Re: [PATCH] builtin/blame: fix out-of-bounds read with excessive `--abbrev`
Hi Patrick,
On Thu, 9 Jan 2025, Patrick Steinhardt wrote:
Show 19 quoted lines
> 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, where as 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>
> ---
> This fixes the issue reported in [1]. Thanks!
Thank you for the quick fix! We will need to adjust it a little more, though:
Show 19 quoted lines
>
> Patrick
>
> [1]: <4d812802-afbc-4635-7a19-73896fcda625@gmx.de>
> ---
> builtin/blame.c | 4 +++-
> t/t8002-blame.sh | 4 ++++
> 2 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/builtin/blame.c b/builtin/blame.c
> index 867032e4c16878ffd56df8a73162b89ca4bd2694..ad91fe9e97f90625dd2708fbd44bf2dd24a337a6 100644
> --- a/builtin/blame.c
> +++ b/builtin/blame.c
> @@ -475,6 +475,8 @@ static void emit_other(struct blame_scoreboard *sb, struct blame_entry *ent, int
> char ch;
> size_t length = (opt & OUTPUT_LONG_OBJECT_NAME) ?
> the_hash_algo->hexsz : (size_t) abbrev;
> + if (length > GIT_MAX_HEXSZ)
> + length = GIT_MAX_HEXSZ;
This causes a subtle change of behavior because there are a couple of conditional code blocks between this change and the `printf()` call decrease `length`, i.e. specifying values larger than the maximal hex size causes potentially-desirable, different behavior (and think about https://www.hyrumslaw.com/).
Show 22 quoted lines
>
> if (opt & OUTPUT_COLOR_LINE) {
> if (cnt > 0) {
> @@ -505,7 +507,7 @@ static void emit_other(struct blame_scoreboard *sb, struct blame_entry *ent, int
> length--;
> putchar('?');
> }
> - fwrite(hex, 1, length, stdout);
> + printf("%.*s", (int)length, 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..fcaba8c11f7ede084e069eefd292f337e8396cb4 100755
> --- a/t/t8002-blame.sh
> +++ b/t/t8002-blame.sh
> @@ -126,6 +126,10 @@ test_expect_success '--no-abbrev works like --abbrev with full length' '
> check_abbrev $hexsz --no-abbrev
> '
>
> +test_expect_success 'blame --abbrev gets truncated' '
> + check_abbrev 9000 --abbrev=$hexsz HEADThis is actually incorrect: it passes `--abbrev=$hexsz` instead of a value that needs to be truncated.
Show 9 quoted lines
> +'
> +
> test_expect_success '--exclude-promisor-objects does not BUG-crash' '
> test_must_fail git blame --exclude-promisor-objects one
> '
>
> ---
> base-commit: 14650065b76b28d3cfa9453356ac5669b19e706e
> change-id: 20250109-b4-pks-blame-truncate-hash-length-c875cac66d71
Here is my proposed fixup:
-- snipsnap -- [PATCH] fixup! builtin/blame: fix out-of-bounds read with excessive `--abbrev`
The test case needs to actually test an excessive `--abbrev` value.
Also, when calling `git blame --abbrev=<N>` with an `N` that is larger than the maximal OID hex size, there is a subtle side effect that makes it behave _differently_ than specifying said maximal hex size: When the command outputs boundary, unblamable or ignored commits' OIDs, those outputs are prefixed with characters indicating this, and the `abbrev` value is used to align the information that comes after the OID, clipping it as needed. Specifying a "too large" abbrev value here tells Git that yes, we want the full OIDs and don't you worry about alignment.
Thanks to SHA-256 being _larger_ than the default SHA-1-based OIDs, and thanks to clipping at `GIT_MAX_HEXSZ`, this change of behavior can only be observed when running the test in SHA-256 mode.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
builtin/blame.c | 9 +++++++--
t/t8002-blame.sh | 2 +-
2 files changed, 8 insertions(+), 3 deletions(-)
Show changes to 2 files +8 −4
builtin/blame.c, t/t8002-blame.sh
diff --git a/builtin/blame.c b/builtin/blame.c
index ad91fe9e97f9..5b4976835066 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -475,8 +475,13 @@ static void emit_other(struct blame_scoreboard *sb, struct blame_entry *ent, int
char ch;
size_t length = (opt & OUTPUT_LONG_OBJECT_NAME) ?
the_hash_algo->hexsz : (size_t) abbrev;
- if (length > GIT_MAX_HEXSZ)
- length = GIT_MAX_HEXSZ;
+
+ /*
+ * Leave enough space for ^, * and ? indicators (boundary,
+ * unblamable, ignored).
+ */
+ if (length > GIT_MAX_HEXSZ + 3)
+ length = GIT_MAX_HEXSZ + 3;
if (opt & OUTPUT_COLOR_LINE) {
if (cnt > 0) {
diff --git a/t/t8002-blame.sh b/t/t8002-blame.sh
index fcaba8c11f7e..71fa70a64679 100755
--- a/t/t8002-blame.sh
+++ b/t/t8002-blame.sh
@@ -127,7 +127,7 @@ test_expect_success '--no-abbrev works like --abbrev with full length' '
'
test_expect_success 'blame --abbrev gets truncated' '
- check_abbrev 9000 --abbrev=$hexsz HEAD
+ check_abbrev 9000 --abbrev=9000 HEAD..
'
test_expect_success '--exclude-promisor-objects does not BUG-crash' '
--
2.48.0.rc0.windows.1
Patrick Steinhardt
