* 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
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2017-11-27 12:13 ` lib/mpi: call cond_resched() from mpi_powm() loop Greg KH
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