Fix NULL pointer dereference when zlib DSO fails to load

SiteRelEnby · t8m · commit 347e3f79a094 · 2026-01-22T18:09:51.000+01:00
When ZLIB_SHARED is defined and DSO_load() fails to load the zlib library, ossl_comp_zlib_init() incorrectly returns 1 (success) while leaving all function pointers (p_compress, p_uncompress, etc.) as NULL. This causes COMP_zlib() and COMP_zlib_oneshot() to return valid-looking COMP_METHOD pointers, but when these methods are used (e.g., during TLS 1.3 certificate decompression), the NULL function pointers are dereferenced, causing a SIGSEGV crash. The bug occurs because the NULL pointer check (lines 297-303) was inside the `if (zlib_dso != NULL)` block, so it was skipped entirely when DSO_load() returned NULL. The fix moves the NULL pointer check outside the conditional block, consistent with how c_brotli.c and c_zstd.c handle this case. Now if the DSO fails to load, all function pointers remain NULL, the check catches this, and the function correctly returns 0 (failure). This also fixes an incorrect cast of p_uncompress from compress_ft to the correct uncompress_ft type. PoC demonstrating the bug: https://github.com/SiteRelEnby/openssl-zlib-poc Fixes #23563 CLA: trivial Reviewed-by: Paul Yang <paulyang.inf@gmail.com> Reviewed-by: Paul Dale <paul.dale@oracle.com> Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org> Reviewed-by: Tomas Mraz <tomas@openssl.org> MergeDate: Thu Jan 22 17:00:50 2026 (Merged from #29699) (cherry picked from commit 045ca33)
diff --git a/crypto/comp/c_zlib.c b/crypto/comp/c_zlib.c
@@ -228,14 +228,14 @@ DEFINE_RUN_ONCE_STATIC(ossl_comp_zlib_init)
         p_deflate = (deflate_ft)DSO_bind_func(zlib_dso, "deflate");
         p_deflateInit_ = (deflateInit__ft)DSO_bind_func(zlib_dso, "deflateInit_");
         p_zError = (zError__ft)DSO_bind_func(zlib_dso, "zError");
+    }
 
-        if (p_compress == NULL || p_inflateEnd == NULL
-            || p_inflate == NULL || p_inflateInit_ == NULL
-            || p_deflateEnd == NULL || p_deflate == NULL
-            || p_deflateInit_ == NULL || p_zError == NULL) {
-            ossl_comp_zlib_cleanup();
-            return 0;
-        }
+    if (p_compress == NULL || p_inflateEnd == NULL
+        || p_inflate == NULL || p_inflateInit_ == NULL
+        || p_deflateEnd == NULL || p_deflate == NULL
+        || p_deflateInit_ == NULL || p_zError == NULL) {
+        ossl_comp_zlib_cleanup();
+        return 0;
     }
 #endif
     return 1;
