the blank line to make clear
> that instruction and annotation belong together).
OK, will do.
Thanks!
-- Steve
Commit-ID: 28696f434fef0efa97534b59986ad33b9c4df7f8
Gitweb: http://git.kernel.org/tip/28696f434fef0efa97534b59986ad33b9c4df7f8
Author: Salman Qazi <[email protected]>
AuthorDate: Mon, 1 Oct 2012 17:29:25 -0700
Committer: Steven Rostedt <[email protected]>
CommitDate: Fri, 2 Nov 2012 11:29:36 -0400
x86: Don't clobber top of pt_regs in nested NMI
The nested NMI modifies the place (instruction, flags and stack)
that the first NMI will iret to. However, the copy of registers
modified is exactly the one that is the part of pt_regs in
the first NMI. This can change the behaviour of the first NMI.
In particular, Google's arch_trigger_all_cpu_backtrace handler
also prints regions of memory surrounding addresses appearing in
registers. This results in handled exceptions, after which nested NMIs
start coming in. These nested NMIs change the value of registers
in pt_regs. This can cause the original NMI handler to produce
incorrect output.
We solve this problem by interchanging the position of the preserved
copy of the iret registers ("saved") and the copy subject to being
trampled by nested NMI ("copied").
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Salman Qazi <[email protected]>
[ Added a needed CFI_ADJUST_CFA_OFFSET ]
Signed-off-by: Steven Rostedt <[email protected]>
---
arch/x86/kernel/entry_64.S | 41 +++++++++++++++++++++++++++--------------
1 files changed, 27 insertions(+), 14 deletions(-)
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index b51b2c7..811795d 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1699,9 +1699,10 @@ nested_nmi:
1:
/* Set up the interrupted NMIs stack to jump to repeat_nmi */
- leaq -6*8(%rsp), %rdx
+ leaq -1*8(%rsp), %rdx
movq %rdx, %rsp
- CFI_ADJUST_CFA_OFFSET 6*8
+ CFI_ADJUST_CFA_OFFSET 1*8
+ leaq -10*8(%rsp), %rdx
pushq_cfi $__KERNEL_DS
pushq_cfi %rdx
pushfq_cfi
@@ -1709,8 +1710,8 @@ nested_nmi:
pushq_cfi $repeat_nmi
/* Put stack back */
- addq $(11*8), %rsp
- CFI_ADJUST_CFA_OFFSET -11*8
+ addq $(6*8), %rsp
+ CFI_ADJUST_CFA_OFFSET -6*8
nested_nmi_out:
popq_cfi %rdx
@@ -1736,18 +1737,18 @@ first_nmi:
* +-------------------------+
* | NMI executing variable |
* +-------------------------+
- * | Saved SS |
- * | Saved Return RSP |
- * | Saved RFLAGS |
- * | Saved CS |
- * | Saved RIP |
- * +-------------------------+
* | copied SS |
* | copied Return RSP |
* | copied RFLAGS |
* | copied CS |
* | copied RIP |
* +-------------------------+
+ * | Saved SS |
+ * | Saved Return RSP |
+ * | Saved RFLAGS |
+ * | Saved CS |
+ * | Saved RIP |
+ * +-------------------------+
* | pt_regs |
* +-------------------------+
*
@@ -1763,9 +1764,14 @@ first_nmi:
/* Set the NMI executing variable on the stack. */
pushq_cfi $1
+ /*
+ * Leave room for the "copied" frame
+ */
+ subq $(5*8), %rsp
+
/* Copy the stack frame to the Saved frame */
.rept 5
- pushq_cfi 6*8(%rsp)
+ pushq_cfi 11*8(%rsp)
.endr
CFI_DEF_CFA_OFFSET SS+8-RIP
@@ -1786,12 +1792,15 @@ repeat_nmi:
* is benign for the non-repeat case, where 1 was pushed just above
* to this very stack slot).
*/
- movq $1, 5*8(%rsp)
+ movq $1, 10*8(%rsp)
/* Make another copy, this one may be modified by nested NMIs */
+ addq $(10*8), %rsp
+ CFI_ADJUST_CFA_OFFSET -10*8
.rept 5
- pushq_cfi 4*8(%rsp)
+ pushq_cfi -6*8(%rsp)
.endr
+ subq $(5*8), %rsp
CFI_DEF_CFA_OFFSET SS+8-RIP
end_repeat_nmi:
@@ -1842,8 +1851,12 @@ nmi_swapgs:
SWAPGS_UNSAFE_STACK
nmi_restore:
RESTORE_ALL 8
+
+ /* Pop the extra iret frame */
+ addq $(5*8), %rsp
+
/* Clear the NMI executing stack variable */
- movq $0, 10*8(%rsp)
+ movq $0, 5*8(%rsp)
jmp irq_return
CFI_ENDPROC
END(nmi)