This is a note to let you know that I've just added the patch titled
lib/mpi: call cond_resched() from mpi_powm() loop
to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
lib-mpi-call-cond_resched-from-mpi_powm-loop.patch
and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 1d9ddde12e3c9bab7f3d3484eb9446315e3571ca Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@xxxxxxxxxx>
Date: Tue, 7 Nov 2017 14:15:27 -0800
Subject: lib/mpi: call cond_resched() from mpi_powm() loop
From: Eric Biggers <ebiggers@xxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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;
c--;
+ cond_resched();
}
i--;
Patches currently in stable-queue which might be from ebiggers@xxxxxxxxxx are
queue-4.4/lib-mpi-call-cond_resched-from-mpi_powm-loop.patch