From: Phillip Wood Date: Wed, 06 Oct 2021 14:01:18 GMT Subject: Re: [PATCH] [RFC] sparse index: fix use-after-free bug in cache_tree_verify() Message-ID: In-Reply-To: <2b1f0e56-5bb4-7f41-5a1e-d8a21096084a@gmail.com> Hi Stolee On 06/10/2021 12:20, Derrick Stolee wrote: > On 10/6/2021 5:29 AM, Phillip Wood via GitGitGadget wrote: >> From: Phillip Wood >> >> In a sparse index it is possible for the tree that is being verified >> to be freed while it is being verified. This happens when >> index_name_pos() looks up a entry that is missing from the index and >> that would be a descendant of a sparse entry. That triggers a call to >> ensure_full_index() which frees the cache tree that is being verified. >> Carrying on trying to verify the tree after this results in a >> use-after-free bug. Instead restart the verification if a sparse index >> is converted to a full index. This bug is triggered by a call to >> reset_head() in "git rebase --apply". Thanks to René Scharfe for his >> help analyzing the problem. > > Thank you for identifying an interesting case! I hadn't thought to > change the mode from --merge to --apply. Thanks, I can't really take much credit for that though - Junio pointed out that my patch converting the merge based rebase to use the same checkout code as the apply based rebase broke a test in seen and René diagnosed the problem. >> In a sparse index it is possible for the tree that is being verified to >> be freed while it is being verified. This is an RFC as I'm not familiar >> with the cache tree code. I'm confused as to why this bug is triggered >> by the sequence >> >> unpack_trees() >> prime_cache_tree() >> write_locked_index() >> >> but not >> >> unpack_trees() >> write_locked_index() >> >> >> as unpack_trees() appears to update the cache tree with >> >> if (!cache_tree_fully_valid(o->result.cache_tree)) >> cache_tree_update(&o->result, >> WRITE_TREE_SILENT | >> WRITE_TREE_REPAIR); >> >> >> and I don't understand why the cache tree from prime_cache_tree() >> results in different behavior. It concerns me that this fix is hiding >> another bug. > > prime_cache_tree() appears to clear the cache tree and start from scratch > from a tree object instead of using the index. > > In particular, prime_cache_tree_rec() does not stop at the sparse-checkout > cone, so the cache tree is the full size at that point. > > When the verify_one() method reaches these nodes that are outside of the > cone, index_name_pos() triggers the index expansion in a way that the > cache-tree that is restricted to the sparse-checkout cone does not. > > Hopefully that helps clear up _why_ this happens. It does thanks - we end up with a full cache tree but a sparse index > There is a remaining issue that "git rebase --apply" will be a lot slower > than "git rebase --merge" because of this construction of a cache-tree > that is much larger than necessary. > > I will make note of this as a potential improvement for the future. I think I'm going to remove the call to prime_cache_tree(). Correct me if I'm wrong but as I understand it unpack_trees() updates the cache tree so the call to prime_cache_tree() is not needed (I think it was copied from builtin/rebase.c which does need to call prime_cache_tree() if it has updated a few paths rather than the whole top-level tree). In any case I've just noticed that one of Victoria's patches[1] looks like it fixes prime_cache_tree() with a sparse index. [1] https://lore.kernel.org/git/78cd85d8dcc790251ce8235e649902cf6adf091a.1633440057.git.gitgitgadget@gmail.com/ >> -static void verify_one(struct repository *r, >> - struct index_state *istate, >> - struct cache_tree *it, >> - struct strbuf *path) >> +static int verify_one(struct repository *r, >> + struct index_state *istate, >> + struct cache_tree *it, >> + struct strbuf *path) >> { >> int i, pos, len = path->len; >> struct strbuf tree_buf = STRBUF_INIT; >> @@ -837,21 +837,30 @@ static void verify_one(struct repository *r, >> >> for (i = 0; i < it->subtree_nr; i++) { >> strbuf_addf(path, "%s/", it->down[i]->name); >> - verify_one(r, istate, it->down[i]->cache_tree, path); >> + if (verify_one(r, istate, it->down[i]->cache_tree, path)) >> + return 1; >> strbuf_setlen(path, len); >> } >> >> if (it->entry_count < 0 || >> /* no verification on tests (t7003) that replace trees */ >> lookup_replace_object(r, &it->oid) != &it->oid) >> - return; >> + return 0; >> >> if (path->len) { >> + /* >> + * If the index is sparse index_name_pos() may trigger >> + * ensure_full_index() which will free the tree that is being >> + * verified. >> + */ >> + int is_sparse = istate->sparse_index; >> pos = index_name_pos(istate, path->buf, path->len); >> + if (is_sparse && !istate->sparse_index) >> + return 1; > > I think this guard is good to have, even if we fix prime_cache_tree() to > avoid triggering expansion here in most cases. > >> if (pos >= 0) { >> verify_one_sparse(r, istate, it, path, pos); >> - return; >> + return 0; >> } >> >> pos = -pos - 1; >> @@ -899,6 +908,7 @@ static void verify_one(struct repository *r, >> oid_to_hex(&new_oid), oid_to_hex(&it->oid)); >> strbuf_setlen(path, len); >> strbuf_release(&tree_buf); >> + return 0; >> } >> >> void cache_tree_verify(struct repository *r, struct index_state *istate) >> @@ -907,6 +917,9 @@ void cache_tree_verify(struct repository *r, struct index_state *istate) >> >> if (!istate->cache_tree) >> return; >> - verify_one(r, istate, istate->cache_tree, &path); >> + if (verify_one(r, istate, istate->cache_tree, &path)) { >> + strbuf_reset(&path); >> + verify_one(r, istate, istate->cache_tree, &path); >> + } > > And this limits us to doing at most two passes. Good. In theory ensure_full_index() will only ever be called once but I wanted to make sure we could not get into an infinite loop. >> test_expect_success 'merge, cherry-pick, and rebase' ' >> init_repos && >> >> - for OPERATION in "merge -m merge" cherry-pick rebase >> + for OPERATION in "merge -m merge" cherry-pick "rebase --apply" "rebase --merge" > > Thank you for the additional test! Thanks for your explanation and looking at the patch Best Wishes Phillip > Thanks, > -Stolee >