linux-kernel - Re: [PATCH] lib/mpi: call cond_resched() from mpi

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Date:   Tue, 7 Nov 2017 10:38:30 -0800 (PST)
From:   Mat Martineau <mathew.j.martineau@...ux.intel.com>
To:     Eric Biggers <ebiggers3@...il.com>
cc:     linux-crypto@...r.kernel.org,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Tudor-Dan Ambarus <tudor.ambarus@...rochip.com>,
        Salvatore Benedetto <salvatore.benedetto@...el.com>,
        keyrings@...r.kernel.org, linux-kernel@...r.kernel.org,
        Eric Biggers <ebiggers@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH] lib/mpi: call cond_resched() from mpi_powm() loop


Eric,

On Mon, 6 Nov 2017, Eric Biggers wrote:

> From: Eric Biggers <ebiggers@...gle.com>
>
> 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.

cond_resched() is in the outer loop and gets called every BITS_PER_LONG 
bits. That seems to be often enough for the system that was taking 10+ 
seconds, and might be ok for slower processors.

Was your intent to call cond_resched() for every bit as you described in 
the commit message?

Thanks for the fix.


Mat


>
> Cc: stable@...r.kernel.org # v4.12+
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
> lib/mpi/mpi-pow.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/lib/mpi/mpi-pow.c b/lib/mpi/mpi-pow.c
> index e24388a863a7..f089a52dbbdb 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"
> @@ -263,6 +264,8 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod)
> 				break;
> 			e = ep[i];
> 			c = BITS_PER_MPI_LIMB;
> +
> +			cond_resched();
> 		}
>
> 		/* We shifted MOD, the modulo reduction argument, left MOD_SHIFT_CNT
> -- 
> 2.15.0
>
>

--
Mat Martineau
Intel OTC