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    Subject[PATCH 5.15 909/917] powerpc/security: Use a mutex for interrupt exit code patching
    Date
    From: Russell Currey <ruscur@russell.cc>

    commit 3c12b4df8d5e026345a19886ae375b3ebc33c0b6 upstream.

    The mitigation-patching.sh script in the powerpc selftests toggles
    all mitigations on and off simultaneously, revealing that rfi_flush
    and stf_barrier cannot safely operate at the same time due to races
    in updating the static key.

    On some systems, the static key code throws a warning and the kernel
    remains functional. On others, the kernel will hang or crash.

    Fix this by slapping on a mutex.

    Fixes: 13799748b957 ("powerpc/64: use interrupt restart table to speed up return from interrupt")
    Cc: stable@vger.kernel.org # v5.14+
    Signed-off-by: Russell Currey <ruscur@russell.cc>
    Acked-by: Nicholas Piggin <npiggin@gmail.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20211027072410.40950-1-ruscur@russell.cc
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/powerpc/lib/feature-fixups.c | 11 +++++++++++
    1 file changed, 11 insertions(+)

    --- a/arch/powerpc/lib/feature-fixups.c
    +++ b/arch/powerpc/lib/feature-fixups.c
    @@ -228,6 +228,7 @@ static void do_stf_exit_barrier_fixups(e

    static bool stf_exit_reentrant = false;
    static bool rfi_exit_reentrant = false;
    +static DEFINE_MUTEX(exit_flush_lock);

    static int __do_stf_barrier_fixups(void *data)
    {
    @@ -253,6 +254,9 @@ void do_stf_barrier_fixups(enum stf_barr
    * low level interrupt exit code before patching. After the patching,
    * if allowed, then flip the branch to allow fast exits.
    */
    +
    + // Prevent static key update races with do_rfi_flush_fixups()
    + mutex_lock(&exit_flush_lock);
    static_branch_enable(&interrupt_exit_not_reentrant);

    stop_machine(__do_stf_barrier_fixups, &types, NULL);
    @@ -264,6 +268,8 @@ void do_stf_barrier_fixups(enum stf_barr

    if (stf_exit_reentrant && rfi_exit_reentrant)
    static_branch_disable(&interrupt_exit_not_reentrant);
    +
    + mutex_unlock(&exit_flush_lock);
    }

    void do_uaccess_flush_fixups(enum l1d_flush_type types)
    @@ -486,6 +492,9 @@ void do_rfi_flush_fixups(enum l1d_flush_
    * without stop_machine, so this could be achieved with a broadcast
    * IPI instead, but this matches the stf sequence.
    */
    +
    + // Prevent static key update races with do_stf_barrier_fixups()
    + mutex_lock(&exit_flush_lock);
    static_branch_enable(&interrupt_exit_not_reentrant);

    stop_machine(__do_rfi_flush_fixups, &types, NULL);
    @@ -497,6 +506,8 @@ void do_rfi_flush_fixups(enum l1d_flush_

    if (stf_exit_reentrant && rfi_exit_reentrant)
    static_branch_disable(&interrupt_exit_not_reentrant);
    +
    + mutex_unlock(&exit_flush_lock);
    }

    void do_barrier_nospec_fixups_range(bool enable, void *fixup_start, void *fixup_end)

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