Commit ce922b7b authored Nov 07, 2017 by Eric Biggers Committed by Greg Kroah-Hartman Nov 30, 2017

lib/mpi: call cond_resched() from mpi_powm() loop



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>

parent 9f088f6a

lib/mpi/mpi-pow.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -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, MPI mod)
		}
		e <<= 1;
		c--;
		cond_resched();
		}

		i--;