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1 – Articles du blog. Licence Creative Commons Les articles de ce blog sont fournis selon les termes de la Licence Creative Commons CC-BY-ND. 8 novembre. Shells Linux et Unix par la pratique (French Edition) Feb 06, by CHRISTOPHE BLAESS · Paperback. $$ More Buying Choices. $ (2 Used. Langages de scripts sous Linux [Christophe Blaess] on *FREE* shipping on qualifying offers.

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Popularity Popularity Featured Price: Let’s write something like:. The most usual case is inserting blarss record in the middle of a sequential ordered file, what implies to make a copy of the original file in a temporary file, while adding new information. The main cyristophe comes from an user running simultaneously multiple instances of a Set-UID root application or establishing various connexions at once with the same daemon, hoping to create a race condition situation, during which the content of a system file could be modified in an unusual way.

Next, it really opens the file and writes the message. The kernel ensures to keep the association to the file content during the lapse of time between the open system call providing a file descriptor and the release of this descriptor using close or when the process ends.

Very often a program needs to temporarily store data in an external file. The ln command -f option forces the creation, even if that name already exists:.

The mistake made in the previous program is therefore to consider christopeh unchanging the association between the name of the file and its content, or at least, constant during the lapse of chtistophe between stat and fopen operations.


Low to High Price: All Formats Paperback Hardcover Sort by: In a directory belonging to us, we create a new link to a system file. The results may vary. According to the author, the specifications don’t guarantee the file creation and he hasn’t been able to chrisrophe every implementation. As we explained in our first article, it would be better for a Set-UID application to temporarily loose its privileges and to open the file using the real UID of the user having called it.


Everything is ready for the attack. The Sticky-Bit represented by the letter t at the end or the octal mode, has a particular meaning when applied to a directory: Let’s remind you must never consider that two operations in a row are always linked unless the kernel manages this. We ask it to write a string into that file.

Christophe BLAESS – Ulule

Within a real application, the race condition only applies during very short lapses of time. When a process wants to write into a file, it asks the kernel to lock that file – or a part of it.

Let’s remind a few principles to avoid this sort of trouble. How to benefit from that? To make our attack even easier, let’s add a line making the process sleeping between the two operations, thus having the time to do the job by chriatophe. Nevertheless, using the temporary storage directory may cause a few problems.

We write a few letters into a file and lock it using the previous program:. And, that is where lies the security hole!

This lapse of time is often extremely short but it isn’t null, then an attacker can benefit from it to change the file’s characteristics.

This is done using the fstat system call this last working like statbut checking a file descriptor rather than a path. An attacker could create a symlink to the name provided by the C library.

It checks the resource is not already used by another process, then it takes over chhristophe uses it as it wants.

[PATCH] Add the xsc field when rtps reads the sched/acct file.

The flock structure important members are the following:. We did succeed in exploiting a race condition in a Set-UID root utility.

Unlike the security holes discussed in ours previous articles, this security problem applies to every application, and christophhe only to Set-UID utilities and system servers or daemons.

Then, of course, you must start opening the file in the wanted mode, calling open don’t forget the third argument when creating a new file. If the argument is an allocated chrietophe, the name is copied there, what requires a string of at least L-tmpnam bytes. Thus, enough to take the example of a physical link to check this association is not at all a permanent one.


To get next an IO flow around the descriptor we’ll use the fdopen function working like fopen while relying on a descriptor rather than on a filename. The second one represents the operation to be done: Unfortunately, a badly written program is able to replace a file content, even if another process, with good behavior, has a lock for writing. High to Low Avg. ComiXology Thousands of Digital Comics.

That is, the standard libraries functions are expected to provide features abstraction The tmpfile function does it. Or, more exactly, it’s within the lapse of time between the reading of the file attributes with stat and its opening with fopen.

However it isn’t possible to create a copy of such a file, since it would require a full read.

This function checks the file doesn’t exist before returning its name. However, only one process can christtophe from a lock for writing at a given time, and no other lock can be provided at the same time, even for reading. Conclusion We flew over most of the security christoophe concerning race conditions to a same resource. If race conditions generate security holes, you must not neglect the holes relying on other resources, such as common variables with different threads, or memory segments shared from shmget.

The idea is to slow down the target process to manage more easily the delay preceding the file modification. The system call stays locked as long as the requested operation remains impossible.

Withoutabox Submit to Film Festivals. Thus, the program becomes:. Christophe Blaess is an independent aeronautics engineer.