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Date: Mon, 6 Nov 2017 22:19:51 -0800 From: Eric Biggers <ebiggers3@...il.com> To: linux-crypto@...r.kernel.org, Herbert Xu <herbert@...dor.apana.org.au> Cc: Tudor-Dan Ambarus <tudor.ambarus@...rochip.com>, Mat Martineau <mathew.j.martineau@...ux.intel.com>, Salvatore Benedetto <salvatore.benedetto@...el.com>, keyrings@...r.kernel.org, linux-kernel@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>, stable@...r.kernel.org Subject: [PATCH] lib/mpi: call cond_resched() from mpi_powm() loop From: Eric Biggers <ebiggers@...gle.com> 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. Cc: stable@...r.kernel.org # v4.12+ Signed-off-by: Eric Biggers <ebiggers@...gle.com> --- lib/mpi/mpi-pow.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c index e24388a863a7..f089a52dbbdb 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 <linux/sched.h> #include <linux/string.h> #include "mpi-internal.h" #include "longlong.h" @@ -263,6 +264,8 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod) break; e = ep[i]; c = BITS_PER_MPI_LIMB; + + cond_resched(); } /* We shifted MOD, the modulo reduction argument, left MOD_SHIFT_CNT -- 2.15.0
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