kernfs: Relax constraint in draining guard

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: kernfs: Relax constraint in draining guard The active reference lifecycle provides the break/unbreak mechanism but the active reference is not truly active after unbreak -- callers don't use it afterwards but it's important for proper pairing of kn->active counting. Assuming this mechanism is in place, the WARN check in kernfs_should_drain_open_files() is too sensitive -- it may transiently catch those (rightful) callers between kernfs_unbreak_active_protection() and kernfs_put_active() as found out by Chen Ridong: kernfs_remove_by_name_ns kernfs_get_active // active=1 __kernfs_remove // active=0x80000002 kernfs_drain ... wait_event //waiting (active == 0x80000001) kernfs_break_active_protection // active = 0x80000001 // continue kernfs_unbreak_active_protection // active = 0x80000002 ... kernfs_should_drain_open_files // warning occurs kernfs_put_active To avoid the false positives (mind panic_on_warn) remove the check altogether. (This is meant as quick fix, I think active reference break/unbreak may be simplified with larger rework.)

References

• https://git.kernel.org/stable/c/2d6a67c2b3b87808a347dc1047b520a9dd177a4f
• https://git.kernel.org/stable/c/6c81f1c7812c61f187bed1b938f1d2e391d503ab
• https://git.kernel.org/stable/c/6bfb154f95d5f0ab7ed056f23aba8c1a94cb3927
• https://git.kernel.org/stable/c/72275c888f8962b406ee9c6885c79bf68cca5a63
• https://git.kernel.org/stable/c/071d8e4c2a3b0999a9b822e2eb8854784a350f8a