From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Date: Fri, 20 Mar 2020 13:27:03 +0000 Subject: Re: [PATCH v5 2/2] KEYS: Avoid false positive ENOMEM error on key read Message-Id: <7dbc524f-6c16-026a-a372-2e80b40eab30@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit List-Id: References: <20200318221457.1330-1-longman@redhat.com> <20200318221457.1330-3-longman@redhat.com> <20200319194650.GA24804@linux.intel.com> <20200320020717.GC183331@linux.intel.com> In-Reply-To: <20200320020717.GC183331@linux.intel.com> To: Jarkko Sakkinen Cc: David Howells , James Morris , "Serge E. Hallyn" , Mimi Zohar , "David S. Miller" , Jakub Kicinski , keyrings@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, linux-integrity@vger.kernel.org, netdev@vger.kernel.org, linux-afs@lists.infradead.org, Sumit Garg , Jerry Snitselaar , Roberto Sassu , Eric Biggers , Chris von Recklinghausen On 3/19/20 10:07 PM, Jarkko Sakkinen wrote: > On Thu, Mar 19, 2020 at 08:07:55PM -0400, Waiman Long wrote: >> On 3/19/20 3:46 PM, Jarkko Sakkinen wrote: >>> On Wed, Mar 18, 2020 at 06:14:57PM -0400, Waiman Long wrote: >>>> + * It is possible, though unlikely, that the key >>>> + * changes in between the up_read->down_read period. >>>> + * If the key becomes longer, we will have to >>>> + * allocate a larger buffer and redo the key read >>>> + * again. >>>> + */ >>>> + if (!tmpbuf || unlikely(ret > tmpbuflen)) { >>> Shouldn't you check that tmpbuflen stays below buflen (why else >>> you had made copy of buflen otherwise)? >> The check above this thunk: >> >> if ((ret > 0) && (ret <= buflen)) { >> >> will make sure that ret will not be larger than buflen. So tmpbuflen >> will never be bigger than buflen. > Ah right, of course, thanks. > > What would go wrong if the condition was instead > ((ret > 0) && (ret <= tmpbuflen))? That if statement is a check to see if the actual key length is longer than the user-supplied buffer (buflen). If that is the case, it will just return the expected length without storing anything into the user buffer. For the case that buflen >= ret > tmpbuflen, the revised check above will incorrectly skip the storing step causing the caller to incorrectly think the key is there in the buffer. Maybe I should clarify that a bit more in the comment. Cheers, Longman