Patch "lib/mpi: call cond_resched() from mpi_powm() loop" has been added to the 4.9-stable tree

Patch "lib/mpi: call cond_resched() from mpi_powm() loop" has been added to the 4.9-stable tree

[gregkh]

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 4.9-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-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 1d9ddde12e3c9bab7f3d3484eb9446315e3571ca Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@google.com>
Date: Tue, 7 Nov 2017 14:15:27 -0800
Subject: lib/mpi: call cond_resched() from mpi_powm() loop

From: Eric Biggers <ebiggers@google.com>

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>

---
 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@google.com are

queue-4.9/lib-mpi-call-cond_resched-from-mpi_powm-loop.patch