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

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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>
This commit is contained in:
Eric Biggers 2017-11-07 14:15:27 -08:00 committed by Herbert Xu
parent 63c53823f0
commit 1d9ddde12e
1 changed files with 2 additions and 0 deletions
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lib/mpi/mpi-pow.c
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@ -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--;