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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This time, after line 20, no change concerning the filename deleting, renaming, linking will affect our program’s behavior; the content of the original physical file will be kept.

Let’s insist on that point: Everything is ready for the attack. Unfortunately, a badly written program is able to replace a file content, even if another process, with good behavior, has a lock for writing.

As we can see, the program starts doing all the needed controls, checking that the file exists, that it belongs to the user and that it’s a normal file. Get to Know Us.

Don’t we forget, the program being Set-UID rootit is allowed to modify any file in the machine. The second type of lock comes from System V, and relies on the fcntl system call which invocation is a bit complicated.

Let’s have a look at the behavior of a Set-UID program having to save data into a file belonging to the user. 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.


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On the other hand, checking that it was impossible to open the file instead of checking if the file really existed, was less acceptable. Get fast, free shipping with Amazon Prime. To avoid a program being sensitive to this kind of situation, it’s necessary to institute an exclusive access mechanism to the file data. Only 15 left in stock – order soon. The third argument is a pointer to a variable of struct flock type, describing the lock.

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However it isn’t possible to create a copy of such a file, since it would require a full read. Our first reaction is to check the file exists before opening it.

We are in a directory belonging to us. There’s a problem loading this menu right now. As we blarss in our first article, nlaess 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. Thus, it’s also possible to program a periodic cleaning using cronthe use of an independant partition formated at boot time, etc. Its first argument is the descriptor of the file you wish to access christkphe an exclusive way, and the second one is a symbolic constant representing the operation to be done.

This is done using the fstat system call this last working like statbut checking a file descriptor rather than a path.


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. And, that is where lies the security hole! This is possible using a specific option of the open system call.

spi-config(1) — spi-tools — Debian testing — Debian Manpages

The general principle of race conditions is the following: Blaaess a library function called lockf close to the system call but not so performing. Langages de scripts sous Linux.

Next, we run our application in the background “to keep the lead”. Usually, the principle relies on a brutal attack, renewing the attempts hundred, thousand or ten thousand times, using scripts to automate the sequence. Therefore, the first step is to use a filename defined for the current program instance. The second one represents the operation to be done: The content of the root line comes from the shadow 5 man page, the most important being the second field to be empty no password.

The Sticky-Bit represented by the letter t at the end or the octal mode, has a particular meaning when applied to a directory: