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    Subject[PATCH 4.15 030/202] [Variant 3/Meltdown] arm64: tls: Avoid unconditional zeroing of tpidrro_el0 for native tasks
    Date
    4.15-stable review patch.  If anyone has any objections, please let me know.

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

    From: Will Deacon <will.deacon@arm.com>


    Commit 18011eac28c7 upstream.

    When unmapping the kernel at EL0, we use tpidrro_el0 as a scratch register
    during exception entry from native tasks and subsequently zero it in
    the kernel_ventry macro. We can therefore avoid zeroing tpidrro_el0
    in the context-switch path for native tasks using the entry trampoline.

    Reviewed-by: Mark Rutland <mark.rutland@arm.com>
    Tested-by: Laura Abbott <labbott@redhat.com>
    Tested-by: Shanker Donthineni <shankerd@codeaurora.org>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/arm64/kernel/process.c | 12 +++++-------
    1 file changed, 5 insertions(+), 7 deletions(-)

    --- a/arch/arm64/kernel/process.c
    +++ b/arch/arm64/kernel/process.c
    @@ -370,16 +370,14 @@ void tls_preserve_current_state(void)

    static void tls_thread_switch(struct task_struct *next)
    {
    - unsigned long tpidr, tpidrro;
    -
    tls_preserve_current_state();

    - tpidr = *task_user_tls(next);
    - tpidrro = is_compat_thread(task_thread_info(next)) ?
    - next->thread.tp_value : 0;
    + if (is_compat_thread(task_thread_info(next)))
    + write_sysreg(next->thread.tp_value, tpidrro_el0);
    + else if (!arm64_kernel_unmapped_at_el0())
    + write_sysreg(0, tpidrro_el0);

    - write_sysreg(tpidr, tpidr_el0);
    - write_sysreg(tpidrro, tpidrro_el0);
    + write_sysreg(*task_user_tls(next), tpidr_el0);
    }

    /* Restore the UAO state depending on next's addr_limit */

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