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authorJeff King <peff@peff.net>2022-08-14 06:29:15 (GMT)
committerJunio C Hamano <gitster@pobox.com>2022-08-15 01:03:36 (GMT)
commit1490d7d82d5bd504b095c4db030a98b8080c596a (patch)
tree71125ae0d7c3a62c0fa0902dde34a181c479e1a4
parentc1fa951d7ea9e943f001ac7c7502995273db5776 (diff)
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is_promisor_object(): fix use-after-free of tree buffer
Since commit fcc07e980b (is_promisor_object(): free tree buffer after parsing, 2021-04-13), we'll always free the buffers attached to a "struct tree" after searching them for promisor links. But there's an important case where we don't want to do so: if somebody else is already using the tree! This can happen during a "rev-list --missing=allow-promisor" traversal in a partial clone that is missing one or more trees or blobs. The backtrace for the free looks like this: #1 free_tree_buffer tree.c:147 #2 add_promisor_object packfile.c:2250 #3 for_each_object_in_pack packfile.c:2190 #4 for_each_packed_object packfile.c:2215 #5 is_promisor_object packfile.c:2272 #6 finish_object__ma builtin/rev-list.c:245 #7 finish_object builtin/rev-list.c:261 #8 show_object builtin/rev-list.c:274 #9 process_blob list-objects.c:63 #10 process_tree_contents list-objects.c:145 #11 process_tree list-objects.c:201 #12 traverse_trees_and_blobs list-objects.c:344 [...] We're in the middle of walking through the entries of a tree object via process_tree_contents(). We see a blob (or it could even be another tree entry) that we don't have, so we call is_promisor_object() to check it. That function loops over all of the objects in the promisor packfile, including the tree we're currently walking. When we're done with it there, we free the tree buffer. But as we return to the walk in process_tree_contents(), it's still holding on to a pointer to that buffer, via its tree_desc iterator, and it accesses the freed memory. Even a trivial use of "--missing=allow-promisor" triggers this problem, as the included test demonstrates (it's just a vanilla --blob:none clone). We can detect this case by only freeing the tree buffer if it was allocated on our behalf. This is a little tricky since that happens inside parse_object(), and it doesn't tell us whether the object was already parsed, or whether it allocated the buffer itself. But by checking for an already-parsed tree beforehand, we can distinguish the two cases. That feels a little hacky, and does incur an extra lookup in the object-hash table. But that cost is fairly minimal compared to actually loading objects (and since we're iterating the whole pack here, we're likely to be loading most objects, rather than reusing cached results). It may also be a good direction for this function in general, as there are other possible optimizations that rely on doing some analysis before parsing: - we could detect blobs and avoid reading their contents; they can't link to other objects, but parse_object() doesn't know that we don't care about checking their hashes. - we could avoid allocating object structs entirely for most objects (since we really only need them in the oidset), which would save some memory. - promisor commits could use the commit-graph rather than loading the object from disk This commit doesn't do any of those optimizations, but I think it argues that this direction is reasonable, rather than relying on parse_object() and trying to teach it to give us more information about whether it parsed. The included test fails reliably under SANITIZE=address just when running "rev-list --missing=allow-promisor". Checking the output isn't strictly necessary to detect the bug, but it seems like a reasonable addition given the general lack of coverage for "allow-promisor" in the test suite. Reported-by: Andrew Olsen <andrew.olsen@koordinates.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
-rw-r--r--packfile.c15
-rwxr-xr-xt/t5616-partial-clone.sh7
2 files changed, 20 insertions, 2 deletions
diff --git a/packfile.c b/packfile.c
index b79cbc8..1556a5c 100644
--- a/packfile.c
+++ b/packfile.c
@@ -2225,7 +2225,17 @@ static int add_promisor_object(const struct object_id *oid,
void *set_)
{
struct oidset *set = set_;
- struct object *obj = parse_object(the_repository, oid);
+ struct object *obj;
+ int we_parsed_object;
+
+ obj = lookup_object(the_repository, oid);
+ if (obj && obj->parsed) {
+ we_parsed_object = 0;
+ } else {
+ we_parsed_object = 1;
+ obj = parse_object(the_repository, oid);
+ }
+
if (!obj)
return 1;
@@ -2247,7 +2257,8 @@ static int add_promisor_object(const struct object_id *oid,
return 0;
while (tree_entry_gently(&desc, &entry))
oidset_insert(set, &entry.oid);
- free_tree_buffer(tree);
+ if (we_parsed_object)
+ free_tree_buffer(tree);
} else if (obj->type == OBJ_COMMIT) {
struct commit *commit = (struct commit *) obj;
struct commit_list *parents = commit->parents;
diff --git a/t/t5616-partial-clone.sh b/t/t5616-partial-clone.sh
index 5cb4153..9c0e422 100755
--- a/t/t5616-partial-clone.sh
+++ b/t/t5616-partial-clone.sh
@@ -49,6 +49,13 @@ test_expect_success 'do partial clone 1' '
test "$(git -C pc1 config --local remote.origin.partialclonefilter)" = "blob:none"
'
+test_expect_success 'rev-list --missing=allow-promisor on partial clone' '
+ git -C pc1 rev-list --objects --missing=allow-promisor HEAD >actual &&
+ git -C pc1 rev-list --objects --missing=print HEAD >expect.raw &&
+ grep -v "^?" expect.raw >expect &&
+ test_cmp expect actual
+'
+
test_expect_success 'verify that .promisor file contains refs fetched' '
ls pc1/.git/objects/pack/pack-*.promisor >promisorlist &&
test_line_count = 1 promisorlist &&