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Subject[PATCH][RFC] Fix breakage in ffs_fs_mount()

There's a bunch of failure exits in ffs_fs_mount() with
seriously broken recovery logics. Most of that appears to stem
from misunderstanding of the ->kill_sb() semantics; unlike
->put_super() it is called for *all* superblocks of given type,
no matter how (in)complete the setup had been. ->put_super()
is called only if ->s_root is not NULL; any failure prior to
setting ->s_root will have the call of ->put_super() skipped.
->kill_sb(), OTOH, awaits every superblock that has come from
sget().

Current behaviour of ffs_fs_mount():

We have struct ffs_sb_fill_data data on stack there. We do
ffs_dev = functionfs_acquire_dev_callback(dev_name);
and store that in data.private_data. Then we call mount_nodev(),
passing it ffs_sb_fill() as a callback. That will either fail
outright, or manage to call ffs_sb_fill(). There we allocate an
instance of struct ffs_data, slap the value of ffs_dev (picked
from data.private_data) into ffs->private_data and overwrite
data.private_data by storing ffs into an overlapping member
(data.ffs_data). Then we store ffs into sb->s_fs_info and attempt
to set the rest of the things up (root inode, root dentry, then
create /ep0 there). Any of those might fail. Should that
happen, we get ffs_fs_kill_sb() called before mount_nodev()
returns. If mount_nodev() fails for any reason whatsoever,
we proceed to
functionfs_release_dev_callback(data.ffs_data);

That's broken in a lot of ways. Suppose the thing has failed in
allocation of e.g. root inode or dentry. We have
functionfs_release_dev_callback(ffs);
ffs_data_put(ffs);
done by ffs_fs_kill_sb() (ffs accessed via sb->s_fs_info), followed by
functionfs_release_dev_callback(ffs);
from ffs_fs_mount() (via data.ffs_data). Note that the second
functionfs_release_dev_callback() has every chance to be done to freed memory.

Suppose we fail *before* root inode allocation. What happens then?
ffs_fs_kill_sb() doesn't do anything to ffs (it's either not called at all,
or it doesn't have a pointer to ffs stored in sb->s_fs_info). And
functionfs_release_dev_callback(data.ffs_data);
is called by ffs_fs_mount(), but here we are in nasal daemon country - we
are reading from a member of union we'd never stored into. In practice,
we'll get what we used to store into the overlapping field, i.e. ffs_dev.
And then we get screwed, since we treat it (struct gfs_ffs_obj * in
disguise, returned by functionfs_acquire_dev_callback()) as struct
ffs_data *, pick what would've been ffs_data ->private_data from it
(*well* past the actual end of the struct gfs_ffs_obj - struct ffs_data
is much bigger) and poke in whatever it points to.

FWIW, there's a minor leak on top of all that in case if ffs_sb_fill()
fails on kstrdup() - ffs is obviously forgotten.

The thing is, there is no point in playing all those games with union.
Just allocate and initialize ffs_data *before* calling mount_nodev() and
pass a pointer to it via data.ffs_data. And once it's stored in
sb->s_fs_info, clear data.ffs_data, so that ffs_fs_mount() knows that
it doesn't need to kill the sucker manually - from that point on
we'll have it done by ->kill_sb().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
--
diff --git a/drivers/usb/gadget/f_fs.c b/drivers/usb/gadget/f_fs.c
index 1a66c5b..0658908 100644
--- a/drivers/usb/gadget/f_fs.c
+++ b/drivers/usb/gadget/f_fs.c
@@ -1034,37 +1034,19 @@ struct ffs_sb_fill_data {
struct ffs_file_perms perms;
umode_t root_mode;
const char *dev_name;
- union {
- /* set by ffs_fs_mount(), read by ffs_sb_fill() */
- void *private_data;
- /* set by ffs_sb_fill(), read by ffs_fs_mount */
- struct ffs_data *ffs_data;
- };
+ struct ffs_data *ffs_data;
};

static int ffs_sb_fill(struct super_block *sb, void *_data, int silent)
{
struct ffs_sb_fill_data *data = _data;
struct inode *inode;
- struct ffs_data *ffs;
+ struct ffs_data *ffs = data->ffs_data;

ENTER();

- /* Initialise data */
- ffs = ffs_data_new();
- if (unlikely(!ffs))
- goto Enomem;
-
ffs->sb = sb;
- ffs->dev_name = kstrdup(data->dev_name, GFP_KERNEL);
- if (unlikely(!ffs->dev_name))
- goto Enomem;
- ffs->file_perms = data->perms;
- ffs->private_data = data->private_data;
-
- /* used by the caller of this function */
- data->ffs_data = ffs;
-
+ data->ffs_data = NULL;
sb->s_fs_info = ffs;
sb->s_blocksize = PAGE_CACHE_SIZE;
sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
@@ -1080,17 +1062,14 @@ static int ffs_sb_fill(struct super_block *sb, void *_data, int silent)
&data->perms);
sb->s_root = d_make_root(inode);
if (unlikely(!sb->s_root))
- goto Enomem;
+ return -ENOMEM;

/* EP0 file */
if (unlikely(!ffs_sb_create_file(sb, "ep0", ffs,
&ffs_ep0_operations, NULL)))
- goto Enomem;
+ return -ENOMEM;

return 0;
-
-Enomem:
- return -ENOMEM;
}

static int ffs_fs_parse_opts(struct ffs_sb_fill_data *data, char *opts)
@@ -1193,6 +1172,7 @@ ffs_fs_mount(struct file_system_type *t, int flags,
struct dentry *rv;
int ret;
void *ffs_dev;
+ struct ffs_data *ffs;

ENTER();

@@ -1200,18 +1180,30 @@ ffs_fs_mount(struct file_system_type *t, int flags,
if (unlikely(ret < 0))
return ERR_PTR(ret);

+ ffs = ffs_data_new();
+ if (unlikely(!ffs))
+ return ERR_PTR(-ENOMEM);
+ ffs->file_perms = data.perms;
+
+ ffs->dev_name = kstrdup(dev_name, GFP_KERNEL);
+ if (unlikely(!ffs->dev_name)) {
+ ffs_data_put(ffs);
+ return ERR_PTR(-ENOMEM);
+ }
+
ffs_dev = functionfs_acquire_dev_callback(dev_name);
- if (IS_ERR(ffs_dev))
- return ffs_dev;
+ if (IS_ERR(ffs_dev)) {
+ ffs_data_put(ffs);
+ return ERR_CAST(ffs_dev);
+ }
+ ffs->private_data = ffs_dev;
+ data.ffs_data = ffs;

- data.dev_name = dev_name;
- data.private_data = ffs_dev;
rv = mount_nodev(t, flags, &data, ffs_sb_fill);
-
- /* data.ffs_data is set by ffs_sb_fill */
- if (IS_ERR(rv))
+ if (IS_ERR(rv) && data.ffs_data) {
functionfs_release_dev_callback(data.ffs_data);
-
+ ffs_data_put(data.ffs_data);
+ }
return rv;
}


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