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 3.18-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: lib-mpi-call-cond_resched-from-mpi_powm-loop.patch and it can be found in the queue-3.18 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-3.18/lib-mpi-call-cond_resched-from-mpi_powm-loop.patch