Fix buffer overrun in unicode string normalization with empty input

PostgreSQL 13 and newer versions are directly impacted by that through
the SQL function normalize(), which would cause a call of this function
to write one byte past its allocation if using in input an empty
string after recomposing the string with NFC and NFKC.  Older versions
(v10~v12) are not directly affected by this problem as the only code
path using normalization is SASLprep in SCRAM authentication that
forbids the case of an empty string, but let's make the code more robust
anyway there so as any out-of-core callers of this function are covered.

The solution chosen to fix this issue is simple, with the addition of a
fast-exit path if the decomposed string is found as empty.  This would
only happen for an empty string as at its lowest level a codepoint would
be decomposed as itself if it has no entry in the decomposition table or
if it has a decomposition size of 0.

Some tests are added to cover this issue in v13~.  Note that an empty
string has always been considered as normalized (grammar "IS NF[K]{C,D}
NORMALIZED", through the SQL function is_normalized()) for all the
operations allowed (NFC, NFD, NFKC and NFKD) since this feature has been
introduced as of 2991ac5.  This behavior is unchanged but some tests are
added in v13~ to check after that.

I have also checked "make normalization-check" in src/common/unicode/,
while on it (works in 13~, and breaks in older stable branches
independently of this commit).

The release notes should just mention this commit for v13~.

Reported-by: Matthijs van der Vleuten
Discussion: https://postgr.es/m/17277-0c527a373794e802@postgresql.org
Backpatch-through: 10

3 files changed, 17 insertions, 3 deletions

diff --git a/src/common/unicode_norm.c b/src/common/unicode_norm.c
index ab5ce593456..cfea34e30b5 100644
--- a/src/common/unicode_norm.c
+++ b/src/common/unicode_norm.c
@@ -349,6 +349,10 @@ unicode_normalize(UnicodeNormalizationForm form, const pg_wchar *input)
 	decomp_chars[decomp_size] = '\0';
 	Assert(decomp_size == current_size);
 
+	/* Leave if there is nothing to decompose */
+	if (decomp_size == 0)
+		return decomp_chars;
+
 	/*
 	 * Now apply canonical ordering.
 	 */
diff --git a/src/test/regress/expected/unicode.out b/src/test/regress/expected/unicode.out
index 2a1e9036966..f2713a23268 100644
--- a/src/test/regress/expected/unicode.out
+++ b/src/test/regress/expected/unicode.out
@@ -8,6 +8,12 @@ SELECT U&'\0061\0308bc' <> U&'\00E4bc' COLLATE "C" AS sanity_check;
  t
 (1 row)
 
+SELECT normalize('');
+ normalize 
+-----------
+ 
+(1 row)
+
 SELECT normalize(U&'\0061\0308\24D1c') = U&'\00E4\24D1c' COLLATE "C" AS test_default;
  test_default 
 --------------
@@ -67,7 +73,8 @@ FROM
   (VALUES (1, U&'\00E4bc'),
           (2, U&'\0061\0308bc'),
           (3, U&'\00E4\24D1c'),
-          (4, U&'\0061\0308\24D1c')) vals (num, val)
+          (4, U&'\0061\0308\24D1c'),
+          (5, '')) vals (num, val)
 ORDER BY num;
  num | val | nfc | nfd | nfkc | nfkd 
 -----+-----+-----+-----+------+------
@@ -75,7 +82,8 @@ ORDER BY num;
    2 | äbc | f   | t   | f    | t
    3 | äⓑc | t   | f   | f    | f
    4 | äⓑc | f   | t   | f    | f
-(4 rows)
+   5 |     | t   | t   | t    | t
+(5 rows)
 
 SELECT is_normalized('abc', 'def');  -- run-time error
 ERROR:  invalid normalization form: def
diff --git a/src/test/regress/sql/unicode.sql b/src/test/regress/sql/unicode.sql
index ccfc6fa77ab..63cd523f85f 100644
--- a/src/test/regress/sql/unicode.sql
+++ b/src/test/regress/sql/unicode.sql
@@ -5,6 +5,7 @@ SELECT getdatabaseencoding() <> 'UTF8' AS skip_test \gset
 
 SELECT U&'\0061\0308bc' <> U&'\00E4bc' COLLATE "C" AS sanity_check;
 
+SELECT normalize('');
 SELECT normalize(U&'\0061\0308\24D1c') = U&'\00E4\24D1c' COLLATE "C" AS test_default;
 SELECT normalize(U&'\0061\0308\24D1c', NFC) = U&'\00E4\24D1c' COLLATE "C" AS test_nfc;
 SELECT normalize(U&'\00E4bc', NFC) = U&'\00E4bc' COLLATE "C" AS test_nfc_idem;
@@ -26,7 +27,8 @@ FROM
   (VALUES (1, U&'\00E4bc'),
           (2, U&'\0061\0308bc'),
           (3, U&'\00E4\24D1c'),
-          (4, U&'\0061\0308\24D1c')) vals (num, val)
+          (4, U&'\0061\0308\24D1c'),
+          (5, '')) vals (num, val)
 ORDER BY num;
 
 SELECT is_normalized('abc', 'def');  -- run-time error