[lkml]   [2017]   [Nov]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[PATCH 4.9 010/138] lib/mpi: call cond_resched() from mpi_powm() loop
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Eric Biggers <>

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 <>
Signed-off-by: Herbert Xu <>
Signed-off-by: Greg Kroah-Hartman <>

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;
+ cond_resched();


 \ /
  Last update: 2017-11-28 13:04    [W:0.605 / U:0.276 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site