In the Linux kernel, the following vulnerability has been resolved: netfs: Fix zeropoint update where i_size > remote_i_size Fix the update of the zero point[*] by netfs_release_folio() when there is uncommitted data in the pagecache b…

Overview

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

netfs: Fix zeropoint update where i_size > remote_i_size

Fix the update of the zero point[*] by netfs_release_folio() when there is uncommitted data in the pagecache beyond the folio being released but the on-server EOF is in this folio (ie. i_size > remote_i_size). The update needs to limit zero_point to remote_i_size, not i_size as i_size is a local phenomenon reflecting updates made locally to the pagecache, not stuff written to the server. remote_i_size tracks the server's i_size.

[*] The zero point is the file position from which we can assume that the server will just return zeros, so we can avoid generating reads.

Note that netfs_invalidate_folio() probably doesn't need fixing as zero_point should be updated by setattr after truncation or fallocate.

Found with:

fsx -q -N 1000000 -p 10000 -o 128000 -l 600000 \
    /xfstest.test/junk --replay-ops=junk.fsxops

using the following as junk.fsxops:

truncate 0x0 0x1bbae 0x82864
write 0x3ef2e 0xf9c8 0x1bbae
write 0x67e05 0xcb5a 0x4e8f6
mapread 0x57781 0x85b6 0x7495f
copy_range 0x5d3d 0x10329 0x54fac 0x7495f
write 0x64710 0x1c2b 0x7495f
mapread 0x64000 0x1000 0x7495f

on cifs with the default cache option.

It shows read-gaps on folio 0x64 failing with a short read (ie. it hits EOF) if the FMODE_READ check is commented out in netfs_perform_write():

            if (//(file->f_mode & FMODE_READ) ||
                netfs_is_cache_enabled(ctx)) {

and no fscache. This was initially found with the generic/522 xfstest.

Affected

• linux_kernel >= 6.10.8, < 6.11
• linux_kernel >= 6.11.1, < 7.0.11
• linux_kernel = 6.11
• linux_kernel = 7.1

Remediation

Upgrade past the affected range:

• linux_kernel 7.0.11