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    Subject[PATCH 4.9 119/223] powerpc/watchpoint: Restore NV GPRs while returning from exception
    Date
    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>
    Cc: stable@vger.kernel.org # 2.6.36+
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    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
    @@ -1505,7 +1505,7 @@ 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
    @@ -1520,7 +1520,12 @@ 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


    #ifdef CONFIG_PPC_STD_MMU_64

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