Re: ovl: uninitialized pointer read in ovl_lookup_real_one

[Miklos Szeredi <miklos@szeredi.hu>]

On Sat, 24 Jul 2021 at 09:36, Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Sat, Jul 24, 2021 at 1:26 AM Colin Ian King <colin.king@canonical.com> wrote:
> >
> > Hi,
> >
> > Static analysis with Coverity has detected an uninitialized pointer read
> > in function ovl_lookup_real_one in fs/overlayfs/export.c
> >
> > The issue was introduced with the following commit:
> >
> > commit 3985b70a3e3f58109dc6ae347eafe6e8610be41e
> > Author: Amir Goldstein <amir73il@gmail.com>
> > Date:   Thu Dec 28 18:36:16 2017 +0200
> >
> >     ovl: decode connected upper dir file handles
> >
> > The analysis is as follows:
> >
> > 365static struct dentry *ovl_lookup_real_one(struct dentry *connected,
> > 366                                          struct dentry *real,
> > 367                                          const struct ovl_layer *layer)
> > 368{
> > 369        struct inode *dir = d_inode(connected);
> > 370        struct dentry *this, *parent = NULL;
> >
> >    1. var_decl: Declaring variable name without initializer.
> >
> > 371        struct name_snapshot name;
> > 372        int err;
> > 373
> > 374        /*
> > 375         * Lookup child overlay dentry by real name. The dir mutex
> > protects us
> > 376         * from racing with overlay rename. If the overlay dentry
> > that is above
> > 377         * real has already been moved to a parent that is not under the
> > 378         * connected overlay dir, we return -ECHILD and restart the
> > lookup of
> > 379         * connected real path from the top.
> > 380         */
> > 381        inode_lock_nested(dir, I_MUTEX_PARENT);
> > 382        err = -ECHILD;
> > 383        parent = dget_parent(real);
> >
> >    2. Condition ovl_dentry_real_at(connected, layer->idx) != parent,
> > taking true branch.
> >
> > 384        if (ovl_dentry_real_at(connected, layer->idx) != parent)
> >
> >    3. Jumping to label fail.
> >
> > 385                goto fail;
> > 386
> > 387        /*
> > 388         * We also need to take a snapshot of real dentry name to
> > protect us
> > 389         * from racing with underlying layer rename. In this case, we
> > don't
> > 390         * care about returning ESTALE, only from dereferencing a
> > free name
> > 391         * pointer because we hold no lock on the real dentry.
> > 392         */
> > 393        take_dentry_name_snapshot(&name, real);
> > 394        this = lookup_one_len(name.name.name, connected, name.name.len);
> > 395        err = PTR_ERR(this);
> > 396        if (IS_ERR(this)) {
> > 397                goto fail;
> > 398        } else if (!this || !this->d_inode) {
> > 399                dput(this);
> > 400                err = -ENOENT;
> > 401                goto fail;
> > 402        } else if (ovl_dentry_real_at(this, layer->idx) != real) {
> > 403                dput(this);
> > 404                err = -ESTALE;
> > 405                goto fail;
> > 406        }
> > 407
> > 408out:
> >
> >    Uninitialized pointer read
> >    6. uninit_use_in_call: Using uninitialized value name.name.name when
> > calling release_dentry_name_snapshot.
> >
> > 409        release_dentry_name_snapshot(&name);
> > 410        dput(parent);
> > 411        inode_unlock(dir);
> > 412        return this;
> > 413
> > 414fail:
> >
> >    4. Condition ___ratelimit(&_rs, <anonymous>), taking false branch
> > .
> > 415        pr_warn_ratelimited("failed to lookup one by real (%pd2,
> > layer=%d, connected=%pd2, err=%i)\n",
> > 416                            real, layer->idx, connected, err);
> > 417        this = ERR_PTR(err);
> >
> >    5. Jumping to label out.
> >
> > 418        goto out;
> > 419}
> >
> > The error exit path on line 395 ends up with an uninitialized structure
> > name being passed to function release_dentry_name_snapshot() on line 409
> > and this accesses the pointer name.name.name, see /fs/dcache.c as follows:
> >
> > 303void release_dentry_name_snapshot(struct name_snapshot *name)
> > 304{
> >
> >    1. read_value: Reading value name->name.name.
> >    2. Condition !!(name->name.name != name->inline_name), taking true
> > branch.
> >
> > 305        if (unlikely(name->name.name != name->inline_name)) {
> > 306                struct external_name *p;
> >
> >    3. Condition 0 /* !!(!__builtin_types_compatible_p() &&
> > !__builtin_types_compatible_p()) */, taking false branch.
> >
> >
> > I suspect name should be initialized in line 371, e.g. name = { } and a
> > null name check should be performed on line 409 before calling
> > release_dentry_name_snapshot, but this seems a bit message as a fix.
> >
>
> Thanks for the report.
> A simpler fix is to move take_dentry_name_snapshot() to top of the
> function before goto fail.

Even simpler:  move the release_dentry_name_snapshot to just after lookup.

Commit 89741437981a ("ovl: fix uninitialized pointer read in
ovl_lookup_real_one()") pushed to #overlayfs-next.

Thanks,
Miklos