All posts by corbet

[$] Timer IDs, CRIU, and ABI challenges

Post Syndicated from corbet original https://lwn.net/Articles/1012490/

The kernel project has usually been willing to make fundamental internal
changes if they lead to a better kernel in the end. The project also,
though, goes out of its way to avoid breaking interfaces that have been
exposed to user space, even if programs come to rely on behavior that was
never documented. Sometimes, those two principles come into conflict,
leading to a situation where fixing problems within the kernel is either
difficult or impossible. This sort of situation has been impeding
performance improvements in the kernel’s POSIX timers implementation for
some time, but it appears that a solution has been found.

Zen and the Art of Microcode Hacking (Google Bug Hunters)

Post Syndicated from corbet original https://lwn.net/Articles/1013136/

The Google Bug Hunters blog has a
detailed description of how a vulnerability in AMD’s microcode-patching
functionality was discovered and exploited; the authors have also released
a set of tools to assist with this kind of research in the future.

Secure hash functions are designed in such a way that there is no
secret key, and there is no way to use knowledge of the
intermediate state in order to generate a collision. However, CMAC
was not designed as a hash function, and therefore it is a weak
hash function against an adversary who has the key. Remember that
every AMD Zen CPU has to have the same AES-CMAC key in order to
successfully calculate the hash of the AMD public key and the
microcode patch contents. Therefore, the key only needs to be
revealed from a single CPU in order to compromise all other CPUs
using the same key. This opens up the potential for hardware
attacks (e.g., reading the key from ROM with a scanning electron
microscope), side-channel attacks (e.g., using Correlation Power
Analysis to leak the key during validation), or other software or
hardware attacks that can somehow reveal the key. In summary, it is
a safe assumption that such a key will not remain secret forever.

Security updates for Tuesday

Post Syndicated from corbet original https://lwn.net/Articles/1012915/

Security updates have been issued by AlmaLinux (kernel), Mageia (x11-server), Red Hat (emacs and webkit2gtk3), SUSE (ffmpeg-7, govulncheck-vulndb, kernel, and skopeo), and Ubuntu (cmark-gfm, erlang, krb5, linux-gcp-6.8, linux-raspi, linux-kvm, lucene-solr, postgresql-12, postgresql-14, postgresql-16, raptor2, spip, tomcat7, and wpa).

[$] Guard pages for file-backed memory

Post Syndicated from corbet original https://lwn.net/Articles/1011366/

One of the many new features packed into the 6.13 kernel release was guard
pages, a hardening mechanism that makes it possible to inject zero-access
pages into a process’s address space in an efficient way. That feature
only supports anonymous (user-space data) pages, though. To make guard
pages more widely useful, Lorenzo Stoakes has put together a patch
set enabling the feature for file-backed pages as well; in the process,
he examined and resolved a long list of potential problems that extending
the feature could encounter. One potential problem was not on his list,
though.

Terms of use and privacy changes for Firefox

Post Syndicated from corbet original https://lwn.net/Articles/1012430/

There is a fair amount of unhappiness on the Internet about the announcement
from Mozilla about a new “terms of
use” agreement and an updated
privacy notice for the Firefox browser.

Firefox will always continue to add new features, improve existing
ones, and test new ideas. We remain dedicated to making Firefox
open source, but we believe that doing so along with an official
Terms of Use will give you more transparency over your rights and
permissions as you use Firefox. And actually asking you to
acknowledge it is an important step, so we’re making it a part of
the standard product experience starting in early March for new
users and later this year for existing ones.

Specifically, the apparent
removal of a promise to not sell users’ personal data has drawn
attention.

(See also: this
analysis by Michael Taggart. “So, is this Mozilla ‘going evil?’
Nah, prolly not. But it is at best clumsy, and a poor showing if they want
me to believe they care about Firefox, rather than the data it can
provide“.)

McKenney: Speaking at Kernel Recipes

Post Syndicated from corbet original https://lwn.net/Articles/1012286/

Paul McKenney has put together a series of
articles on how to improve one’s ability to give a good talk at a
technical conference.

On the other hand, (1) presentation skills stay with you through
life, and (2) small improvements in presentation skills over months
or years can provide you with great advantages longer term. An old
saying credited to Thomas Edison claims a breakdown of 1%
inspiration and 99% perspiration. However, my own experience with
RCU has instead been 0.1% inspiration, 9.9% perspiration, and 90%
communication. Had I been unable to communicate effectively,
others would have extreme difficulty using RCU, as in even more
difficulty than they do now.

There is a lot of speaking experience distilled into this set of posts.

[$] A hole in FineIBT protection

Post Syndicated from corbet original https://lwn.net/Articles/1011680/

Intel’s indirect
branch tracking (IBT) is a hardware-implemented control-flow-integrity
mechanism that makes it harder for an attacker to gain control of the
system by way of a corrupted indirect branch. FineIBT is a software
extension to IBT that is meant to improve its protection. Recently,
though, Jennifer Miller reported a novel way to bypass
FineIBT by taking advantage of how the kernel’s system-call entry point is
constructed. In response, Peter Zijlstra is working on some FineIBT
enhancements to close that hole and make IBT more secure in general.

[$] LWN.net Weekly Edition for February 27, 2025

Post Syndicated from corbet original https://lwn.net/Articles/1011011/

Inside this week’s LWN.net Weekly Edition:

  • Front: Tail calls in CPython; BPF cancellation; Slabs, sheaves, and barns; Atomic block writes; Large filesystem block sizes; EPEL 10 for older CPUs; pytest-mh; Open-source battery.
  • Briefs: DMA discussion; Armbian 25.2; Gentoo qcow2; Aqualung 2.0; Emacs 30.1; Rust 1.85.0; Quotes; …
  • Announcements: Newsletters, conferences, security updates, patches, and more.

Two new site features: full-text RSS and automatic dark mode

Post Syndicated from corbet original https://lwn.net/Articles/1011830/

One of the often-requested LWN site features that has languished the
longest on our to-do list is full-text RSS feeds. We are happy to announce
that, finally, there is a set of such feeds available; the full set can be
seen on our feeds page. This is a
subscriber-only feature, and it works by creating a unique fetch URL for
each user. We will, of course, be counting on our readers to not share
those URLs.

Another feature we have had requests for is to automatically present the
site in dark-mode colors when a reader’s browser has been configured to
prefer it. That feature, too, is now available. In this case, we had to
think about the interaction between automatic selection and the color
customization that the site has long had. The conclusion we reached is
that, if custom colors have been configured for an account, they will win
out over the automatic selection. There is a new preference in the customization area to change this
default if desired.

Both of these features — and the other enhancements we have made recently —
were enabled by the support of LWN’s subscribers. By making it possible to
bring in new staff last year, you created the space to improve the site
experience while keeping up with the writing. We thank all of you for your
support.

A change in maintenance for the kernel’s DMA-mapping layer

Post Syndicated from corbet original https://lwn.net/Articles/1011819/

The conversation around the merging of a set of Rust abstractions for the
kernel’s DMA-mapping layer has mostly settled after Linus Torvalds made it clear that the code would be
accepted. One other consequence of this decision, though, is that
Christoph Hellwig has quietly stepped down from the
maintenance of the DMA-mapping code. Marek Szyprowski will be the
maintainer of that layer going forward. Hellwig has maintained that code
for many years; his contributions will be missed.

Security updates for Tuesday

Post Syndicated from corbet original https://lwn.net/Articles/1011764/

Security updates have been issued by AlmaLinux (libpq, postgresql:13, postgresql:15, and postgresql:16), Debian (nodejs and php-nesbot-carbon), Mageia (neomutt), Red Hat (python3.11-urllib3 and tuned), SUSE (crun, ovmf, pam_pkcs11, qemu, and webkit2gtk3), and Ubuntu (iniparser, libcap2, linux, linux-hwe, linux, linux-hwe-5.4, linux, linux-lowlatency, linux-azure, linux-azure-5.4, linux-bluefield, linux-gcp, linux-gcp-5.4, linux-ibm-5.4, linux-azure, linux-azure-fde, linux-gkeop, linux-nvidia, linux-oracle, linux-azure-5.15, linux-azure-fde-5.15, linux-oracle-5.15, linux-gcp, linux-gcp-5.15, linux-gke, linux-kvm, linux-lowlatency-hwe-5.15, and linux-xilinx-zynqmp).

[$] Slabs, sheaves, and barns

Post Syndicated from corbet original https://lwn.net/Articles/1010667/

The kernel’s slab allocator is responsible for the allocation of small
(usually sub-page) chunks of memory. For many workloads, the speed of
object allocation and freeing is one of the key factors in overall
performance, so it is not surprising that a lot of effort has gone into
optimizing the slab allocator over time. Now that the kernel is down to a single slab allocator, the
memory-management developers have free rein to add complexity to it; the
latest move in that direction is the per-CPU
sheaves patch set from slab maintainer Vlastimil Babka.