[PATCH 4.4 09/96] lib/mpi: call cond_resched() from mpi_powm() loop

Greg Kroah-Hartman
2017-11-28T10:28:23Z

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--;