• Eric Biggers's avatar
    lib/mpi: call cond_resched() from mpi_powm() loop · ce922b7b
    Eric Biggers authored
    commit 1d9ddde1 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: default avatarEric Biggers <ebiggers@google.com>
    Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    ce922b7b
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