3.16.76-rc1 review patch. If anyone has any objections, please let me know.
------------------
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