On Tue, Nov 14, 2017 at 10:46:16PM +0000, Linux Kernel Mailing List wrote: > Web: > https://git.kernel.org/torvalds/c/1d9ddde12e3c9bab7f3d3484eb9446315e3571ca > Commit: 1d9ddde12e3c9bab7f3d3484eb9446315e3571ca > Parent: 63c53823f00f0ffd13e8c86b05c1486614a2df85 > Refname: refs/heads/master > Author: Eric Biggers <[email protected]> > AuthorDate: Tue Nov 7 14:15:27 2017 -0800 > Committer: Herbert Xu <[email protected]> > CommitDate: Fri Nov 10 19:20:26 2017 +0800 > > lib/mpi: call cond_resched() from mpi_powm() loop > > 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. > > Cc: <[email protected]> # v4.12+ > Signed-off-by: Eric Biggers <[email protected]> > Signed-off-by: Herbert Xu <[email protected]> > --- > lib/mpi/mpi-pow.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c > index e24388a863a7..468fb7cd1221 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" > @@ -256,6 +257,7 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod) > } > e <<= 1; > c--; > + cond_resched(); > } > > i--;
Note, I've applied this to kernels older than 4.12 as well, as the issue seems to also be there as well, right? thanks, greg k-h
