[PATCH v3 1/2] builtin/blame: fix out-of-bounds read with excessive `--abbrev`

From: Patrick Steinhardt <ps@pks.im>
Date: Jan 10, 2025, 11:26 UTC
Message-ID: <20250110-b4-pks-blame-truncate-hash-length-v3-1-e61f25b68f30@pks.im>
In-Reply-To: <20250110-b4-pks-blame-truncate-hash-length-v3-0-e61f25b68f30@pks.im>

In 6411a0a896 (builtin/blame: fix type of `length` variable when emitting object ID, 2024-12-06) we have fixed the type of the `length` variable. In order to avoid a cast from `size_t` to `int` in the call to printf(3p) with the "%.*s" formatter we have converted the code to instead use fwrite(3p), which accepts the length as a `size_t`.

It was reported though that this makes us read over the end of the OID array when the provided `--abbrev=` length exceeds the length of the object ID. This is because fwrite(3p) of course doesn't stop when it sees a NUL byte, whereas printf(3p) does.

Fix the bug by reverting back to printf(3p) and culling the provided length to `GIT_MAX_HEXSZ` to keep it from overflowing when cast to an `int`.

Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/blame.c  | 3 ++-
 t/t8002-blame.sh | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/builtin/blame.c b/builtin/blame.c
index 867032e4c16878ffd56df8a73162b89ca4bd2694..d7630ac89cb7bd6e9ce5d72c6a98aa433b3b12da 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -505,7 +505,8 @@ static void emit_other(struct blame_scoreboard *sb, struct blame_entry *ent, int
 			length--;
 			putchar('?');
 		}
-		fwrite(hex, 1, length, stdout);
+
+		printf("%.*s", (int)(length < GIT_MAX_HEXSZ ? length : GIT_MAX_HEXSZ), hex);
 		if (opt & OUTPUT_ANNOTATE_COMPAT) {
 			const char *name;
 			if (opt & OUTPUT_SHOW_EMAIL)
diff --git a/t/t8002-blame.sh b/t/t8002-blame.sh
index 0147de304b4d104cc7f05ea1f8d68f1a07ceb80d..b3f8b63d2e6744dd434f38fd9f10b56cd432141b 100755
--- a/t/t8002-blame.sh
+++ b/t/t8002-blame.sh
@@ -126,6 +126,14 @@ test_expect_success '--no-abbrev works like --abbrev with full length' '
 	check_abbrev $hexsz --no-abbrev
 '
 
+test_expect_success 'blame --abbrev gets truncated' '
+	check_abbrev $hexsz --abbrev=9000 HEAD
+'
+
+test_expect_success 'blame --abbrev gets truncated with boundary commit' '
+	check_abbrev $hexsz --abbrev=9000 ^HEAD
+'
+
 test_expect_success '--exclude-promisor-objects does not BUG-crash' '
 	test_must_fail git blame --exclude-promisor-objects one
 '

-- 
2.48.0.rc2.279.g1de40edade.dirty