Fix buffer overrun in unicode string normalization with empty input

michaelpq · michaelpq · commit 13c8adf90e9d · 2021-11-11T15:01:54.000+09:00
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
diff --git 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
@@ -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 
 --------------
@@ -67,15 +73,17 @@ 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 
 -----+-----+-----+-----+------+------
    1 | äbc | t   | f   | t    | f
    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
@@ -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
