linux-kernel - array underflow in receive

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Message-ID: <20120327071036.GA19008@elgon.mountain>
Date:	Tue, 27 Mar 2012 10:10:36 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Philipp Reisner <philipp.reisner@...bit.com>
Cc:	drbd-user@...ts.linbit.com, linux-kernel@...r.kernel.org
Subject: array underflow in receive_SyncParam()?

I had a question about the following code:

drivers/block/drbd/drbd_receiver.c
  2808                  if (apv == 88) {
  2809                          if (data_size > SHARED_SECRET_MAX) {
  2810                                  dev_err(DEV, "verify-alg too long, "
  2811                                      "peer wants %u, accepting only %u byte\n",
  2812                                                  data_size, SHARED_SECRET_MAX);
  2813                                  return false;
  2814                          }
  2815  
  2816                          if (drbd_recv(mdev, p->verify_alg, data_size) != data_size)
  2817                                  return false;
  2818  
  2819                          /* we expect NUL terminated string */
  2820                          /* but just in case someone tries to be evil */
  2821                          D_ASSERT(p->verify_alg[data_size-1] == 0);
  2822                          p->verify_alg[data_size-1] = 0;
                                              ^^^^^^^^^
Is it possible for data_size to be zero here leading to an array
underflow?  We test for overflows, but I don't see any place where we
test for zero.

regards,
dan carpenter
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