threads / patch / 62874

patchapply: detect overflow when parsing hunk header

Subject: [PATCH] apply: detect overflow when parsing hunk header

## tl;dr

2 messages between Jan 30, 2025 and Jan 30, 2025. Diffs are folded; open one to read it.

replies: 1people: 2as markdown or json

Phillip Wood via GitGitGadget· Jan 30, 2025, 11:08 UTC · lore

From: Phillip Wood <phillip.wood@dunelm.org.uk>

"git apply" uses strtoul() to parse the numbers in the hunk header but silently ignores overflows. As LONG_MAX is a legitimate return value for strtoul() we need to set errno to zero before the call to strtoul() and check that it is still zero afterwards. The error message we display is not particularly helpful as it does not say what was wrong. However, it seems pretty unlikely that users are going to trigger this error in practice and we can always improve it later if needed.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
    apply: detect overflow when parsing hunk header
    
    We should do something similar in "git add -p" but I'll wait to see what
    happens with
    https://lore.kernel.org/git/20250126125638.3089-2-soekkle@freenet.de/
    first

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1858%2Fphillipwood%2Fapply-detect-hunk-header-overflow-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1858/phillipwood/apply-detect-hunk-header-overflow-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1858

 apply.c               |  3 +++
 t/t4100-apply-stat.sh | 13 +++++++++++++
 2 files changed, 16 insertions(+)

Show changes to 2 files +16 −0

apply.c, t/t4100-apply-stat.sh

diff --git a/apply.c b/apply.c
index 4a7b6120ac8..b124678b93b 100644
--- a/apply.c
+++ b/apply.c
@@ -1423,7 +1423,10 @@ static int parse_num(const char *line, unsigned long *p)
 
 	if (!isdigit(*line))
 		return 0;
+	errno = 0;
 	*p = strtoul(line, &ptr, 10);
+	if (errno)
+		return 0;
 	return ptr - line;
 }
 
diff --git a/t/t4100-apply-stat.sh b/t/t4100-apply-stat.sh
index 146e73d8f55..a5664f3eb3c 100755
--- a/t/t4100-apply-stat.sh
+++ b/t/t4100-apply-stat.sh
@@ -38,4 +38,17 @@ incomplete (1)
 incomplete (2)
 EOF
 
+test_expect_success 'applying a hunk header which overflows fails' '
+	cat >patch <<-\EOF &&
+	diff -u a/file b/file
+	--- a/file
+	+++ b/file
+	@@ -98765432109876543210 +98765432109876543210 @@
+	-a
+	+b
+	EOF
+	test_must_fail git apply patch 2>err &&
+	echo "error: corrupt patch at line 4" >expect &&
+	test_cmp expect err
+'
 test_done

base-commit: fbe8d3079d4a96aeb4e4529cc93cc0043b759a05

-- 
gitgitgadget

Junio C Hamano· Jan 30, 2025, 22:17 UTC · re: Phillip Wood via GitGitGadget · lore

Re: [PATCH] apply: detect overflow when parsing hunk header

"Phillip Wood via GitGitGadget" <gitgitgadget@gmail.com> writes:

Show 9 quoted lines

> From: Phillip Wood <phillip.wood@dunelm.org.uk>
>
> "git apply" uses strtoul() to parse the numbers in the hunk header but
> silently ignores overflows. As LONG_MAX is a legitimate return value for
> strtoul() we need to set errno to zero before the call to strtoul() and
> check that it is still zero afterwards. The error message we display is
> not particularly helpful as it does not say what was wrong.  However, it
> seems pretty unlikely that users are going to trigger this error in
> practice and we can always improve it later if needed.

Thanks. We made an effort to use a type that is a bit wider than "int", but we apparently ignored that the Git userbase will become a lot wider some day and unfriendly and/or hostile folks would start feeding malicious input to us X-<. The check presented here look good, and the fact that there was only one change needed shows how well designed the base code was ;-)

Will queue.  Thanks.

Show 35 quoted lines

> @@ -1423,7 +1423,10 @@ static int parse_num(const char *line, unsigned long *p)
>  
>  	if (!isdigit(*line))
>  		return 0;
> +	errno = 0;
>  	*p = strtoul(line, &ptr, 10);
> +	if (errno)
> +		return 0;
>  	return ptr - line;
>  }
>  
> diff --git a/t/t4100-apply-stat.sh b/t/t4100-apply-stat.sh
> index 146e73d8f55..a5664f3eb3c 100755
> --- a/t/t4100-apply-stat.sh
> +++ b/t/t4100-apply-stat.sh
> @@ -38,4 +38,17 @@ incomplete (1)
>  incomplete (2)
>  EOF
>  
> +test_expect_success 'applying a hunk header which overflows fails' '
> +	cat >patch <<-\EOF &&
> +	diff -u a/file b/file
> +	--- a/file
> +	+++ b/file
> +	@@ -98765432109876543210 +98765432109876543210 @@
> +	-a
> +	+b
> +	EOF
> +	test_must_fail git apply patch 2>err &&
> +	echo "error: corrupt patch at line 4" >expect &&
> +	test_cmp expect err
> +'
>  test_done
>
> base-commit: fbe8d3079d4a96aeb4e4529cc93cc0043b759a05