Entry was gained via a machine that goes by the name of Hera.
The breach was discovered on August 28 and, while the kernel team believes the source code repositories were not affected, investigations are underway to check and also to bolster security across the project infrastructure.
Update, September 1, 1.35pm AEST: The British technology news site, The Register, reports that the intrusion went undetected for 17 days. This is based on an email it obtained which was sent to developers by John Hawley, the chief sysadmin of kernel.org.
The breach is believed to have occurred via a compromised user credential; how the attacker or attackers used that to gain superuser status is not yet known.
The attacker(s) had modified ssh files (openssh, openssh-server and openssh-clients) and these were running. In addition, a trojan startup file had been added to the startup scripts on Hera.
The kernel team has logged user interactions as well as some exploit code.
The trojan was initially discovered due to error messages apparently coming from a package, xnest, that was not installed; if similar behaviour was observed elsewhere developers were advised to investigate. However the presence of such messages did not make it clear that the machine was compromised, susceptible or not.
The kernel team has now taken servers offline to back-up and reinstall; a full reinstall would be done on all servers at kernel.org. Authorities in the US and Europe had been notified.
Code in the git distributed revision control system and tarballs is being analysed to check for any modifications.
"However, it's also useful to note that the potential damage of cracking kernel.org is far less than typical software repositories," the advisory pointed out.
"That's because kernel development takes place using the git distributed revision control system, designed by Linus Torvalds. For each of the nearly 40,000 files in the Linux kernel, a cryptographically secure SHA-1 hash is calculated to uniquely define the exact contents of that file.
"Git is designed so that the name of each version of the kernel depends upon the complete development history leading up to that version. Once it is published, it is not possible to change the old versions without it being noticed."
The advisory added: "Those files and the corresponding hashes exist not just on the kernel.org machine and its mirrors, but on the hard drives of each several thousand kernel developers, distribution maintainers, and other users of kernel.org.
"Any tampering with any file in the kernel.org repository would immediately be noticed by each developer as they updated their personal repository, which most do daily."