From: Timo Sirainen <timo.sirainen@open-xchange.com>
Date: Mon, 13 Apr 2026 10:38:50 +0000 (+0200)
Subject: [PATCH 04/12] lib: Key str_hash/strcase_hash with a random seed to prevent HashDoS
X-Git-Tag: archive/raspbian/1%2.4.1+dfsg1-6+rpi1+deb13u7^2~37
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=fe785bf32e7cb61c7a9944f6eac9c55e54ec2f20;p=dovecot.git

[PATCH 04/12] lib: Key str_hash/strcase_hash with a random seed to prevent HashDoS

hash_init() fills a process-wide uint64_t hash_iv via random_fill()
at lib_init() time (after random_init()). str_hash() and strcase_hash()
pass hash_iv as the xxh64 seed so an attacker cannot predict bucket
placement and manufacture collision chains.

Gbp-Pq: Name 0004-lib-Key-str_hash-strcase_hash-with-a-random-seed-to-.patch
---

diff --git a/src/lib/hash.c b/src/lib/hash.c
index 61fe86e..83eeed4 100644
--- a/src/lib/hash.c
+++ b/src/lib/hash.c
@@ -5,6 +5,7 @@
 #include "lib.h"
 #include "hash.h"
 #include "primes.h"
+#include "randgen.h"
 #include "xxh64.h"
 
 #include <ctype.h>
@@ -59,6 +60,8 @@ enum hash_table_operation{
 	HASH_TABLE_OP_RESIZE
 };
 
+uint64_t hash_iv;
+
 static bool hash_table_resize(struct hash_table *table, bool grow);
 
 void hash_table_create(struct hash_table **table_r, pool_t node_pool,
@@ -520,9 +523,14 @@ void hash_table_copy(struct hash_table *dest, struct hash_table *src)
 	hash_table_thaw(dest);
 }
 
+void hash_init(void)
+{
+	random_fill(&hash_iv, sizeof(hash_iv));
+}
+
 unsigned int str_hash(const char *p)
 {
-	return xxh64_to_32(xxh64_data(p, strlen(p), 0));
+	return xxh64_to_32(xxh64_data(p, strlen(p), hash_iv));
 }
 
 unsigned int str_stable_hash(const char *p)
@@ -535,7 +543,7 @@ unsigned int strcase_hash(const char *p)
 	struct xxh64_context ctx;
 	unsigned char c;
 
-	xxh64_init(&ctx, 0);
+	xxh64_init(&ctx, hash_iv);
 	while (*p != '\0') {
 		c = (unsigned char)i_toupper(*p++);
 		xxh64_loop(&ctx, &c, 1);
diff --git a/src/lib/hash.h b/src/lib/hash.h
index 84d9c52..534c25f 100644
--- a/src/lib/hash.h
+++ b/src/lib/hash.h
@@ -14,6 +14,9 @@ typedef unsigned int hash_callback_t(const void *p);
 /* Returns 0 if the pointers are equal. */
 typedef int hash_cmp_callback_t(const void *p1, const void *p2);
 
+/* Random per-process IV to use for hash functions */
+extern uint64_t hash_iv;
+
 /* Create a new hash table. If initial_size is 0, the default value is used.
    table_pool is used to allocate/free large hash tables, node_pool is used
    for smaller allocations and can also be alloconly pool. The pools must not
@@ -168,9 +171,12 @@ void hash_table_copy(struct hash_table *dest, struct hash_table *src);
 #define hash_table_copy(table1, table2) \
 	hash_table_copy((table1)._table, (table2)._table)
 
-/* hash function for strings */
+/* Hash function for strings. These are safe against collision attacks. They
+   are initialized with a per-process random key. */
 unsigned int str_hash(const char *p) ATTR_PURE;
 unsigned int strcase_hash(const char *p) ATTR_PURE;
+/* Like str_hash(), but there is no per-process random key. This isn't as safe
+   against collision attacks. */
 unsigned int str_stable_hash(const char *p) ATTR_PURE;
 
 /* fast hash function which uppercases a-z. Does not work well
@@ -181,4 +187,6 @@ unsigned int strfastcase_hash(const char *p) ATTR_PURE;
 /* a generic hash for a given memory block */
 unsigned int mem_hash(const void *p, unsigned int size) ATTR_PURE;
 
+void hash_init(void);
+
 #endif
diff --git a/src/lib/lib.c b/src/lib/lib.c
index 4df939b..4a4372f 100644
--- a/src/lib/lib.c
+++ b/src/lib/lib.c
@@ -5,6 +5,7 @@
 #include "array.h"
 #include "event-filter.h"
 #include "env-util.h"
+#include "hash.h"
 #include "hostpid.h"
 #include "ipwd.h"
 #include "process-title.h"
@@ -184,6 +185,7 @@ void lib_init(void)
 {
 	i_assert(!lib_initialized);
 	random_init();
+	hash_init();
 	data_stack_init();
 	hostpid_init();
 	lib_open_non_stdio_dev_null();