fix: clear exec_env_tls when destroying exec_env

[teamchong]

Problem

When wasm_exec_env_destroy() is called, exec_env_tls (thread-local storage used by signal handlers for hardware bounds checking) may still point to the exec_env being destroyed. On subsequent WASM executions in the same thread, if a signal occurs (e.g., SIGSEGV for bounds checking), the signal handler accesses freed memory and crashes.

Solution

Clear exec_env_tls if it points to the exec_env being destroyed. This is a simple defensive check that prevents dangling pointer issues.

#ifdef OS_ENABLE_HW_BOUND_CHECK
    WASMExecEnv *current_tls = wasm_runtime_get_exec_env_tls();
    if (current_tls == exec_env) {
        wasm_runtime_set_exec_env_tls(NULL);
    }
#endif

Use Case

Daemon-style execution patterns (like Cloudflare Workers) where the same thread runs multiple WASM modules sequentially without forking. Each module creates its own exec_env, runs, then destroys it. Without this fix, the TLS can point to a destroyed exec_env, causing crashes on subsequent runs.

Testing

• Tested in production daemon-style execution with 100+ consecutive AOT runs without crashes
• No regression in existing tests expected (the fix only adds a NULL check)

Related

• Is it safe to have two `exec_env`s at the same time? #2381

[lum1n0us]

We are hoping to get more details about how the host side is using WAMR's APIs, especially regarding getting/setting exec_env_tls and calling WASM functions. A reproducible case would be great.

From my perspective, if you follow the pattern mentioned here, every call to a WASM function would have the proper exec_env_tls, and runtime_signal_handler() will not encounter a dangling pointer.

#include "core/iwasm/common/wasm_runtime_common.h"
call_worker ()
{
    exec_env_backup = wasm_runtime_get_exec_env_tls();
    wasm_runtime_set_exec_env_tls(NULL);  // clear
    call worker wasm module  // pass local exec_env, then call_wasm_with_hw_bound_check() will wasm_runtime_set_exec_env_tls(exec_env), and clean it like wasm_runtime_set_exec_env_tls(NULL) when finished execution.
    wasm_runtime_set_exec_env_tls(exec_env_backup ); // restore
}

[teamchong]

We are hoping to get more details about how the host side is using WAMR's APIs, especially regarding getting/setting exec_env_tls and calling WASM functions. A reproducible case would be great.

From my perspective, if you follow the pattern mentioned here, every call to a WASM function would have the proper exec_env_tls, and runtime_signal_handler() will not encounter a dangling pointer.

#include "core/iwasm/common/wasm_runtime_common.h"
call_worker ()
{
    exec_env_backup = wasm_runtime_get_exec_env_tls();
    wasm_runtime_set_exec_env_tls(NULL);  // clear
    call worker wasm module  // pass local exec_env, then call_wasm_with_hw_bound_check() will wasm_runtime_set_exec_env_tls(exec_env), and clean it like wasm_runtime_set_exec_env_tls(NULL) when finished execution.
    wasm_runtime_set_exec_env_tls(exec_env_backup ); // restore
}

thanks for the feedback! I've added a reproducible test case that demonstrates the bug.

The Bug

The issue is in invoke_native_with_hw_bound_check (both aot_runtime.c and wasm_runtime.c):

// exec_env_tls is SET here
wasm_runtime_set_exec_env_tls(exec_env);

// Early return WITHOUT clearing exec_env_tls!
if (!wasm_runtime_detect_native_stack_overflow(exec_env)) {
   return false;  // BUG: TLS never cleared
}

When the native stack overflow check fails, the function returns early without clearing exec_env_tls. If the application then destroys the exec_env and creates a new one, subsequent WASM calls fail with "invalid exec env" because exec_env_tls still points to the destroyed exec_env.

Reproducible Test Case

Added tests/standalone/test-exec-env-tls/ with a test that:

1. Creates exec_env_A
2. Sets native_stack_boundary high to trigger overflow check failure
3. Calls WASM → fails with "native stack overflow", but TLS is not cleared
4. Destroys exec_env_A → TLS is now a dangling pointer
5. Creates exec_env_B
6. Calls WASM → fails with "invalid exec env" (without fix)

About the save/restore pattern

The save/restore pattern you mentioned would work if the application explicitly manages TLS. However, in this case:

• The bug is inside WAMR's invoke_native_with_hw_bound_check function
• The application just calls the public wasm_runtime_call_wasm API
• The early return path doesn't clear TLS, leaving it in an inconsistent state

The fix is defensive cleanup in wasm_exec_env_destroy()
if TLS points to the exec_env being destroyed, clear it. This handles any case where TLS wasn't properly cleared.

[lum1n0us]

The test/standalone is about to be archived. It's better to use a few unit test cases. Here are the reference

I believe it would be beneficial to clear exec_env_tls in the early return branch as well, for both wasm_runtime and aot_runtime.

diff --git a/core/iwasm/interpreter/wasm_runtime.c b/core/iwasm/interpreter/wasm_runtime.c
index a59bc9257..cac3730bf 100644
--- a/core/iwasm/interpreter/wasm_runtime.c
+++ b/core/iwasm/interpreter/wasm_runtime.c
@@ -3618,6 +3618,7 @@ call_wasm_with_hw_bound_check(WASMModuleInstance *module_inst,
        native stack to run the following codes before actually calling
        the aot function in invokeNative function. */
     if (!wasm_runtime_detect_native_stack_overflow(exec_env)) {
+        wasm_runtime_set_exec_env_tls(NULL);
         return;
     }

[teamchong]

The test/standalone is about to be archived. It's better to use a few unit test cases. Here are the reference

I believe it would be beneficial to clear exec_env_tls in the early return branch as well, for both wasm_runtime and aot_runtime.

diff --git a/core/iwasm/interpreter/wasm_runtime.c b/core/iwasm/interpreter/wasm_runtime.c
index a59bc9257..cac3730bf 100644
--- a/core/iwasm/interpreter/wasm_runtime.c
+++ b/core/iwasm/interpreter/wasm_runtime.c
@@ -3618,6 +3618,7 @@ call_wasm_with_hw_bound_check(WASMModuleInstance *module_inst,
        native stack to run the following codes before actually calling
        the aot function in invokeNative function. */
     if (!wasm_runtime_detect_native_stack_overflow(exec_env)) {
+        wasm_runtime_set_exec_env_tls(NULL);
         return;
     }

Thanks. I've updated the PR.