[PATCH 3.16 17/47] powerpc/watchpoint: Restore NV GPRs while returning from exception

Ben Hutchings
2019-10-25T18:10:26Z

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

commit f474c28fbcbe42faca4eb415172c07d76adcb819 upstream.

powerpc hardware 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. eg, 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. eg, 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>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/powerpc/kernel/exceptions-64s.S | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -1630,7 +1630,7 @@ handle_page_fault:
addi r3,r1,STACK_FRAME_OVERHEAD
bl do_page_fault
cmpdi r3,0
- beq+ 12f
+ beq+ ret_from_except_lite
bl save_nvgprs
mr r5,r3
addi r3,r1,STACK_FRAME_OVERHEAD
@@ -1645,7 +1645,12 @@ handle_dabr_fault:
ld r5,_DSISR(r1)
addi r3,r1,STACK_FRAME_OVERHEAD
bl do_break
-12: b ret_from_except_lite
+ /*
+ * do_break() may have changed the NV GPRS while handling a breakpoint.
+ * If so, we need to restore them with their updated values. Don't use
+ * ret_from_except_lite here.
+ */
+ b ret_from_except

/* We have a page fault that hash_page could handle but HV refused