From 249e8dc73ea54aa09d828cf83f5a5de6bf5702f9 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Thu, 21 Mar 2019 05:28:44 -0400 Subject: [PATCH 1/2] refs/files-backend: handle packed transaction prepare failure In files_transaction_prepare(), if we have to delete some refs, we use a subordinate packed_transaction to do so. It's rare for that sub-transaction's prepare step to fail, since we hold the packed-refs lock. But if it does, we trigger a BUG() due to these steps: - we've attached the packed transaction to the files transaction as backend_data->packed_transaction - when the prepare step fails, the packed transaction cleans itself up, putting itself into the CLOSED state - the error value from preparing the packed transaction lets us know in files_transaction_prepare() that we should also clean up and return an error. We call files_transaction_cleanup(), which tries to abort backend_data->packed_transaction. Since it's already CLOSED, that triggers an assertion in ref_transaction_abort(). We can fix that by disconnecting the packed transaction from the outer files transaction, and then free-ing (not aborting!) it ourselves. A few other options/alternatives I considered: - we could just make it a noop to abort a CLOSED transaction. But that seems less safe, since clearly this code expects (and enforces) a particular set of state transitions. - we could have files_transaction_cleanup() selectively call abort() vs free() based on the state of the on the packed transaction. That's basically a more restricted version of the above, but also potentially unsafe. - instead of disconnecting backend_data->packed_transaction on error, we could wait to install it until we successfully prepare. That might make the flow a little simpler, but it introduces a hassle. Earlier parts of files_transaction_prepare() that encounter an error will jump to the cleanup label, and expect that cleaning up the outer transaction will clean up the packed transaction, too. We'd have to adjust those sites to clean up the packed transaction. Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- refs/files-backend.c | 10 ++++++++++ t/t1404-update-ref-errors.sh | 16 ++++++++++++++++ 2 files changed, 26 insertions(+) diff --git a/refs/files-backend.c b/refs/files-backend.c index dd8abe9185..0a2929bf6a 100644 --- a/refs/files-backend.c +++ b/refs/files-backend.c @@ -2696,6 +2696,16 @@ static int files_transaction_prepare(struct ref_store *ref_store, if (is_packed_transaction_needed(refs->packed_ref_store, packed_transaction)) { ret = ref_transaction_prepare(packed_transaction, err); + /* + * A failure during the prepare step will abort + * itself, but not free. Do that now, and disconnect + * from the files_transaction so it does not try to + * abort us when we hit the cleanup code below. + */ + if (ret) { + ref_transaction_free(packed_transaction); + backend_data->packed_transaction = NULL; + } } else { /* * We can skip rewriting the `packed-refs` diff --git a/t/t1404-update-ref-errors.sh b/t/t1404-update-ref-errors.sh index 6b6a8e2292..970c5c36b9 100755 --- a/t/t1404-update-ref-errors.sh +++ b/t/t1404-update-ref-errors.sh @@ -618,4 +618,20 @@ test_expect_success 'delete fails cleanly if packed-refs file is locked' ' test_cmp unchanged actual ' +test_expect_success 'delete fails cleanly if packed-refs.new write fails' ' + # Setup and expectations are similar to the test above. + prefix=refs/failed-packed-refs && + git update-ref $prefix/foo $C && + git pack-refs --all && + git update-ref $prefix/foo $D && + git for-each-ref $prefix >unchanged && + # This should not happen in practice, but it is an easy way to get a + # reliable error (we open with create_tempfile(), which uses O_EXCL). + : >.git/packed-refs.new && + test_when_finished "rm -f .git/packed-refs.new" && + test_must_fail git update-ref -d $prefix/foo && + git for-each-ref $prefix >actual && + test_cmp unchanged actual +' + test_done From d3322eb28b142a893fbc03142724bb54e95990a6 Mon Sep 17 00:00:00 2001 From: Jeff King Date: Thu, 21 Mar 2019 05:28:54 -0400 Subject: [PATCH 2/2] refs/files-backend: don't look at an aborted transaction When deleting refs, we hold packed-refs.lock and prepare a packed transaction to drop the refs from the packed-refs file. If it turns out that we don't need to rewrite the packed refs (e.g., because none of the deletions were present in the file), then we abort the transaction. If that abort succeeds, then the transaction struct will have been freed, and we set our local pointer to NULL so we don't look at it again. However, if it fails, then the struct will _still_ have been freed (because ref_transaction_abort() always frees). But we don't clean up the pointer, and will jump to our cleanup code, which will try to abort it again, causing a use-after-free. It's actually impossible for this to trigger in practice, since packed_transaction_abort() will never return anything but success. But let's fix it anyway, since that's more than we should assume about the packed-refs code (after all, we are already bothering to check for an error result which cannot be triggered). Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- refs/files-backend.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/refs/files-backend.c b/refs/files-backend.c index 0a2929bf6a..09608e6c43 100644 --- a/refs/files-backend.c +++ b/refs/files-backend.c @@ -2712,12 +2712,16 @@ static int files_transaction_prepare(struct ref_store *ref_store, * file. But we do need to leave it locked, so * that somebody else doesn't pack a reference * that we are trying to delete. + * + * We need to disconnect our transaction from + * backend_data, since the abort (whether successful or + * not) will free it. */ + backend_data->packed_transaction = NULL; if (ref_transaction_abort(packed_transaction, err)) { ret = TRANSACTION_GENERIC_ERROR; goto cleanup; } - backend_data->packed_transaction = NULL; } }