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    Subject[PATCH 4.9 010/138] lib/mpi: call cond_resched() from mpi_powm() loop
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

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

    From: Eric Biggers <ebiggers@google.com>

    commit 1d9ddde12e3c9bab7f3d3484eb9446315e3571ca upstream.

    On a non-preemptible kernel, if KEYCTL_DH_COMPUTE is called with the
    largest permitted inputs (16384 bits), the kernel spends 10+ seconds
    doing modular exponentiation in mpi_powm() without rescheduling. If all
    threads do it, it locks up the system. Moreover, it can cause
    rcu_sched-stall warnings.

    Notwithstanding the insanity of doing this calculation in kernel mode
    rather than in userspace, fix it by calling cond_resched() as each bit
    from the exponent is processed. It's still noninterruptible, but at
    least it's preemptible now.

    Do the cond_resched() once per bit rather than once per MPI limb because
    each limb might still easily take 100+ milliseconds on slow CPUs.

    Signed-off-by: Eric Biggers <ebiggers@google.com>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    lib/mpi/mpi-pow.c | 2 ++
    1 file changed, 2 insertions(+)

    --- a/lib/mpi/mpi-pow.c
    +++ b/lib/mpi/mpi-pow.c
    @@ -26,6 +26,7 @@
    * however I decided to publish this code under the plain GPL.
    */

    +#include <linux/sched.h>
    #include <linux/string.h>
    #include "mpi-internal.h"
    #include "longlong.h"
    @@ -256,6 +257,7 @@ int mpi_powm(MPI res, MPI base, MPI exp,
    }
    e <<= 1;
    c--;
    + cond_resched();
    }

    i--;

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