CVE-2026-90263

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

btrfs: check if root is readonly when setting posix acl

For a filesystem which has btrfs read-only property set to true, all
write operations including acl and xattr should be denied. However, acl
can still be set even if btrfs ro property is true.

This happens because no function on the set_acl code path checks the root
is readonly or not. It was checked in btrfs_setxattr_trans() but got
removed in commit 353c2ea735e4 ("btrfs: remove redundant readonly root
check in btrfs_setxattr_trans")

That commit didn't check if all the callers properly check the root's
read-only flag. A previous fix is commit b51111271b03 ("btrfs: check if
root is readonly while setting security xattr").

Always check if the root is read-only before performing the set acl
operation.