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

[Greg KH <gregkh@linuxfoundation.org>]

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