Commit 1d9ddde1 authored by Eric Biggers's avatar Eric Biggers Committed by Herbert Xu

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: 's avatarEric Biggers <ebiggers@google.com>
Signed-off-by: 's avatarHerbert Xu <herbert@gondor.apana.org.au>
parent 63c53823
......@@ -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--;
......
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