1 files changed, 21 insertions, 5 deletions
diff --git a/security/sandbox/linux/SandboxInfo.cpp b/security/sandbox/linux/SandboxInfo.cpp
index 3d71e55921..aefd9dac2d 100644
--- a/security/sandbox/linux/SandboxInfo.cpp
+++ b/security/sandbox/linux/SandboxInfo.cpp
@@ -134,11 +134,21 @@ static bool CanCreateUserNamespace() {
   // the new capabilities (in this case, cloning another namespace) to
   // detect AppArmor policies that allow CLONE_NEWUSER but don't allow
   // doing anything useful with it.
-  pid_t pid = syscall(__NR_clone, SIGCHLD | CLONE_NEWUSER | CLONE_NEWPID,
-                      nullptr, nullptr, nullptr, nullptr);
+  //
+  // Bug 1884347: There's a new AppArmor feature which can result in
+  // unsharing NEWUSER and NEWPID (or NEWNET etc.) in one syscall
+  // being allowed, but further use of capabilities will be blocked
+  // afterwards.  That may be a bug, but we need to handle it.
+  pid_t pid = syscall(__NR_clone, SIGCHLD | CLONE_NEWUSER, nullptr, nullptr,
+                      nullptr, nullptr);
   if (pid == 0) {
-    // In the child.  Do as little as possible.
-    _exit(0);
+    // The exact meaning of `unshare(CLONE_NEWPID)` is slightly
+    // counterintuitive but in this case it doesn't matter.  This just
+    // needs to be some operation that attempts to use capabilities,
+    // to check if it's blocked by an LSM.
+    int rv = unshare(CLONE_NEWPID);
+    // Exit with status 0 on success, 1 on failure.
+    _exit(rv == 0 ? 0 : 1);
   }
   if (pid == -1) {
     // Failure.
@@ -149,11 +159,17 @@ static bool CanCreateUserNamespace() {
     return false;
   }
   // Otherwise, in the parent and successful.
-  bool waitpid_ok = HANDLE_EINTR(waitpid(pid, nullptr, 0)) == pid;
+  int wstatus;
+  bool waitpid_ok = HANDLE_EINTR(waitpid(pid, &wstatus, 0)) == pid;
   MOZ_ASSERT(waitpid_ok);
   if (!waitpid_ok) {
     return false;
   }
+  // Check for failures reported by the child process.
+  if (!WIFEXITED(wstatus) || WEXITSTATUS(wstatus) != 0) {
+    setenv(kCacheEnvName, "0", 1);
+    return false;
+  }
   setenv(kCacheEnvName, "1", 1);
   return true;
 }