[Linux-kernel-mentees] [PATCH] fs: fix KMSAN uninit-value bug by initializing nd in do_file_open_root

[Al Viro]

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.