| 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--; |
