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A Linux bug was recently uncovered by a young Norwegian programmer that, when exploited by a simple C program, could crash most Linux 2.4 or 2.6 distributions running on an x86 architecture.

“Using this exploit to crash Linux systems requires the (ab)user to have shell access or other means of uploading and running the program—like cgi-bin and FTP access,” reports the discoverer, Øyvind Sæther.

“The program works on any normal user account, and root access is not required,” Sæther reported. “This exploit has been reported used to take down several ‘lame free-shell providers’ servers. [Running code you know will damage a system intentionally and hacking in general] is illegal in most parts of the world and strongly discouraged.”

Along with the code needed to use the exploit, Sæther also posted several patches to 2.4 and 2.6 kernels that will keep the exploit from crashing systems.

Several security problems have been uncovered in Linux over the past year. The most serious was uncovered in February by the Polish security nonprofit organization iSEC Security Research.

The biggest of these security holes, called “Linux kernel do_mremap VMA limit local privilege escalation vulnerability” by iSEC, could have enabled a cracker to achieve full super-user and full administration privileges. In each case, fixes were quickly delivered by the Linux open-source community.

This latest security hole, however, can be used to crash a system, but it doesn’t give an attacker any other control of a Linux system.

Technically, the problem exists because the Linux kernel’s signal handler isn’t handling floating-point (FP) exceptions correctly. Linux’s creator, Linus Torvalds, said, “There’s a path into the kernel where if there is a pending FP error, the kernel will end up taking an FP exception, and it will continue to take the FP exception forever. Duh.”

Torvalds already has the problem well in hand, he said. “I fixed it in my [source code] tree a few days ago, so it’s in the current snapshots, and if I wasn’t in the middle of a move [to Portland, Ore.] I’d have released a 2.6.7 already. As it is, I’ll hopefully have it done by tomorrow [June 15].

Eric Raymond, president of the Open Source Initiative, added, “It isn’t a big deal. This one can be trivially fixed. This fixable kernel crasher doesn’t cause any new problems.”

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