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    Subject[PATCH 4.4 002/168] arm64: debug: Ensure debug handlers check triggering exception level
    Date
    From: Will Deacon <will.deacon@arm.com>

    commit 6bd288569b50bc89fa5513031086746968f585cb upstream.

    Debug exception handlers may be called for exceptions generated both by
    user and kernel code. In many cases, this is checked explicitly, but
    in other cases things either happen to work by happy accident or they
    go slightly wrong. For example, executing 'brk #4' from userspace will
    enter the kprobes code and be ignored, but the instruction will be
    retried forever in userspace instead of delivering a SIGTRAP.

    Fix this issue in the most stable-friendly fashion by simply adding
    explicit checks of the triggering exception level to all of our debug
    exception handlers.

    Cc: <stable@vger.kernel.org>
    Reviewed-by: Mark Rutland <mark.rutland@arm.com>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/arm64/kernel/kgdb.c | 15 ++++++++++++---
    1 file changed, 12 insertions(+), 3 deletions(-)

    --- a/arch/arm64/kernel/kgdb.c
    +++ b/arch/arm64/kernel/kgdb.c
    @@ -215,22 +215,31 @@ int kgdb_arch_handle_exception(int excep

    static int kgdb_brk_fn(struct pt_regs *regs, unsigned int esr)
    {
    + if (user_mode(regs))
    + return DBG_HOOK_ERROR;
    +
    kgdb_handle_exception(1, SIGTRAP, 0, regs);
    - return 0;
    + return DBG_HOOK_HANDLED;
    }

    static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int esr)
    {
    + if (user_mode(regs))
    + return DBG_HOOK_ERROR;
    +
    compiled_break = 1;
    kgdb_handle_exception(1, SIGTRAP, 0, regs);

    - return 0;
    + return DBG_HOOK_HANDLED;
    }

    static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
    {
    + if (user_mode(regs))
    + return DBG_HOOK_ERROR;
    +
    kgdb_handle_exception(1, SIGTRAP, 0, regs);
    - return 0;
    + return DBG_HOOK_HANDLED;
    }

    static struct break_hook kgdb_brkpt_hook = {

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