From 1d9ddde12e3c9bab7f3d3484eb9446315e3571ca Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Tue, 7 Nov 2017 14:15:27 -0800 Subject: lib/mpi: call cond_resched() from mpi_powm() loop 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. Cc: # v4.12+ Signed-off-by: Eric Biggers Signed-off-by: Herbert Xu --- lib/mpi/mpi-pow.c | 2 ++ 1 file changed, 2 insertions(+) (limited to 'lib/mpi') diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c index e24388a863a7..468fb7cd1221 100644 --- 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 #include #include "mpi-internal.h" #include "longlong.h" @@ -256,6 +257,7 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod) } e <<= 1; c--; + cond_resched(); } i--; -- cgit v1.2.3-70-g09d2