Latest debdiff for 24.04 ** Patch added: "lp2155048-noble.debdiff" https://bugs.launchpad.net/ubuntu/+source/rust-virtiofsd/+bug/2155048/+attachment/5979484/+files/lp2155048-noble.debdiff
** Description changed: [ Impact ] Users running VMs with `virtiofs` mounts where there is a page-size mismatch between the host and guest kernels (e.g., a 4K page-size host and a 64K page-size guest) experience "Cannot allocate memory" (`ENOMEM`) errors when attempting to read directories. This occurs because `virtiofsd` computes `max_pages` based on the host's page size, but the guest converts `max_pages` back to bytes using its own (larger) page size. As a result, the guest sends a `READDIR` request that exceeds `virtiofsd`'s `MAX_BUFFER_SIZE`, causing the daemon to reject the request. The fix resolves this by capping the amount of directory data generated locally to `MAX_BUFFER_SIZE` instead of rejecting the oversized request, as `READDIR` is permitted to return fewer bytes than requested. [ Test Plan ] # Set up repos and download packages 0. Grab a fresh arm64 machine 1. add deb-src to ubuntu.sources 2. sudo apt build-dep linux 3. sudo apt install virtme-ng qemu-system-arm libncurses-dev 4. sudo apt install virtiofsd # the package in question - # 24.04 EXTRA SETUP (NOT NECESSARY FOR 26.04) + # 24.04 EXTRA SETUP (NOT NECESSARY FOR 26.04+) 4.1. sudo apt install python3-pip flake8 pylint cargo rustc 4.2. sudo apt remove virtme-ng # gone too soon :( 4.3. pip install --break-system-packages uv # now it's getting spicy 4.4. git clone https://github.com/arighi/virtme-ng.git && cd virtme-ng 4.5. BUILD_VIRTME_NG_INIT=1 uv tool install . # Set up vng and build kernel 5. sudo usermod -aG kvm ubuntu && newgrp kvm # replace `ubuntu` with your user 6. git clone https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ && cd linux 7. wait 10000 years 8. set up kernel config: a. vng --kconfig b. make menuconfig c. kernel features -> page size -> set to 64k -> save -> exit 9. vng --build # Run the reproducer (from upstream merge request) 10. getconf PAGE_SIZE # should print 4096 on host 11. vng --run arch/arm64/boot/Image --memory 16G --cpus 4 --overlay-rwdir /tmp --exec 'uname -r; getconf PAGE_SIZE; ls /tmp; echo "ls rc=$?"; python3 -c "import os; print(len(os.listdir(\"/tmp\")))"' 12. There should be no errors in the output of 11. # Reproducer output 12a. Unpatched package output (fail): ubuntu@kamek:~/linux$ vng --run arch/arm64/boot/Image --memory 16G --cpus 4 --overlay-rwdir /tmp --exec 'uname -r; getconf PAGE_SIZE; ls /tmp; echo "ls rc=$?"; python3 -c "import os; print(len(os.listdir(\"/tmp\")))"' 7.1.0-rc7-virtme 65536 ls: general io error: Cannot allocate memory (os error 12) ls rc=1 Traceback (most recent call last): File "<string>", line 1, in <module> import os; print(len(os.listdir("/tmp"))) ~~~~~~~~~~^^^^^^^^ OSError: [Errno 12] Cannot allocate memory: '/tmp' 12b. Patched package output (success): ubuntu@kamek:~/linux$ vng --run arch/arm64/boot/Image --memory 16G --cpus 4 --overlay-rwdir /tmp --exec 'uname -r; getconf PAGE_SIZE; ls /tmp; echo "ls rc=$?"; python3 -c "import os; print(len(os.listdir(\"/tmp\")))"' 7.1.0-rc7-virtme 65536 hsperfdata_root snap-private-tmp systemd-private-119639c5e17c430ca33b233f937d2924-ModemManager.service-EmreAg systemd-private-119639c5e17c430ca33b233f937d2924-chrony.service-DJCwIj systemd-private-119639c5e17c430ca33b233f937d2924-fwupd.service-DSIi3M systemd-private-119639c5e17c430ca33b233f937d2924-polkit.service-w28tHD systemd-private-119639c5e17c430ca33b233f937d2924-systemd-logind.service-bWi2S2 virtme_retvjz9uo9n ls rc=0 12 [ Where problems could occur ] This patch modifies the returned value of `READDIR`. If other programs incorrectly assume `READDIR` will never give short reads and do not handle the short read case correctly, this patch could lead to data corruption. However, this hypothetical depends on OTHER privileged programs behaving incorrectly, which is not the responsibility of virtiofsd. This patch strictly improves code correctness, so problems are unlikely. [ Other Info ] The whole rigmarole with building virtme-ng on 24.04 is necessary because the version available in the Noble repos (1.22) is too old to support virtiofs - mounts in the first place. 1.22 uses 9p exclusively on ARMv8; virtiofs support - was added in 1.30 for ARMv8. + mounts in the first place. 1.22 uses 9p exclusively on ARM64; virtiofs support + was added in 1.30 for ARM64. Additionally, we can't install virtme-ng via pip directly because the provided - `virtme-ng-init` initializer is compiled for AMD64, not ARMv8. Building vng + `virtme-ng-init` initializer is compiled for AMD64, not ARM64. Building vng from source was the most reproducible way I found to test this package. Finally, for Noble, a newer kernel may be required. See below as to why. `sudo apt install linux-image-7.0.0-14-generic --install-suggests` Extra info from upstream MR: --- The virtiofsd-side failure is easiest to reproduce after applying the kernel patch that backs uncached readdir output with pages: https://lore.kernel.org/all/[email protected]/ Without that kernel patch, the guest may fail earlier in the kernel before the oversized READDIR request reaches virtiofsd. ... (omitted for brevity) ... The page-size mismatch is what exposes the issue. virtiofsd computes max_pages from the host page size, while the guest converts max_pages back to bytes using the guest page size. With a 4K host and 64K guest, the guest can send a READDIR size larger than virtiofsd's MAX_BUFFER_SIZE. READDIR can return less than requested, so this patch caps the amount of directory data generated locally instead of rejecting the request. --- - Target Releases: Ubuntu 24.04 and 26.04. + Target Releases: Ubuntu 24.04, 26.04, and 26.10. Upstream Commit: d24cda8a325d server: do not reject oversized readdir requests Upstream Link: https://gitlab.com/virtio-fs/virtiofsd/-/commit/d24cda8a325d2a9ae1adc5acda57515ed2e8e1d2 Upstream Merge Request: https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/316 -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2155048 Title: [SRU] Backport request - server: do not reject oversized readdir requests To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/rust-virtiofsd/+bug/2155048/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
