CVE-2026-89704

In the Linux kernel, the following vulnerability has been resolved:

nfsd: sample writeback error cursor before async COPY loop

_nfsd_copy_file_range() samples dst->f_wb_err into "since"

after the copy loop, then uses it to detect writeback errors

via filemap_check_wb_err() once vfs_fsync_range() returns.

Because the nfsd_file cache reuses a single struct file

across requests targeting the same inode, a concurrent

COMMIT or stable WRITE on dst advances dst->f_wb_err to the

current mapping->wb_err via file_check_and_advance_wb_err()

during its own vfs_fsync_range(). If that advancement lands

between the writeback error appearing in mapping->wb_err

and the COPY worker sampling "since", the worker captures

the already-advanced cursor, errseq_check() sees cur ==

since and returns zero, and NFSD4_COPY_F_COMMITTED is set

even though writeback failed. CB_OFFLOAD then encodes

wr_stable_how = FILE_SYNC4, the client treats the copied

data as durable, and the failure becomes silent data loss.

Sample since once at the start of the function. The cursor

then reflects state in effect before this COPY issues any

writes, and filemap_check_wb_err() detects any error that

occurs during the copy regardless of which thread first

observes it. This matches the pattern used by

nfsd_vfs_write() and nfsd4_clone_file_range().