On Tue, 12 May 2015, Jan Kara wrote:
> Date: Tue, 12 May 2015 14:24:30 +0200
> From: Jan Kara <jack@xxxxxxx>
> To: Lukas Czerner <lczerner@xxxxxxxxxx>
> Cc: tytso@xxxxxxx, linux-ext4@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 1/2] ext4: fix NULL pointer dereference when journal
> restart fails
>
> On Wed 06-05-15 10:05:13, Lukas Czerner wrote:
> > Currently when journal restart fails, we'll have the h_transaction of
> > the handle set to NULL to indicate that the handle has been effectively
> > aborted. However it's not _really_ aborted and we need to treat this
> > differently because an abort indicates critical error which might not be
> > the case here.
> >
> > So we handle this situation quietly in the jbd2_journal_stop() and just
> > free the handle and exit because everything else has been done before we
> > attempted (and failed) to restart the journal.
> >
> > Unfortunately there are a number of problems with that approach
> > introduced with commit
> >
> > 41a5b913197c "jbd2: invalidate handle if jbd2_journal_restart()
> > fails"
> >
> > First of all in ext4 jbd2_journal_stop() will be called through
> > __ext4_journal_stop() where we would try to get a hold of the superblock
> > by dereferencing h_transaction which in this case would lead to NULL
> > pointer dereference and crash.
> >
> > In addition we're going to free the handle regardless of the refcount
> > which is bad as well, because other's up the call chain will still
> > reference the handle so we might potentially reference already freed
> > memory.
> >
> > I think that setting the h_transaction to NULL is just a hack and we can
> > do this properly by introducing h_stopped flag to the handle structure.
> No, it isn't a hack. It is actually clearly representing what has
> happened. By the time we set h_transaction to NULL, handle isn't associated
> with that transaction in any way (we decremented transaction->t_updates and
> thus the transaction can happily commit and be freed). So keeping
> h_transaction set is just hiding the real problem and possibly causing
> use-after-free issues.
It's funny you say that since the current solution which is "not a
hack" has caused both NULL pointer dereference and use-after-free
issues :)
It was simply not expected by the code to get a h_transaction unset
among other problems. But I do understand your point and that we may
be asking for problems in the future by leaving h_transaction set
even though the handle has been disconnected from the transaction
and hence can be eventually freed.
I'll rework it.
Thanks!
-Lukas
>
> Now the problems you describe above are real. IMO we should treat failed
> jbd2_journal_restart() as a handle abort as close as possible - after all
> jbd2_journal_restart() can fail only in circumstances when we'd abort the
> handle anyway. Fixing jbd2_journal_stop() to handle refcount properly is
> easy enough. We could fix ext4_journal_stop() to just silently call
> jbd2_journal_stop() for handle that is already detached and thus avoid the
> NULL pointer dereference.
>
> If we really wanted to avoid NULL h_transaction, the clean fix for that
> would IMHO be to replace jbd2_journal_restart() with jbd2_journal_stop()
> and jbd2_journal_start() and return new handle from jbd2_journal_restart().
> But that would need some benchmarking to verify it doesn't regress
> something and it would be a pain to propagate new handle out of all the
> callers of ext4_journal_restart().
>
> Honza
>
> > Now we use h_stopped flag to indicate that the handle has already
> > been stopped so there is nothing else to do in jbd2_journal_stop() other
> > than decrease the refcount, or free the handle structure (in case refcount
> > drops to zero) and exit which exactly fits our case. So if we fail to start
> > the handle in jbd2__journal_restart() instead of setting h_transaction to
> > NULL we set the h_stopped flag and let the jbd2_journal_stop() deal with
> > this.
> >
> > This will also fix the NULL dereference because we no longer free the
> > h_transaction.
> >
> > Moreover remove all the WARN_ON's when we're dealing with already
> > stopped handle. Again the situation is quite similar with the aborted
> > handle and it is possible to get an already stopped handle, in this case
> > we do not want to emit an backtrace.
> >
> > Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
> > ---
> > fs/jbd2/transaction.c | 45 ++++++++++++++++++++++++++-------------------
> > include/linux/jbd2.h | 20 +++++++++++++++++++-
> > 2 files changed, 45 insertions(+), 20 deletions(-)
> >
> > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> > index 5f09370..34bd0c5 100644
> > --- a/fs/jbd2/transaction.c
> > +++ b/fs/jbd2/transaction.c
> > @@ -361,6 +361,7 @@ repeat:
> > handle->h_transaction = transaction;
> > handle->h_requested_credits = blocks;
> > handle->h_start_jiffies = jiffies;
> > + handle->h_stopped = 0;
> > atomic_inc(&transaction->t_updates);
> > atomic_inc(&transaction->t_handle_count);
> > jbd_debug(4, "Handle %p given %d credits (total %d, free %lu)\n",
> > @@ -551,8 +552,7 @@ int jbd2_journal_extend(handle_t *handle, int nblocks)
> > int result;
> > int wanted;
> >
> > - WARN_ON(!transaction);
> > - if (is_handle_aborted(handle))
> > + if (is_handle_aborted_or_stopped(handle))
> > return -EROFS;
> > journal = transaction->t_journal;
> >
> > @@ -627,11 +627,10 @@ int jbd2__journal_restart(handle_t *handle, int nblocks, gfp_t gfp_mask)
> > tid_t tid;
> > int need_to_start, ret;
> >
> > - WARN_ON(!transaction);
> > /* If we've had an abort of any type, don't even think about
> > * actually doing the restart! */
> > - if (is_handle_aborted(handle))
> > - return 0;
> > + if (is_handle_aborted_or_stopped(handle))
> > + return -EROFS;
> > journal = transaction->t_journal;
> >
> > /*
> > @@ -653,7 +652,7 @@ int jbd2__journal_restart(handle_t *handle, int nblocks, gfp_t gfp_mask)
> > wake_up(&journal->j_wait_updates);
> > tid = transaction->t_tid;
> > spin_unlock(&transaction->t_handle_lock);
> > - handle->h_transaction = NULL;
> > + jbd2_journal_stop_handle(handle);
> > current->journal_info = NULL;
> >
> > jbd_debug(2, "restarting handle %p\n", handle);
> > @@ -785,8 +784,7 @@ do_get_write_access(handle_t *handle, struct journal_head *jh,
> > int need_copy = 0;
> > unsigned long start_lock, time_lock;
> >
> > - WARN_ON(!transaction);
> > - if (is_handle_aborted(handle))
> > + if (is_handle_aborted_or_stopped(handle))
> > return -EROFS;
> > journal = transaction->t_journal;
> >
> > @@ -849,7 +847,7 @@ repeat:
> > unlock_buffer(bh);
> >
> > error = -EROFS;
> > - if (is_handle_aborted(handle)) {
> > + if (is_handle_aborted_or_stopped(handle)) {
> > jbd_unlock_bh_state(bh);
> > goto out;
> > }
> > @@ -1051,9 +1049,8 @@ int jbd2_journal_get_create_access(handle_t *handle, struct buffer_head *bh)
> > int err;
> >
> > jbd_debug(5, "journal_head %p\n", jh);
> > - WARN_ON(!transaction);
> > err = -EROFS;
> > - if (is_handle_aborted(handle))
> > + if (is_handle_aborted_or_stopped(handle))
> > goto out;
> > journal = transaction->t_journal;
> > err = 0;
> > @@ -1266,8 +1263,7 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
> > struct journal_head *jh;
> > int ret = 0;
> >
> > - WARN_ON(!transaction);
> > - if (is_handle_aborted(handle))
> > + if (is_handle_aborted_or_stopped(handle))
> > return -EROFS;
> > journal = transaction->t_journal;
> > jh = jbd2_journal_grab_journal_head(bh);
> > @@ -1397,8 +1393,7 @@ int jbd2_journal_forget (handle_t *handle, struct buffer_head *bh)
> > int err = 0;
> > int was_modified = 0;
> >
> > - WARN_ON(!transaction);
> > - if (is_handle_aborted(handle))
> > + if (is_handle_aborted_or_stopped(handle))
> > return -EROFS;
> > journal = transaction->t_journal;
> >
> > @@ -1530,8 +1525,19 @@ int jbd2_journal_stop(handle_t *handle)
> > tid_t tid;
> > pid_t pid;
> >
> > - if (!transaction)
> > - goto free_and_exit;
> > + if (is_handle_stopped(handle)) {
> > + /*
> > + * Handle is stopped so there is nothing to do other than
> > + * decrease a refcount, or free the handle if refcount
> > + * drops to zero
> > + */
> > + if (--handle->h_ref > 0) {
> > + jbd_debug(4, "h_ref %d -> %d\n", handle->h_ref + 1,
> > + handle->h_ref);
> > + return err;
> > + } else
> > + goto free_and_exit;
> > + }
> > journal = transaction->t_journal;
> >
> > J_ASSERT(journal_current_handle() == handle);
> > @@ -1665,7 +1671,9 @@ int jbd2_journal_stop(handle_t *handle)
> >
> > if (handle->h_rsv_handle)
> > jbd2_journal_free_reserved(handle->h_rsv_handle);
> > +
> > free_and_exit:
> > + jbd2_journal_stop_handle(handle);
> > jbd2_free_handle(handle);
> > return err;
> > }
> > @@ -2373,8 +2381,7 @@ int jbd2_journal_file_inode(handle_t *handle, struct jbd2_inode *jinode)
> > transaction_t *transaction = handle->h_transaction;
> > journal_t *journal;
> >
> > - WARN_ON(!transaction);
> > - if (is_handle_aborted(handle))
> > + if (is_handle_aborted_or_stopped(handle))
> > return -EROFS;
> > journal = transaction->t_journal;
> >
> > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> > index 20e7f78..349975e 100644
> > --- a/include/linux/jbd2.h
> > +++ b/include/linux/jbd2.h
> > @@ -441,6 +441,7 @@ struct jbd2_journal_handle
> > unsigned int h_jdata: 1; /* force data journaling */
> > unsigned int h_reserved: 1; /* handle with reserved credits */
> > unsigned int h_aborted: 1; /* fatal error on handle */
> > + unsigned int h_stopped: 1; /* handle is already stopped */
> > unsigned int h_type: 8; /* for handle statistics */
> > unsigned int h_line_no: 16; /* for handle statistics */
> >
> > @@ -1266,18 +1267,35 @@ static inline int is_journal_aborted(journal_t *journal)
> > return journal->j_flags & JBD2_ABORT;
> > }
> >
> > +static inline int is_handle_stopped(handle_t *handle)
> > +{
> > + return handle->h_stopped;
> > +}
> > +
> > static inline int is_handle_aborted(handle_t *handle)
> > {
> > - if (handle->h_aborted || !handle->h_transaction)
> > + if (handle->h_aborted)
> > return 1;
> > return is_journal_aborted(handle->h_transaction->t_journal);
> > }
> >
> > +static inline int is_handle_aborted_or_stopped(handle_t *handle)
> > +{
> > + if (is_handle_aborted(handle) || is_handle_stopped(handle))
> > + return 1;
> > + return 0;
> > +}
> > +
> > static inline void jbd2_journal_abort_handle(handle_t *handle)
> > {
> > handle->h_aborted = 1;
> > }
> >
> > +static inline void jbd2_journal_stop_handle(handle_t *handle)
> > +{
> > + handle->h_stopped = 1;
> > +}
> > +
> > #endif /* __KERNEL__ */
> >
> > /* Comparison functions for transaction IDs: perform comparisons using
> > --
> > 1.8.3.1
> >
> > --
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