* Re: lib/mpi: call cond_resched() from mpi_powm() loop [not found] <20171114224616.A6B8521C52@pdx-korg-gitolite-1.ci.codeaurora.org> @ 2017-11-27 12:13 ` Greg KH 2017-11-27 18:24 ` Eric Biggers 0 siblings, 1 reply; 2+ messages in thread From: Greg KH @ 2017-11-27 12:13 UTC (permalink / raw) To: Eric Biggers, Herbert Xu; +Cc: Linux Kernel Mailing List 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 <ebiggers@google.com> > AuthorDate: Tue Nov 7 14:15:27 2017 -0800 > Committer: Herbert Xu <herbert@gondor.apana.org.au> > 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: <stable@vger.kernel.org> # v4.12+ > Signed-off-by: Eric Biggers <ebiggers@google.com> > Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au> > --- > 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 ^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: lib/mpi: call cond_resched() from mpi_powm() loop 2017-11-27 12:13 ` lib/mpi: call cond_resched() from mpi_powm() loop Greg KH @ 2017-11-27 18:24 ` Eric Biggers 0 siblings, 0 replies; 2+ messages in thread From: Eric Biggers @ 2017-11-27 18:24 UTC (permalink / raw) To: Greg KH; +Cc: Herbert Xu, Linux Kernel Mailing List On Mon, Nov 27, 2017 at 01:13:05PM +0100, Greg KH wrote: > 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 <ebiggers@google.com> > > AuthorDate: Tue Nov 7 14:15:27 2017 -0800 > > Committer: Herbert Xu <herbert@gondor.apana.org.au> > > 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: <stable@vger.kernel.org> # v4.12+ > > Signed-off-by: Eric Biggers <ebiggers@google.com> > > Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au> > > --- > > 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 Indeed, I was going to point out that the "v4.12+" was wrong but you already figured it out. The code is reachable through RSA (via asymmetric_keys) in older kernel versions. Thanks! Eric ^ permalink raw reply [flat|nested] 2+ messages in thread
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2017-11-27 12:13 ` lib/mpi: call cond_resched() from mpi_powm() loop Greg KH
2017-11-27 18:24 ` Eric Biggers
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