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

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Date:   Fri, 10 Nov 2017 10:41:05 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
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 Fri, Nov 10, 2017 at 10:37:30PM +1100, Herbert Xu wrote:
> 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.
> -- 

If it's not too late can you fix the stable line to be just

	Cc: stable@...r.kernel.org

As Mat pointed out KEYCTL_DH_COMPUTE was actually introduced in v4.7.  Also I
think the code is also reachable through RSA by adding an x509 certificate using
the "asymmetric" key type, although that appears to be limited to 4096-bit
inputs rather than 16384 bits.

Eric