Subject: [PATCH] [RFC] sparse index: fix use-after-free bug in cache_tree_verify()

From: "Phillip Wood via GitGitGadget" <[email-masked]>
Date: Wed, 06 Oct 2021 09:29:51 +0000

From: Phillip Wood <[email-masked]>

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.

==74345==ERROR: AddressSanitizer: heap-use-after-free on address 0x606000001b20 at pc 0x557cbe82d3a2 bp 0x7ffdfee08090 sp 0x7ffdfee08080
READ of size 4 at 0x606000001b20 thread T0
    #0 0x557cbe82d3a1 in verify_one /home/phil/src/git/cache-tree.c:863
    #1 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #2 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #3 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #4 0x557cbe830a2b in cache_tree_verify /home/phil/src/git/cache-tree.c:910
    #5 0x557cbea53741 in write_locked_index /home/phil/src/git/read-cache.c:3250
    #6 0x557cbeab7fdd in reset_head /home/phil/src/git/reset.c:87
    #7 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #8 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #9 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #10 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #11 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #12 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #13 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)
    #14 0x557cbe5bcb8d in _start (/home/phil/src/git/git+0x1b9b8d)

0x606000001b20 is located 0 bytes inside of 56-byte region [0x606000001b20,0x606000001b58)
freed by thread T0 here:
    #0 0x7fdd4bacff19 in __interceptor_free /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:127
    #1 0x557cbe82af60 in cache_tree_free /home/phil/src/git/cache-tree.c:35
    #2 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #3 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #4 0x557cbe82aee5 in cache_tree_free /home/phil/src/git/cache-tree.c:31
    #5 0x557cbeb2557a in ensure_full_index /home/phil/src/git/sparse-index.c:310
    #6 0x557cbea45c4a in index_name_stage_pos /home/phil/src/git/read-cache.c:588
    #7 0x557cbe82ce37 in verify_one /home/phil/src/git/cache-tree.c:850
    #8 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #9 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #10 0x557cbe82ca9d in verify_one /home/phil/src/git/cache-tree.c:840
    #11 0x557cbe830a2b in cache_tree_verify /home/phil/src/git/cache-tree.c:910
    #12 0x557cbea53741 in write_locked_index /home/phil/src/git/read-cache.c:3250
    #13 0x557cbeab7fdd in reset_head /home/phil/src/git/reset.c:87
    #14 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #15 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #16 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #17 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #18 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #19 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #20 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

previously allocated by thread T0 here:
    #0 0x7fdd4bad0459 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
    #1 0x557cbebc1807 in xcalloc /home/phil/src/git/wrapper.c:140
    #2 0x557cbe82b7d8 in cache_tree /home/phil/src/git/cache-tree.c:17
    #3 0x557cbe82b7d8 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:763
    #4 0x557cbe82b837 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:764
    #5 0x557cbe82b837 in prime_cache_tree_rec /home/phil/src/git/cache-tree.c:764
    #6 0x557cbe8304e1 in prime_cache_tree /home/phil/src/git/cache-tree.c:779
    #7 0x557cbeab7fa7 in reset_head /home/phil/src/git/reset.c:85
    #8 0x557cbe72147f in cmd_rebase builtin/rebase.c:2074
    #9 0x557cbe5bd151 in run_builtin /home/phil/src/git/git.c:461
    #10 0x557cbe5bd151 in handle_builtin /home/phil/src/git/git.c:714
    #11 0x557cbe5c0503 in run_argv /home/phil/src/git/git.c:781
    #12 0x557cbe5c0503 in cmd_main /home/phil/src/git/git.c:912
    #13 0x557cbe5bad28 in main /home/phil/src/git/common-main.c:52
    #14 0x7fdd4b82eb24 in __libc_start_main (/usr/lib/libc.so.6+0x27b24)

Signed-off-by: Phillip Wood <[email-masked]>
---
    [RFC] sparse index: fix use-after-free bug in cache_tree_verify()
    
    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.
    
    Best Wishes
    
    Phillip

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1053%2Fphillipwood%2Fwip%2Fsparse-index-fix-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1053/phillipwood/wip/sparse-index-fix-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1053

 cache-tree.c                             | 29 +++++++++++++++++-------
 t/t1092-sparse-checkout-compatibility.sh |  2 +-
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/cache-tree.c b/cache-tree.c
index 90919f9e345..7bdbbc24268 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -826,10 +826,10 @@ static void verify_one_sparse(struct repository *r,
 		    path->buf);
 }
 
-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;
 
 		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);
+	}
 	strbuf_release(&path);
 }
diff --git a/t/t1092-sparse-checkout-compatibility.sh b/t/t1092-sparse-checkout-compatibility.sh
index 886e78715fe..85d5279b33c 100755
--- a/t/t1092-sparse-checkout-compatibility.sh
+++ b/t/t1092-sparse-checkout-compatibility.sh
@@ -484,7 +484,7 @@ test_expect_success 'checkout and reset (mixed) [sparse]' '
 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"
 	do
 		test_all_match git checkout -B temp update-deep &&
 		test_all_match git $OPERATION update-folder1 &&

base-commit: cefe983a320c03d7843ac78e73bd513a27806845
-- 
gitgitgadget