On Fri, Jun 9, 2023 at 3:12 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> On Fri, Jun 09, 2023 at 02:57:20PM +0300, Amir Goldstein wrote:
> > On Fri, Jun 9, 2023 at 2:32 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Jun 09, 2023 at 10:32:37AM +0300, Amir Goldstein wrote:
> > > > Overlayfs and cachefiles use open_with_fake_path() to allocate internal
> > > > files, where overlayfs also puts a "fake" path in f_path - a path which
> > > > is not on the same fs as f_inode.
> > > >
> > > > Allocate a container struct file_fake for those internal files, that
> > > > will be used to hold the fake path qlong with the real path.
> > > >
> > > > This commit does not populate the extra fake_path field and leaves the
> > > > overlayfs internal file's f_path fake.
> > > >
> > > > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> > > > ---
> > > > fs/cachefiles/namei.c | 2 +-
> > > > fs/file_table.c | 85 +++++++++++++++++++++++++++++++++++--------
> > > > fs/internal.h | 5 ++-
> > > > fs/namei.c | 2 +-
> > > > fs/open.c | 11 +++---
> > > > fs/overlayfs/file.c | 2 +-
> > > > include/linux/fs.h | 13 ++++---
> > > > 7 files changed, 90 insertions(+), 30 deletions(-)
> > > >
> > > > diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
> > > > index 82219a8f6084..a71bdf2d03ba 100644
> > > > --- a/fs/cachefiles/namei.c
> > > > +++ b/fs/cachefiles/namei.c
> > > > @@ -561,7 +561,7 @@ static bool cachefiles_open_file(struct cachefiles_object *object,
> > > > path.mnt = cache->mnt;
> > > > path.dentry = dentry;
> > > > file = open_with_fake_path(&path, O_RDWR | O_LARGEFILE | O_DIRECT,
> > > > - d_backing_inode(dentry), cache->cache_cred);
> > > > + &path, cache->cache_cred);
> > > > if (IS_ERR(file)) {
> > > > trace_cachefiles_vfs_error(object, d_backing_inode(dentry),
> > > > PTR_ERR(file),
> > > > diff --git a/fs/file_table.c b/fs/file_table.c
> > > > index 372653b92617..adc2a92faa52 100644
> > > > --- a/fs/file_table.c
> > > > +++ b/fs/file_table.c
> > > > @@ -44,18 +44,48 @@ static struct kmem_cache *filp_cachep __read_mostly;
> > > >
> > > > static struct percpu_counter nr_files __cacheline_aligned_in_smp;
> > > >
> > > > +/* Container for file with optional fake path to display in /proc files */
> > > > +struct file_fake {
> > > > + struct file file;
> > > > + struct path fake_path;
> > > > +};
> > > > +
> > > > +static inline struct file_fake *file_fake(struct file *f)
> > > > +{
> > > > + return container_of(f, struct file_fake, file);
> > > > +}
> > > > +
> > > > +/* Returns fake_path if one exists, f_path otherwise */
> > > > +const struct path *file_fake_path(struct file *f)
> > > > +{
> > > > + struct file_fake *ff = file_fake(f);
> > > > +
> > > > + if (!(f->f_mode & FMODE_FAKE_PATH) || !ff->fake_path.dentry)
> > > > + return &f->f_path;
> > > > +
> > > > + return &ff->fake_path;
> > > > +}
> > > > +EXPORT_SYMBOL(file_fake_path);
> > > > +
> > > > static void file_free_rcu(struct rcu_head *head)
> > > > {
> > > > struct file *f = container_of(head, struct file, f_rcuhead);
> > > >
> > > > put_cred(f->f_cred);
> > > > - kmem_cache_free(filp_cachep, f);
> > > > + if (f->f_mode & FMODE_FAKE_PATH)
> > > > + kfree(file_fake(f));
> > > > + else
> > > > + kmem_cache_free(filp_cachep, f);
> > > > }
> > > >
> > > > static inline void file_free(struct file *f)
> > > > {
> > > > + struct file_fake *ff = file_fake(f);
> > > > +
> > > > security_file_free(f);
> > > > - if (!(f->f_mode & FMODE_NOACCOUNT))
> > > > + if (f->f_mode & FMODE_FAKE_PATH)
> > > > + path_put(&ff->fake_path);
> > > > + else
> > > > percpu_counter_dec(&nr_files);
> > > > call_rcu(&f->f_rcuhead, file_free_rcu);
> > > > }
> > > > @@ -131,20 +161,15 @@ static int __init init_fs_stat_sysctls(void)
> > > > fs_initcall(init_fs_stat_sysctls);
> > > > #endif
> > > >
> > > > -static struct file *__alloc_file(int flags, const struct cred *cred)
> > > > +static int init_file(struct file *f, int flags, const struct cred *cred)
> > > > {
> > > > - struct file *f;
> > > > int error;
> > > >
> > > > - f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);
> > > > - if (unlikely(!f))
> > > > - return ERR_PTR(-ENOMEM);
> > > > -
> > > > f->f_cred = get_cred(cred);
> > > > error = security_file_alloc(f);
> > > > if (unlikely(error)) {
> > > > file_free_rcu(&f->f_rcuhead);
> > > > - return ERR_PTR(error);
> > > > + return error;
> > > > }
> > > >
> > > > atomic_long_set(&f->f_count, 1);
> > > > @@ -155,6 +180,22 @@ static struct file *__alloc_file(int flags, const struct cred *cred)
> > > > f->f_mode = OPEN_FMODE(flags);
> > > > /* f->f_version: 0 */
> > > >
> > > > + return 0;
> > > > +}
> > > > +
> > > > +static struct file *__alloc_file(int flags, const struct cred *cred)
> > > > +{
> > > > + struct file *f;
> > > > + int error;
> > > > +
> > > > + f = kmem_cache_zalloc(filp_cachep, GFP_KERNEL);
> > > > + if (unlikely(!f))
> > > > + return ERR_PTR(-ENOMEM);
> > > > +
> > > > + error = init_file(f, flags, cred);
> > > > + if (unlikely(error))
> > > > + return ERR_PTR(error);
> > > > +
> > > > return f;
> > > > }
> > > >
> > > > @@ -201,18 +242,32 @@ struct file *alloc_empty_file(int flags, const struct cred *cred)
> > > > }
> > > >
> > > > /*
> > > > - * Variant of alloc_empty_file() that doesn't check and modify nr_files.
> > > > + * Variant of alloc_empty_file() that allocates a file_fake container
> > > > + * and doesn't check and modify nr_files.
> > > > *
> > > > * Should not be used unless there's a very good reason to do so.
> > > > */
> > > > -struct file *alloc_empty_file_noaccount(int flags, const struct cred *cred)
> > > > +struct file *alloc_empty_file_fake(const struct path *fake_path, int flags,
> > > > + const struct cred *cred)
> > > > {
> > > > - struct file *f = __alloc_file(flags, cred);
> > > > + struct file_fake *ff;
> > > > + int error;
> > > >
> > > > - if (!IS_ERR(f))
> > > > - f->f_mode |= FMODE_NOACCOUNT;
> > > > + ff = kzalloc(sizeof(struct file_fake), GFP_KERNEL);
> > > > + if (unlikely(!ff))
> > > > + return ERR_PTR(-ENOMEM);
> > > >
> > > > - return f;
> > > > + error = init_file(&ff->file, flags, cred);
> > > > + if (unlikely(error))
> > > > + return ERR_PTR(error);
> > > > +
> > > > + ff->file.f_mode |= FMODE_FAKE_PATH;
> > > > + if (fake_path) {
> > > > + path_get(fake_path);
> > > > + ff->fake_path = *fake_path;
> > > > + }
> > >
> > > Hm, I see that this check is mostly done for vfs_tmpfile_open() which
> > > only fills in file->f_path in vfs_tmpfile() but leaves ff->fake_path
> > > NULL.
> > >
> > > So really I think having FMODE_FAKE_PATH set but ff->fake_path be NULL
> > > is an invitation for NULL derefs sooner or later. I would simply
> > > document that it's required to set ff->fake_path. For callers such as
> > > vfs_tmpfile_open() it can just be path itself. IOW, vfs_tmpfile_open()
> > > should set ff->fake_path to file->f_path.
> >
> > Makes sense.
> > I also took the liberty to re-arrange vfs_tmpfile_open() without the
> > unneeded if (!error) { nesting depth.
>
> Yes, please. I had a rough sketch just for my own amusement...
>
> fs/namei.c
> struct file *vfs_tmpfile_open(struct mnt_idmap *idmap,
> const struct path *parentpath, umode_t mode,
> int open_flag, const struct cred *cred)
> {
> struct file *file;
> int error;
>
> file = alloc_empty_file_fake(open_flag, cred);
> if (IS_ERR(file))
> return file;
>
> error = vfs_tmpfile(idmap, parentpath, file, mode);
> if (error) {
> fput(file);
> return ERR_PTR(error);
> }
>
> return file_set_fake_path(file, &file->f_path);
FYI, this is not enough to guarantee that the fake_path cannot
be empty, for example in fput() above.
So I did keep the real_path empty in this case in v3 and
I have an accessor that verifies that real_path is not empty
before returning it.
Thanks,
Amir.
