Re: [PATCH] lib/mpi: call cond_resched() from mpi_powm() loop

[Mat Martineau]

Eric,

On Mon, 6 Nov 2017, Eric Biggers wrote:

> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxx # v4.12+
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
> 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