On Thu, Sep 17, 2020 at 01:22:38AM +0100, Al Viro wrote: > On Tue, Sep 15, 2020 at 10:41:57PM -0700, Eric Biggers wrote: > > > Looking at the actual KMSAN report, it looks like it's nameidata::dir_mode or > > nameidata::dir_uid that is uninitialized. You need to figure out the correct > > solution, not just blindly initialize with zeroes -- that could hide a bug. > > Is there a bug that is preventing these fields from being initialized to the > > correct values, are these fields being used when they shouldn't be, etc... > > False positive, and this is the wrong place to shut it up. > > ->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... thanks, greg k-h
