Bug 2536776 (CVE-2026-90260)

Summary: CVE-2026-90260 kernel: btrfs: zoned: don't clobber the extent buffer when zeroing it out
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akhatavk, aos-team-art-private, asdas, dpaolell, jdelft, jupierce, lgarciaa, mbiarnes, ppalepu, ppostler, prdhamdh, rhel-process-autobot, sghai, sidsharm, suppawar, vlaad, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's Btrfs zoned filesystem. When a freed-but-still-dirty tree block is written out as zeros, the btree_csum_one_bio() function improperly clears the extent buffer's in-memory content. This can lead to corruption of the extent tree, potentially causing system instability or a denial of service (DoS) due to an "unable to find ref" error or an assertion failure.
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Description OSIDB Bzimport 2026-09-17 19:32:04 UTC 
In the Linux kernel, the following vulnerability has been resolved:

btrfs: zoned: don't clobber the extent buffer when zeroing it out

On a zoned filesystem a freed-but-still-dirty tree block is written out
as zeros (EXTENT_BUFFER_ZONED_ZEROOUT) only to keep the zone write
pointer advancing. btree_csum_one_bio() implemented this by memzeroing
the extent buffer's own folios before submission.

That destroys the in-memory buffer while it may still be referenced. In
particular btrfs_free_tree_block() can run on it afterwards and reads
the header to add a delayed reference; once the header has been zeroed
it frees bytenr 0 and corrupts the extent tree (the
btrfs_header_bytenr(buf) != 0 ASSERT in btrfs_free_tree_block(), or an
"unable to find ref" abort). It is flaky and reproduces under fsstress,
e.g. generic/461 and generic/013.

Write the zeros to disk from the shared zero page instead and leave the
extent buffer content untouched, so any later reference - including the
delayed reference from btrfs_free_tree_block() - still sees a valid
header. end_bbio_meta_write() now clears writeback on the buffer's own
folios, as the bio no longer carries them.
Comment 1 gkamathe 2026-09-23 21:27:16 UTC 
Upstream advisory:
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2026/CVE-2026-90260.mbox