news
Kernel Space: Linux and BSD
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Himanshu Anand ☛ I Missed One TLB Shootdown and Somehow Ended Up Controlling a Page Table
I used a public Linux kernel bug to make one CPU keep using a memory mapping after another CPU had removed it. I then persuaded Linux to recycle the physical page behind that stale mapping as a page table. My unprivileged process could still write to the old page, while Linux now treated the same bytes as part of its memory map.
That chain produced one real flag from Google’s kernelCTF service. This was a 1/n day exploit, not a new vulnerability: the bug, reproducer and fix were already public. My work was the long and occasionally silly trip from “stale read” to a complete exploit.
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Ruben Schade ☛ ZFS silently discarding invalid acltype
This surprised me last week, so I’m passing it on for what it’s worth. I used my standard FreeBSD Ansible playbook to define a new zpool on a Linux host, and in the process inadvertently set the acltype to nfsv4, which Linux doesn’t support.
At least, I thought? I checked the pool: [...]
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Graphics Stack
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AMD Will Enable HDMI 2.1 by Default in Linux 7.4 With VRR and ALLM
For years, using HDMI to connect a PC with AMD graphics to a new TV or monitor on Linux was frustrating. Although the hardware had more than enough support for HDMI 2.1, licensing restrictions from the HDMI Forum prevented FRL (Fixed Rate Link) technology from being included in open-source drivers such as AMDGPU. This left users stuck with lower resolutions and refresh rates or forced them to depend only on DisplayPort.
This is finally changing. AMD has prepared a set of changes for the Linux 7.4 kernel that will not only enable HDMI 2.1 FRL by default, but will also add the full set of key gaming technologies such as FreeSync, VRR (Variable Refresh Rate) and ALLM (Auto Low Latency Mode).
The addition of FRL is the real driving force behind all of this. This feature replaces the old TMDS standard from HDMI 2.0 and makes it possible to use the full bandwidth of the cable. In everyday use, this means gaming at 4K and 120 Hz (or higher), with greater color depth and DSC compression without stuttering.
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