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

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 <[email protected]> know about it.

From: Eric Biggers <[email protected]>
Date: Tue, 7 Nov 2017 14:15:27 -0800
Subject: lib/mpi: call cond_resched() from mpi_powm() loop

From: Eric Biggers <[email protected]>

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.

---
 lib/mpi/mpi-pow.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -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 [email protected] are

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