LinuxLists.cc - [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception

2019-06-06 07:32:06

Subject: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception

2019-06-06 08:22:57

Subject: Re: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception

Ravi Bangoria wrote:
> Powerpc hw triggers watchpoint before executing the instruction.
> To make trigger-after-execute behavior, kernel emulates the
> instruction. If the instruction is 'load something into non-
> volatile register', exception handler should restore emulated
> register state while returning back, otherwise there will be
> register state corruption. Ex, Adding a watchpoint on a list
> can corrput the list:
>
> # cat /proc/kallsyms | grep kthread_create_list
> c00000000121c8b8 d kthread_create_list
>
> Add watchpoint on kthread_create_list->next:
>
> # perf record -e mem:0xc00000000121c8c0
>
> Run some workload such that new kthread gets invoked. Ex, I
> just logged out from console:
>
> list_add corruption. next->prev should be prev (c000000001214e00), \
> but was c00000000121c8b8. (next=c00000000121c8b8).
> WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0
> CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69
> ...
> NIP __list_add_valid+0xb4/0xc0
> LR __list_add_valid+0xb0/0xc0
> Call Trace:
> __list_add_valid+0xb0/0xc0 (unreliable)
> __kthread_create_on_node+0xe0/0x260
> kthread_create_on_node+0x34/0x50
> create_worker+0xe8/0x260
> worker_thread+0x444/0x560
> kthread+0x160/0x1a0
> ret_from_kernel_thread+0x5c/0x70
>
> Signed-off-by: Ravi Bangoria <[email protected]>
> ---
> arch/powerpc/kernel/exceptions-64s.S | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>

Awesome catch - this one has had a glorious run...
Fixes: 5aae8a5370802 ("powerpc, hw_breakpoints: Implement hw_breakpoints for 64-bit server processors")

Reviewed-by: Naveen N. Rao <[email protected]>

- Naveen

2019-06-06 08:34:03

by Ravi Bangoria

[permalink] [raw]

Subject: Re: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception

On 6/6/19 12:59 PM, Ravi Bangoria wrote:
> Powerpc hw triggers watchpoint before executing the instruction.
> To make trigger-after-execute behavior, kernel emulates the
> instruction. If the instruction is 'load something into non-
> volatile register', exception handler should restore emulated
> register state while returning back, otherwise there will be
> register state corruption. Ex, Adding a watchpoint on a list
> can corrput the list:
>
> # cat /proc/kallsyms | grep kthread_create_list
> c00000000121c8b8 d kthread_create_list
>
> Add watchpoint on kthread_create_list->next:

s/kthread_create_list->next/kthread_create_list->prev/

2019-06-07 01:05:59

by Michael Neuling

[permalink] [raw]

Subject: Re: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception

On Thu, 2019-06-06 at 12:59 +0530, Ravi Bangoria wrote:
> Powerpc hw triggers watchpoint before executing the instruction.
> To make trigger-after-execute behavior, kernel emulates the
> instruction. If the instruction is 'load something into non-
> volatile register', exception handler should restore emulated
> register state while returning back, otherwise there will be
> register state corruption. Ex, Adding a watchpoint on a list
> can corrput the list:
>
> # cat /proc/kallsyms | grep kthread_create_list
> c00000000121c8b8 d kthread_create_list
>
> Add watchpoint on kthread_create_list->next:
>
> # perf record -e mem:0xc00000000121c8c0
>
> Run some workload such that new kthread gets invoked. Ex, I
> just logged out from console:
>
> list_add corruption. next->prev should be prev (c000000001214e00), \
> but was c00000000121c8b8. (next=c00000000121c8b8).
> WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0
> CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69
> ...
> NIP __list_add_valid+0xb4/0xc0
> LR __list_add_valid+0xb0/0xc0
> Call Trace:
> __list_add_valid+0xb0/0xc0 (unreliable)
> __kthread_create_on_node+0xe0/0x260
> kthread_create_on_node+0x34/0x50
> create_worker+0xe8/0x260
> worker_thread+0x444/0x560
> kthread+0x160/0x1a0
> ret_from_kernel_thread+0x5c/0x70
>
> Signed-off-by: Ravi Bangoria <[email protected]>

How long has this been around? Should we be CCing stable?

Mikey

> ---
> arch/powerpc/kernel/exceptions-64s.S | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/exceptions-64s.S
> b/arch/powerpc/kernel/exceptions-64s.S
> index 9481a11..96de0d1 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -1753,7 +1753,7 @@ handle_dabr_fault:
> ld r5,_DSISR(r1)
> addi r3,r1,STACK_FRAME_OVERHEAD
> bl do_break
> -12: b ret_from_except_lite
> +12: b ret_from_except
>
>
> #ifdef CONFIG_PPC_BOOK3S_64

2019-06-07 03:43:12

by Ravi Bangoria

[permalink] [raw]

Subject: Re: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception

On 6/7/19 6:20 AM, Michael Neuling wrote:
> On Thu, 2019-06-06 at 12:59 +0530, Ravi Bangoria wrote:
>> Powerpc hw triggers watchpoint before executing the instruction.
>> To make trigger-after-execute behavior, kernel emulates the
>> instruction. If the instruction is 'load something into non-
>> volatile register', exception handler should restore emulated
>> register state while returning back, otherwise there will be
>> register state corruption. Ex, Adding a watchpoint on a list
>> can corrput the list:
>>
>> # cat /proc/kallsyms | grep kthread_create_list
>> c00000000121c8b8 d kthread_create_list
>>
>> Add watchpoint on kthread_create_list->next:
>>
>> # perf record -e mem:0xc00000000121c8c0
>>
>> Run some workload such that new kthread gets invoked. Ex, I
>> just logged out from console:
>>
>> list_add corruption. next->prev should be prev (c000000001214e00), \
>> but was c00000000121c8b8. (next=c00000000121c8b8).
>> WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0
>> CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69
>> ...
>> NIP __list_add_valid+0xb4/0xc0
>> LR __list_add_valid+0xb0/0xc0
>> Call Trace:
>> __list_add_valid+0xb0/0xc0 (unreliable)
>> __kthread_create_on_node+0xe0/0x260
>> kthread_create_on_node+0x34/0x50
>> create_worker+0xe8/0x260
>> worker_thread+0x444/0x560
>> kthread+0x160/0x1a0
>> ret_from_kernel_thread+0x5c/0x70
>>
>> Signed-off-by: Ravi Bangoria <[email protected]>
>
> How long has this been around? Should we be CCing stable?

"bl .save_nvgprs" was added in the commit 5aae8a5370802 ("powerpc, hw_breakpoints:
Implement hw_breakpoints for 64-bit server processors"), which was merged in
v2.6.36.

2019-06-07 05:54:13

by Michael Ellerman

[permalink] [raw]

Subject: Re: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception

Ravi Bangoria <[email protected]> writes:

> Powerpc hw triggers watchpoint before executing the instruction.
> To make trigger-after-execute behavior, kernel emulates the
> instruction. If the instruction is 'load something into non-
> volatile register', exception handler should restore emulated
> register state while returning back, otherwise there will be
> register state corruption. Ex, Adding a watchpoint on a list
> can corrput the list:
>
> # cat /proc/kallsyms | grep kthread_create_list
> c00000000121c8b8 d kthread_create_list
>
> Add watchpoint on kthread_create_list->next:
>
> # perf record -e mem:0xc00000000121c8c0
>
> Run some workload such that new kthread gets invoked. Ex, I
> just logged out from console:
>
> list_add corruption. next->prev should be prev (c000000001214e00), \
> but was c00000000121c8b8. (next=c00000000121c8b8).
> WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0
> CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69
> ...
> NIP __list_add_valid+0xb4/0xc0
> LR __list_add_valid+0xb0/0xc0
> Call Trace:
> __list_add_valid+0xb0/0xc0 (unreliable)
> __kthread_create_on_node+0xe0/0x260
> kthread_create_on_node+0x34/0x50
> create_worker+0xe8/0x260
> worker_thread+0x444/0x560
> kthread+0x160/0x1a0
> ret_from_kernel_thread+0x5c/0x70

This all depends on what code the compiler generates for the list
access. Can you include a disassembly of the relevant code in your
kernel so we have an example of the bad case.

> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
> index 9481a11..96de0d1 100644
> --- a/arch/powerpc/kernel/exceptions-64s.S
> +++ b/arch/powerpc/kernel/exceptions-64s.S
> @@ -1753,7 +1753,7 @@ handle_dabr_fault:
> ld r5,_DSISR(r1)
> addi r3,r1,STACK_FRAME_OVERHEAD
> bl do_break
> -12: b ret_from_except_lite
> +12: b ret_from_except

This probably warrants a comment explaining why we can't use the (badly
named) "lite" version.

cheers

2019-06-07 06:37:53

by Ravi Bangoria

[permalink] [raw]

Subject: Re: [PATCH] Powerpc/Watchpoint: Restore nvgprs while returning from exception

On 6/7/19 11:20 AM, Michael Ellerman wrote:
> Ravi Bangoria <[email protected]> writes:
>
>> Powerpc hw triggers watchpoint before executing the instruction.
>> To make trigger-after-execute behavior, kernel emulates the
>> instruction. If the instruction is 'load something into non-
>> volatile register', exception handler should restore emulated
>> register state while returning back, otherwise there will be
>> register state corruption. Ex, Adding a watchpoint on a list
>> can corrput the list:
>>
>> # cat /proc/kallsyms | grep kthread_create_list
>> c00000000121c8b8 d kthread_create_list
>>
>> Add watchpoint on kthread_create_list->next:
>>
>> # perf record -e mem:0xc00000000121c8c0
>>
>> Run some workload such that new kthread gets invoked. Ex, I
>> just logged out from console:
>>
>> list_add corruption. next->prev should be prev (c000000001214e00), \
>> but was c00000000121c8b8. (next=c00000000121c8b8).
>> WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0
>> CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69
>> ...
>> NIP __list_add_valid+0xb4/0xc0
>> LR __list_add_valid+0xb0/0xc0
>> Call Trace:
>> __list_add_valid+0xb0/0xc0 (unreliable)
>> __kthread_create_on_node+0xe0/0x260
>> kthread_create_on_node+0x34/0x50
>> create_worker+0xe8/0x260
>> worker_thread+0x444/0x560
>> kthread+0x160/0x1a0
>> ret_from_kernel_thread+0x5c/0x70
>
> This all depends on what code the compiler generates for the list
> access.

True. list corruption is just an example. But any load instruction that uses
non-volatile register and hits a watchpoint, will result in register state
corruption.

> Can you include a disassembly of the relevant code in your
> kernel so we have an example of the bad case.

Register state from WARN_ON():

GPR00: c00000000059a3a0 c000007ff23afb50 c000000001344e00 0000000000000075
GPR04: 0000000000000000 0000000000000000 0000001852af8bc1 0000000000000000
GPR08: 0000000000000001 0000000000000007 0000000000000006 00000000000004aa
GPR12: 0000000000000000 c000007ffffeb080 c000000000137038 c000005ff62aaa00
GPR16: 0000000000000000 0000000000000000 c000007fffbe7600 c000007fffbe7370
GPR20: c000007fffbe7320 c000007fffbe7300 c000000001373a00 0000000000000000
GPR24: fffffffffffffef7 c00000000012e320 c000007ff23afcb0 c000000000cb8628
GPR28: c00000000121c8b8 c000000001214e00 c000007fef5b17e8 c000007fef5b17c0

Snippet from __kthread_create_on_node:

c000000000136be8: ed ff a2 3f addis r29,r2,-19
c000000000136bec: c0 7a bd eb ld r29,31424(r29)
if (!__list_add_valid(new, prev, next))
c000000000136bf0: 78 f3 c3 7f mr r3,r30
c000000000136bf4: 78 e3 85 7f mr r5,r28
c000000000136bf8: 78 eb a4 7f mr r4,r29
c000000000136bfc: fd 36 46 48 bl c00000000059a2f8 <__list_add_valid+0x8>

Watchpoint hit at 0xc000000000136bec.

addis r29,r2,-19
=> r29 = 0xc000000001344e00 + (-19 << 16)
=> r29 = 0xc000000001214e00

ld r29,31424(r29)
=> r29 = *(0xc000000001214e00 + 31424)
=> r29 = *(0xc00000000121c8c0)

0xc00000000121c8c0 is where we placed a watchpoint and thus this instruction was
emulated by emulate_step. But because handle_dabr_fault did not restore emulated
register state, r29 still contains stale value in above register state.

2019-06-11 03:35:21

by Ravi Bangoria

[permalink] [raw]

Subject: [PATCH RESEND] Powerpc/Watchpoint: Restore nvgprs while returning from exception

Powerpc hw triggers watchpoint before executing the instruction. To
make trigger-after-execute behavior, kernel emulates the instruction.
If the instruction is 'load something into non-volatile register',
exception handler should restore emulated register state while
returning back, otherwise there will be register state corruption.
Ex, Adding a watchpoint on a list can corrput the list:

# cat /proc/kallsyms | grep kthread_create_list
c00000000121c8b8 d kthread_create_list

Add watchpoint on kthread_create_list->prev:

# perf record -e mem:0xc00000000121c8c0

Run some workload such that new kthread gets invoked. Ex, I just
logged out from console:

list_add corruption. next->prev should be prev (c000000001214e00), \
but was c00000000121c8b8. (next=c00000000121c8b8).
WARNING: CPU: 59 PID: 309 at lib/list_debug.c:25 __list_add_valid+0xb4/0xc0
CPU: 59 PID: 309 Comm: kworker/59:0 Kdump: loaded Not tainted 5.1.0-rc7+ #69
...
NIP __list_add_valid+0xb4/0xc0
LR __list_add_valid+0xb0/0xc0
Call Trace:
__list_add_valid+0xb0/0xc0 (unreliable)
__kthread_create_on_node+0xe0/0x260
kthread_create_on_node+0x34/0x50
create_worker+0xe8/0x260
worker_thread+0x444/0x560
kthread+0x160/0x1a0
ret_from_kernel_thread+0x5c/0x70

List corruption happened because it uses 'load into non-volatile
register' instruction:

Snippet from __kthread_create_on_node:

c000000000136be8: addis r29,r2,-19
c000000000136bec: ld r29,31424(r29)
if (!__list_add_valid(new, prev, next))
c000000000136bf0: mr r3,r30
c000000000136bf4: mr r5,r28
c000000000136bf8: mr r4,r29
c000000000136bfc: bl c00000000059a2f8 <__list_add_valid+0x8>

Register state from WARN_ON():

GPR00: c00000000059a3a0 c000007ff23afb50 c000000001344e00 0000000000000075
GPR04: 0000000000000000 0000000000000000 0000001852af8bc1 0000000000000000
GPR08: 0000000000000001 0000000000000007 0000000000000006 00000000000004aa
GPR12: 0000000000000000 c000007ffffeb080 c000000000137038 c000005ff62aaa00
GPR16: 0000000000000000 0000000000000000 c000007fffbe7600 c000007fffbe7370
GPR20: c000007fffbe7320 c000007fffbe7300 c000000001373a00 0000000000000000
GPR24: fffffffffffffef7 c00000000012e320 c000007ff23afcb0 c000000000cb8628
GPR28: c00000000121c8b8 c000000001214e00 c000007fef5b17e8 c000007fef5b17c0

Watchpoint hit at 0xc000000000136bec.

addis r29,r2,-19
=> r29 = 0xc000000001344e00 + (-19 << 16)
=> r29 = 0xc000000001214e00

ld r29,31424(r29)
=> r29 = *(0xc000000001214e00 + 31424)
=> r29 = *(0xc00000000121c8c0)

0xc00000000121c8c0 is where we placed a watchpoint and thus this
instruction was emulated by emulate_step. But because handle_dabr_fault
did not restore emulated register state, r29 still contains stale
value in above register state.

Fixes: 5aae8a5370802 ("powerpc, hw_breakpoints: Implement hw_breakpoints for 64-bit server processors")
Signed-off-by: Ravi Bangoria <[email protected]>
Cc: [email protected] # 2.6.36+
Reviewed-by: Naveen N. Rao <[email protected]>
---
arch/powerpc/kernel/exceptions-64s.S | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 6b86055e5251..0e649d980ec3 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -1761,7 +1761,7 @@ handle_dabr_fault:
ld r5,_DSISR(r1)
addi r3,r1,STACK_FRAME_OVERHEAD
bl do_break
-12: b ret_from_except_lite
+12: b ret_from_except

#ifdef CONFIG_PPC_BOOK3S_64
--
2.20.1