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    Subject[PATCH 3.18 16/47] arm64: avoid returning from bad_mode
    Date
    3.18-stable review patch.  If anyone has any objections, please let me know.

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

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

    commit 7d9e8f71b989230bc613d121ca38507d34ada849 upstream.

    Generally, taking an unexpected exception should be a fatal event, and
    bad_mode is intended to cater for this. However, it should be possible
    to contain unexpected synchronous exceptions from EL0 without bringing
    the kernel down, by sending a SIGILL to the task.

    We tried to apply this approach in commit 9955ac47f4ba1c95 ("arm64:
    don't kill the kernel on a bad esr from el0"), by sending a signal for
    any bad_mode call resulting from an EL0 exception.

    However, this also applies to other unexpected exceptions, such as
    SError and FIQ. The entry paths for these exceptions branch to bad_mode
    without configuring the link register, and have no kernel_exit. Thus, if
    we take one of these exceptions from EL0, bad_mode will eventually
    return to the original user link register value.

    This patch fixes this by introducing a new bad_el0_sync handler to cater
    for the recoverable case, and restoring bad_mode to its original state,
    whereby it calls panic() and never returns. The recoverable case
    branches to bad_el0_sync with a bl, and returns to userspace via the
    usual ret_to_user mechanism.

    Signed-off-by: Mark Rutland <mark.rutland@arm.com>
    Fixes: 9955ac47f4ba1c95 ("arm64: don't kill the kernel on a bad esr from el0")
    Reported-by: Mark Salter <msalter@redhat.com>
    Cc: 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/entry.S | 4 ++--
    arch/arm64/kernel/traps.c | 28 ++++++++++++++++++++++++----
    2 files changed, 26 insertions(+), 6 deletions(-)

    --- a/arch/arm64/kernel/entry.S
    +++ b/arch/arm64/kernel/entry.S
    @@ -551,8 +551,8 @@ el0_inv:
    mov x0, sp
    mov x1, #BAD_SYNC
    mrs x2, esr_el1
    - adr lr, ret_to_user
    - b bad_mode
    + bl bad_el0_sync
    + b ret_to_user
    ENDPROC(el0_sync)

    .align 6
    --- a/arch/arm64/kernel/traps.c
    +++ b/arch/arm64/kernel/traps.c
    @@ -308,16 +308,33 @@ asmlinkage long do_ni_syscall(struct pt_
    }

    /*
    - * bad_mode handles the impossible case in the exception vector.
    + * bad_mode handles the impossible case in the exception vector. This is always
    + * fatal.
    */
    asmlinkage void bad_mode(struct pt_regs *regs, int reason, unsigned int esr)
    {
    - siginfo_t info;
    - void __user *pc = (void __user *)instruction_pointer(regs);
    console_verbose();

    pr_crit("Bad mode in %s handler detected, code 0x%08x\n",
    handler[reason], esr);
    +
    + die("Oops - bad mode", regs, 0);
    + local_irq_disable();
    + panic("bad mode");
    +}
    +
    +/*
    + * bad_el0_sync handles unexpected, but potentially recoverable synchronous
    + * exceptions taken from EL0. Unlike bad_mode, this returns.
    + */
    +asmlinkage void bad_el0_sync(struct pt_regs *regs, int reason, unsigned int esr)
    +{
    + siginfo_t info;
    + void __user *pc = (void __user *)instruction_pointer(regs);
    + console_verbose();
    +
    + pr_crit("Bad EL0 synchronous exception detected on CPU%d, code 0x%08x\n",
    + smp_processor_id(), esr);
    __show_regs(regs);

    info.si_signo = SIGILL;
    @@ -325,7 +342,10 @@ asmlinkage void bad_mode(struct pt_regs
    info.si_code = ILL_ILLOPC;
    info.si_addr = pc;

    - arm64_notify_die("Oops - bad mode", regs, &info, 0);
    + current->thread.fault_address = 0;
    + current->thread.fault_code = 0;
    +
    + force_sig_info(info.si_signo, &info, current);
    }

    void __pte_error(const char *file, int line, unsigned long val)

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