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

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Date:   Fri, 10 Nov 2017 22:37:30 +1100
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-crypto@...r.kernel.org,
        Tudor-Dan Ambarus <tudor.ambarus@...rochip.com>,
        Mat Martineau <mathew.j.martineau@...ux.intel.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

On Mon, Nov 06, 2017 at 10:19:51PM -0800, 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.
> 
> Cc: stable@...r.kernel.org # v4.12+
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt