lkml.org 
[lkml]   [2017]   [Nov]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: lib/mpi: call cond_resched() from mpi_powm() loop
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

\
 
 \ /
  Last update: 2017-11-27 19:25    [W:0.036 / U:5.100 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site