CVE-2026-23157 SUSE CVE security@suse.de SUSE Security Team SUSE CVE-2026-23157 Interim 1 1 2026-03-05T00:17:12Z current 2026-03-05T00:17:12Z 2026-03-05T00:17:12Z cve-database/bin/generate-cvrf-cve.pl 2020-12-27T01:00:00Z CVE-2026-23157 In the Linux kernel, the following vulnerability has been resolved: btrfs: do not strictly require dirty metadata threshold for metadata writepages [BUG] There is an internal report that over 1000 processes are waiting at the io_schedule_timeout() of balance_dirty_pages(), causing a system hang and trigger a kernel coredump. The kernel is v6.4 kernel based, but the root problem still applies to any upstream kernel before v6.18. [CAUSE] From Jan Kara for his wisdom on the dirty page balance behavior first. This cgroup dirty limit was what was actually playing the role here because the cgroup had only a small amount of memory and so the dirty limit for it was something like 16MB. Dirty throttling is responsible for enforcing that nobody can dirty (significantly) more dirty memory than there's dirty limit. Thus when a task is dirtying pages it periodically enters into balance_dirty_pages() and we let it sleep there to slow down the dirtying. When the system is over dirty limit already (either globally or within a cgroup of the running task), we will not let the task exit from balance_dirty_pages() until the number of dirty pages drops below the limit. So in this particular case, as I already mentioned, there was a cgroup with relatively small amount of memory and as a result with dirty limit set at 16MB. A task from that cgroup has dirtied about 28MB worth of pages in btrfs btree inode and these were practically the only dirty pages in that cgroup. So that means the only way to reduce the dirty pages of that cgroup is to writeback the dirty pages of btrfs btree inode, and only after that those processes can exit balance_dirty_pages(). Now back to the btrfs part, btree_writepages() is responsible for writing back dirty btree inode pages. The problem here is, there is a btrfs internal threshold that if the btree inode's dirty bytes are below the 32M threshold, it will not do any writeback. This behavior is to batch as much metadata as possible so we won't write back those tree blocks and then later re-COW them again for another modification. This internal 32MiB is higher than the existing dirty page size (28MiB), meaning no writeback will happen, causing a deadlock between btrfs and cgroup: - Btrfs doesn't want to write back btree inode until more dirty pages - Cgroup/MM doesn't want more dirty pages for btrfs btree inode Thus any process touching that btree inode is put into sleep until the number of dirty pages is reduced. Thanks Jan Kara a lot for the analysis of the root cause. [ENHANCEMENT] Since kernel commit b55102826d7d ("btrfs: set AS_KERNEL_FILE on the btree_inode"), btrfs btree inode pages will only be charged to the root cgroup which should have a much larger limit than btrfs' 32MiB threshold. So it should not affect newer kernels. But for all current LTS kernels, they are all affected by this problem, and backporting the whole AS_KERNEL_FILE may not be a good idea. Even for newer kernels I still think it's a good idea to get rid of the internal threshold at btree_writepages(), since for most cases cgroup/MM has a better view of full system memory usage than btrfs' fixed threshold. For internal callers using btrfs_btree_balance_dirty() since that function is already doing internal threshold check, we don't need to bother them. But for external callers of btree_writepages(), just respect their requests and write back whatever they want, ignoring the internal btrfs threshold to avoid such deadlock on btree inode dirty page balancing. The CVRF data is provided by SUSE under the Creative Commons License 4.0 with Attribution (CC-BY-4.0). https://www.suse.com/support/security/rating/ SUSE Security Ratings SUSE Linux Enterprise Module for Basesystem 15 SP7 SUSE Linux Enterprise Module for Development Tools 15 SP7 SUSE Linux Enterprise Workstation Extension 15 SP7 SUSE Linux Enterprise High Availability Extension 15 SP7 SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS SUSE Linux Enterprise High Performance Computing 15 SP5-LTSS SUSE Linux Enterprise Module for Basesystem 15 SP7 SUSE Linux Enterprise Module for Development Tools 15 SP7 SUSE Linux Enterprise Module for Legacy 15 SP7 SUSE Linux Enterprise Module for Public Cloud 15 SP7 SUSE Linux Enterprise Live Patching 12 SP5 SUSE Linux Enterprise Live Patching 15 SP4 SUSE Linux Enterprise Live Patching 15 SP5 SUSE Linux Enterprise Live Patching 15 SP7 SUSE Linux Enterprise Micro 5.0 SUSE Linux Enterprise Micro 5.1 SUSE Linux Enterprise Micro 5.2 SUSE Linux 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as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 dlm-kmp-default as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 gfs2-kmp-default as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 kernel-default as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 kernel-default-devel as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 ocfs2-kmp-default as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 reiserfs-kmp-default as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 kernel-devel as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 kernel-macros as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 kernel-source as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP5 cluster-md-kmp-default as a component of SUSE Linux Enterprise Server for SAP Applications 15 SP6 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kernel-macros as a component of SUSE Linux Enterprise Server for SAP applications 16.0 kernel-source as a component of SUSE Linux Enterprise Server for SAP applications 16.0 kernel-source-vanilla as a component of SUSE Linux Enterprise Server for SAP applications 16.0 kernel-devel-azure as a component of SUSE Linux Enterprise Server for SAP applications 16.0 kernel-source-azure as a component of SUSE Linux Enterprise Server for SAP applications 16.0 kernel-default-extra as a component of SUSE Linux Enterprise Workstation Extension 15 SP7 kernel-default as a component of SUSE Linux Enterprise Workstation Extension 15 SP7 kernel-source as a component of SUSE Linux Enterprise Workstation Extension 15 SP7 kernel-default as a component of SUSE Linux Micro 6.0 kernel-devel as a component of SUSE Linux Micro 6.0 kernel-macros as a component of SUSE Linux Micro 6.0 kernel-source as a component of SUSE Linux Micro 6.0 kernel-devel-rt as a component of SUSE Linux Micro 6.0 kernel-source-rt as a 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Leap 15.6 kernel-default-vdso as a component of openSUSE Leap 15.6 kselftests-kmp-default as a component of openSUSE Leap 15.6 ocfs2-kmp-default as a component of openSUSE Leap 15.6 reiserfs-kmp-default as a component of openSUSE Leap 15.6 kernel-devel as a component of openSUSE Leap 15.6 kernel-macros as a component of openSUSE Leap 15.6 kernel-source as a component of openSUSE Leap 15.6 kernel-source-vanilla as a component of openSUSE Leap 15.6 In the Linux kernel, the following vulnerability has been resolved: btrfs: do not strictly require dirty metadata threshold for metadata writepages [BUG] There is an internal report that over 1000 processes are waiting at the io_schedule_timeout() of balance_dirty_pages(), causing a system hang and trigger a kernel coredump. The kernel is v6.4 kernel based, but the root problem still applies to any upstream kernel before v6.18. [CAUSE] From Jan Kara for his wisdom on the dirty page balance behavior first. This cgroup dirty limit was what was actually playing the role here because the cgroup had only a small amount of memory and so the dirty limit for it was something like 16MB. Dirty throttling is responsible for enforcing that nobody can dirty (significantly) more dirty memory than there's dirty limit. Thus when a task is dirtying pages it periodically enters into balance_dirty_pages() and we let it sleep there to slow down the dirtying. When the system is over dirty limit already (either globally or within a cgroup of the running task), we will not let the task exit from balance_dirty_pages() until the number of dirty pages drops below the limit. So in this particular case, as I already mentioned, there was a cgroup with relatively small amount of memory and as a result with dirty limit set at 16MB. A task from that cgroup has dirtied about 28MB worth of pages in btrfs btree inode and these were practically the only dirty pages in that cgroup. So that means the only way to reduce the dirty pages of that cgroup is to writeback the dirty pages of btrfs btree inode, and only after that those processes can exit balance_dirty_pages(). Now back to the btrfs part, btree_writepages() is responsible for writing back dirty btree inode pages. The problem here is, there is a btrfs internal threshold that if the btree inode's dirty bytes are below the 32M threshold, it will not do any writeback. This behavior is to batch as much metadata as possible so we won't write back those tree blocks and then later re-COW them again for another modification. This internal 32MiB is higher than the existing dirty page size (28MiB), meaning no writeback will happen, causing a deadlock between btrfs and cgroup: - Btrfs doesn't want to write back btree inode until more dirty pages - Cgroup/MM doesn't want more dirty pages for btrfs btree inode Thus any process touching that btree inode is put into sleep until the number of dirty pages is reduced. Thanks Jan Kara a lot for the analysis of the root cause. [ENHANCEMENT] Since kernel commit b55102826d7d ("btrfs: set AS_KERNEL_FILE on the btree_inode"), btrfs btree inode pages will only be charged to the root cgroup which should have a much larger limit than btrfs' 32MiB threshold. So it should not affect newer kernels. But for all current LTS kernels, they are all affected by this problem, and backporting the whole AS_KERNEL_FILE may not be a good idea. Even for newer kernels I still think it's a good idea to get rid of the internal threshold at btree_writepages(), since for most cases cgroup/MM has a better view of full system memory usage than btrfs' fixed threshold. For internal callers using btrfs_btree_balance_dirty() since that function is already doing internal threshold check, we don't need to bother them. But for external callers of btree_writepages(), just respect their requests and write back whatever they want, ignoring the internal btrfs threshold to avoid such deadlock on btree inode dirty page balancing. CVE-2026-23157 moderate 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H