[PATCH] dir: do not apply prefix to negative pathspecs

common_prefix_len() derives the common prefix solely from positive
pathspecs, skipping those marked with PATHSPEC_EXCLUDE. However,
match_pathspec_with_flags() also passes that prefix when matching the
negative pathspecs.

A negative pathspec may be shorter than the prefix. In that case,
match_pathspec_item() advances item->match beyond its allocation and
subtracts the prefix from item->len, producing a negative matchlen. It
then dereferences the out-of-bounds pointer. If the resulting byte is
not NUL, matchlen is converted to size_t when passed to ps_strncmp(),
which may cause a much larger out-of-bounds read.

The problem can be reproduced with AddressSanitizer:

    make SANITIZE=address CFLAGS="-g -O0" git
    git init test &&
    cd test &&
    DIR=$(printf "a%.0s" {1..150}) &&
    mkdir -p "$DIR" &&
    touch "$DIR/f.txt" &&
    git add -A &&
    git commit -m test &&
    ../git ls-files -- "$DIR/" ":(exclude)xy"

This reports a heap-buffer-overflow. Without AddressSanitizer, the
output may depend on the contents of memory following the negative
pathspec.

Fix the bug by using a zero prefix when matching negative pathspecs.
Add a regression test that combines a positive pathspec with a longer
common prefix and a shorter, unrelated negative pathspec.

Signed-off-by: Yannik Tausch <redacted>
---

Note that I already sent information about this issue to the git security
mailing list on July 31, where I was informed that a fix of this kind does
not require an embargo.

 dir.c                       | 2 +-
 t/t6132-pathspec-exclude.sh | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/dir.c b/dir.c
index 95d8a1cce9..7072715389 100644
--- a/dir.c
+++ b/dir.c

@@ -593,7 +593,7 @@ static int match_pathspec_with_flags(struct index_state *istate,
 	if (!(ps->magic & PATHSPEC_EXCLUDE) || !positive)
 		return positive;
 	negative = do_match_pathspec(istate, ps, name, namelen,
-				     prefix, seen,
+				     0, seen,
 				     flags | DO_MATCH_EXCLUDE);
 	return negative ? 0 : positive;
 }

diff --git a/t/t6132-pathspec-exclude.sh b/t/t6132-pathspec-exclude.sh
index 9fdafeb1e9..ad919cc739 100755
--- a/t/t6132-pathspec-exclude.sh
+++ b/t/t6132-pathspec-exclude.sh

@@ -183,6 +183,15 @@ EOF
 	test_cmp expect actual
 '
 
+test_expect_success 'negative pathspec shorter than positive pathspec prefix' '
+	git ls-files -- sub/sub/ ":(exclude)sub2" >actual &&
+	cat <<-\EOF >expect &&
+	sub/sub/file
+	sub/sub/sub/file
+	EOF
+	test_cmp expect actual
+'
+
 test_expect_success 'multiple exclusions' '
 	git ls-files -- ":^*/file2" ":^sub2" >actual &&
 	cat <<-\EOF >expect &&