Eric, On Mon, 6 Nov 2017, Eric Biggers wrote:
From: Eric Biggers <ebiggers@xxxxxxxxxx> 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.
cond_resched() is in the outer loop and gets called every BITS_PER_LONG bits. That seems to be often enough for the system that was taking 10+ seconds, and might be ok for slower processors.
Was your intent to call cond_resched() for every bit as you described in the commit message?
Thanks for the fix. Mat
Cc: stable@xxxxxxxxxxxxxxx # v4.12+ Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> --- 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
-- Mat Martineau Intel OTC
