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

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

[Ravi Bangoria]

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 <ravi.bangoria@linux.ibm.com>
---
 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
-- 
1.8.3.1

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

[Naveen N. Rao]

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 <ravi.bangoria@linux.ibm.com>
> ---
>  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 <naveen.n.rao@linux.vnet.ibm.com>


- Naveen

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

[Ravi Bangoria]

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/

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

[Michael Neuling]

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 <ravi.bangoria@linux.ibm.com>

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

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

[Ravi Bangoria]

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 <ravi.bangoria@linux.ibm.com>
> 
> 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.

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

[Michael Ellerman]

Ravi Bangoria <ravi.bangoria@linux.ibm.com> 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

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

[Ravi Bangoria]

On 6/7/19 11:20 AM, Michael Ellerman wrote:
> Ravi Bangoria <ravi.bangoria@linux.ibm.com> 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.

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

[Ravi Bangoria]

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 <ravi.bangoria@linux.ibm.com>
Cc: stable@vger.kernel.org # 2.6.36+
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 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