https://git.ruby-lang.org/ruby.git/commit/?id=bcc2bb28b0

From bcc2bb28b04054106f4a36e8fd69b2af6ecb033a Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Tue, 17 Aug 2021 22:01:57 +0900
Subject: Fix stack buffer overflow

https://hackerone.com/reports/1306859
---
 include/ruby/internal/memory.h | 6 +++---
 random.c                       | 7 ++-----
 2 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/include/ruby/internal/memory.h b/include/ruby/internal/memory.h
index aa3464465da..242892d50f0 100644
--- a/include/ruby/internal/memory.h
+++ b/include/ruby/internal/memory.h
@@ -284,7 +284,7 @@ typedef uint128_t DSIZE_T; https://github.com/ruby/ruby/blob/trunk/include/ruby/internal/memory.h#L284
  * @return      A pointer on stack.
  */
 #define ALLOCA_N(type,n) \
-    RBIMPL_CAST((type *)alloca(rbimpl_size_mul_or_raise(sizeof(type), (n))))
+    RBIMPL_CAST((type *)(!(n) ? NULL : alloca(rbimpl_size_mul_or_raise(sizeof(type), (n)))))
 
 /**
  * Identical to #RB_ALLOCV_N(), except it  implicitly assumes the type of array
@@ -297,7 +297,7 @@ typedef uint128_t DSIZE_T; https://github.com/ruby/ruby/blob/trunk/include/ruby/internal/memory.h#L297
  */
 #define RB_ALLOCV(v, n)        \
     ((n) < RUBY_ALLOCV_LIMIT ? \
-     ((v) = 0, alloca(n)) :    \
+     ((v) = 0, !(n) ? NULL : alloca(n)) : \
      rb_alloc_tmp_buffer(&(v), (n)))
 
 /**
@@ -330,7 +330,7 @@ typedef uint128_t DSIZE_T; https://github.com/ruby/ruby/blob/trunk/include/ruby/internal/memory.h#L330
 #define RB_ALLOCV_N(type, v, n)                             \
     RBIMPL_CAST((type *)                                     \
         (((size_t)(n) < RUBY_ALLOCV_LIMIT / sizeof(type)) ? \
-         ((v) = 0, alloca((n) * sizeof(type))) :            \
+         ((v) = 0, !(n) ? NULL : alloca((n) * sizeof(type))) : \
          rb_alloc_tmp_buffer2(&(v), (n), sizeof(type))))
 
 /**
diff --git a/random.c b/random.c
index b873d0593ed..580c1f26eb2 100644
--- a/random.c
+++ b/random.c
@@ -365,15 +365,12 @@ rand_init(const rb_random_interface_t *rng, rb_random_t *rnd, VALUE seed) https://github.com/ruby/ruby/blob/trunk/random.c#L365
     int sign;
 
     len = rb_absint_numwords(seed, 32, NULL);
+    if (len == 0) len = 1;
     buf = ALLOCV_N(uint32_t, buf0, len);
     sign = rb_integer_pack(seed, buf, len, sizeof(uint32_t), 0,
         INTEGER_PACK_LSWORD_FIRST|INTEGER_PACK_NATIVE_BYTE_ORDER);
     if (sign < 0)
         sign = -sign;
-    if (len == 0) {
-        buf[0] = 0;
-        len = 1;
-    }
     if (len > 1) {
         if (sign != 2 && buf[len-1] == 1) /* remove leading-zero-guard */
             len--;
@@ -883,7 +880,7 @@ rand_mt_init(rb_random_t *rnd, const uint32_t *buf, size_t len) https://github.com/ruby/ruby/blob/trunk/random.c#L880
 {
     struct MT *mt = &((rb_random_mt_t *)rnd)->mt;
     if (len <= 1) {
-        init_genrand(mt, buf[0]);
+        init_genrand(mt, len ? buf[0] : 0);
     }
     else {
         init_by_array(mt, buf, (int)len);
-- 
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