In the Linux kernel, the following vulnerability has been resolved:
fuse: fix invalidate lock leak on setattr writeback failure
fuse_do_setattr() takes filemap_invalidate_lock() for a DAX truncate
(fault_blocked = true) and releases it at the out:/error: labels. But
when a writeback flush is also needed, a write_inode_now() failure
returns directly and leaks the lock, so any later fault or truncate on
the file stalls on the stale rwsem.
For example, truncate(2) on a setuid file reaches fuse_do_setattr()
with both ATTR_SIZE and ATTR_MODE set:
truncate(2)
└─ do_truncate()
├─ dentry_needs_remove_privs() # S_ISUID
└─ notify_change() # KILL_SUID -> ATTR_MODE
└─ fuse_setattr() # no killpriv:
│ # ia_valid |= ATTR_MODE
└─ fuse_do_setattr()
├─ filemap_invalidate_lock() # IS_DAX && is_truncate
└─ write_inode_now() # is_wb && ATTR_MODE
└─ if (err) # e.g. daemon -> -EIO
return err # <- lock leaked
Fix this by adding an unlock label that releases the lock before
returning the error, and use it for the fuse_dax_break_layouts()
failure path as well.