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    Subject[PATCH 5.16 0066/1039] arm64: errata: Fix exec handling in erratum 1418040 workaround
    Date
    From: D Scott Phillips <scott@os.amperecomputing.com>

    commit 38e0257e0e6f4fef2aa2966b089b56a8b1cfb75c upstream.

    The erratum 1418040 workaround enables CNTVCT_EL1 access trapping in EL0
    when executing compat threads. The workaround is applied when switching
    between tasks, but the need for the workaround could also change at an
    exec(), when a non-compat task execs a compat binary or vice versa. Apply
    the workaround in arch_setup_new_exec().

    This leaves a small window of time between SET_PERSONALITY and
    arch_setup_new_exec where preemption could occur and confuse the old
    workaround logic that compares TIF_32BIT between prev and next. Instead, we
    can just read cntkctl to make sure it's in the state that the next task
    needs. I measured cntkctl read time to be about the same as a mov from a
    general-purpose register on N1. Update the workaround logic to examine the
    current value of cntkctl instead of the previous task's compat state.

    Fixes: d49f7d7376d0 ("arm64: Move handling of erratum 1418040 into C code")
    Cc: <stable@vger.kernel.org> # 5.9.x
    Signed-off-by: D Scott Phillips <scott@os.amperecomputing.com>
    Reviewed-by: Marc Zyngier <maz@kernel.org>
    Link: https://lore.kernel.org/r/20211220234114.3926-1-scott@os.amperecomputing.com
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/arm64/kernel/process.c | 39 ++++++++++++++++-----------------------
    1 file changed, 16 insertions(+), 23 deletions(-)

    --- a/arch/arm64/kernel/process.c
    +++ b/arch/arm64/kernel/process.c
    @@ -439,34 +439,26 @@ static void entry_task_switch(struct tas

    /*
    * ARM erratum 1418040 handling, affecting the 32bit view of CNTVCT.
    - * Assuming the virtual counter is enabled at the beginning of times:
    - *
    - * - disable access when switching from a 64bit task to a 32bit task
    - * - enable access when switching from a 32bit task to a 64bit task
    + * Ensure access is disabled when switching to a 32bit task, ensure
    + * access is enabled when switching to a 64bit task.
    */
    -static void erratum_1418040_thread_switch(struct task_struct *prev,
    - struct task_struct *next)
    +static void erratum_1418040_thread_switch(struct task_struct *next)
    {
    - bool prev32, next32;
    - u64 val;
    -
    - if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040))
    - return;
    -
    - prev32 = is_compat_thread(task_thread_info(prev));
    - next32 = is_compat_thread(task_thread_info(next));
    -
    - if (prev32 == next32 || !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
    + if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_1418040) ||
    + !this_cpu_has_cap(ARM64_WORKAROUND_1418040))
    return;

    - val = read_sysreg(cntkctl_el1);
    -
    - if (!next32)
    - val |= ARCH_TIMER_USR_VCT_ACCESS_EN;
    + if (is_compat_thread(task_thread_info(next)))
    + sysreg_clear_set(cntkctl_el1, ARCH_TIMER_USR_VCT_ACCESS_EN, 0);
    else
    - val &= ~ARCH_TIMER_USR_VCT_ACCESS_EN;
    + sysreg_clear_set(cntkctl_el1, 0, ARCH_TIMER_USR_VCT_ACCESS_EN);
    +}

    - write_sysreg(val, cntkctl_el1);
    +static void erratum_1418040_new_exec(void)
    +{
    + preempt_disable();
    + erratum_1418040_thread_switch(current);
    + preempt_enable();
    }

    /*
    @@ -501,7 +493,7 @@ __notrace_funcgraph struct task_struct *
    contextidr_thread_switch(next);
    entry_task_switch(next);
    ssbs_thread_switch(next);
    - erratum_1418040_thread_switch(prev, next);
    + erratum_1418040_thread_switch(next);
    ptrauth_thread_switch_user(next);

    /*
    @@ -611,6 +603,7 @@ void arch_setup_new_exec(void)
    current->mm->context.flags = mmflags;
    ptrauth_thread_init_user();
    mte_thread_init_user();
    + erratum_1418040_new_exec();

    if (task_spec_ssb_noexec(current)) {
    arch_prctl_spec_ctrl_set(current, PR_SPEC_STORE_BYPASS,

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