path: root/lib
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authorEric Biggers <ebiggers@google.com>2017-11-07 14:15:27 -0800
committerHerbert Xu <herbert@gondor.apana.org.au>2017-11-10 19:20:26 +0800
commit1d9ddde12e3c9bab7f3d3484eb9446315e3571ca (patch)
treeb951622c25b368b0224012e2a463aee35c92d7da /lib
parent63c53823f00f0ffd13e8c86b05c1486614a2df85 (diff)
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>
Diffstat (limited to 'lib')
1 files changed, 2 insertions, 0 deletions
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;
+ cond_resched();

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