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    Subject[PATCH 4.16 094/279] arm64: only advance singlestep for user instruction traps
    Date
    4.16-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Mark Rutland <mark.rutland@arm.com>

    [ Upstream commit 9478f1927e6ef9ef5e1ad761af1c98aa8e40b7f5 ]

    Our arm64_skip_faulting_instruction() helper advances the userspace
    singlestep state machine, but this is also called by the kernel BRK
    handler, as used for WARN*().

    Thus, if we happen to hit a WARN*() while the user singlestep state
    machine is in the active-no-pending state, we'll advance to the
    active-pending state without having executed a user instruction, and
    will take a step exception earlier than expected when we return to
    userspace.

    Let's fix this by only advancing the state machine when skipping a user
    instruction.

    Signed-off-by: Mark Rutland <mark.rutland@arm.com>
    Cc: Andrey Konovalov <andreyknvl@google.com>
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Cc: Will Deacon <will.deacon@arm.com>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/arm64/kernel/traps.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- a/arch/arm64/kernel/traps.c
    +++ b/arch/arm64/kernel/traps.c
    @@ -243,7 +243,8 @@ void arm64_skip_faulting_instruction(str
    * If we were single stepping, we want to get the step exception after
    * we return from the trap.
    */
    - user_fastforward_single_step(current);
    + if (user_mode(regs))
    + user_fastforward_single_step(current);
    }

    static LIST_HEAD(undef_hook);

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