On Sat, Sep 19, 2020 at 04:44:51PM +0200, Greg KH wrote: > > ->dir_uid and ->dir_mode are set when link_path_walk() resolves the pathname > > to directory + final component. They are used when deciding whether to reject > > a trailing symlink (on fs.protected_symlinks setups) and whether to allow > > creation in sticky directories (on fs.protected_regular and fs.protected_fifos > > setups). Both operations really need the results of successful link_path_walk(). > > > > I don't see how that could be not a false positive. If we hit the use in > > may_create_in_sticky(), we'd need the combination of > > * pathname that consists only of slashes (or it will be initialized) > > * LAST_NORM in nd->last_type, which is flat-out impossible, since > > we are left with LAST_ROOT for such pathnames. The same goes for > > may_follow_link() use - we need WALK_TRAILING in flags to hit it in the > > first place, which can come from two sources - > > return walk_component(nd, WALK_TRAILING); > > in lookup_last() (and walk_component() won't go anywhere near the > > call chain leading to may_follow_link() without LAST_NORM in nd->last_type) > > and > > res = step_into(nd, WALK_TRAILING, dentry, inode, seq); > > in open_last_lookups(), which also won't go anywhere near that line without > > LAST_NORM in the nd->last_type. > > > > IOW, unless we manage to call that without having called link_path_walk() > > at all or after link_path_walk() returning an error, we shouldn't hit > > that. And if we *do* go there without link_path_walk() or with an error > > from link_path_walk(), we have a much worse problem. > > > > I want to see the details of reproducer. If it's for real, we have a much > > more serious problem; if it's a false positive, the right place to deal > > with it would be elsewhere (perhaps on return from link_path_walk() with > > a slashes-only pathname), but in any case it should only be done after we > > manage to understand what's going on. > > Reproducer is pretty simple: > https://syzkaller.appspot.com/text?tag=ReproC&x=13974b2f100000 > > Now if that is actually valid or not, I don't know... Lovely... That would get an empty path and non-directory for a starting point, but it should end up with LAST_ROOT in nd->last_type. Which should not be able to reach the readers of those fields... Which kernel had that been on?
