From: Eric Biggers <ebigg...@google.com>
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.
Cc: sta...@vger.kernel.org # v4.12+
Signed-off-by: Eric Biggers <ebigg...@google.com>
---
lib/mpi/mpi-pow.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c
index e24388a863a7..f089a52dbbdb 100644
--- 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"
@@ -263,6 +264,8 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod)
break;
e = ep[i];
c = BITS_PER_MPI_LIMB;
+
+ cond_resched();
}
/* We shifted MOD, the modulo reduction argument, left
MOD_SHIFT_CNT
--
2.15.0
