futex: Don't leak robust_list pointer on exec race

Summary

In the Linux kernel, the following vulnerability has been resolved: futex: Don't leak robust_list pointer on exec race sys_get_robust_list() and compat_get_robust_list() use ptrace_may_access() to check if the calling task is allowed to access another task's robust_list pointer. This check is racy against a concurrent exec() in the target process. During exec(), a task may transition from a non-privileged binary to a privileged one (e.g., setuid binary) and its credentials/memory mappings may change. If get_robust_list() performs ptrace_may_access() before this transition, it may erroneously allow access to sensitive information after the target becomes privileged. A racy access allows an attacker to exploit a window during which ptrace_may_access() passes before a target process transitions to a privileged state via exec(). For example, consider a non-privileged task T that is about to execute a setuid-root binary. An attacker task A calls get_robust_list(T) while T is still unprivileged. Since ptrace_may_access() checks permissions based on current credentials, it succeeds. However, if T begins exec immediately afterwards, it becomes privileged and may change its memory mappings. Because get_robust_list() proceeds to access T->robust_list without synchronizing with exec() it may read user-space pointers from a now-privileged process. This violates the intended post-exec access restrictions and could expose sensitive memory addresses or be used as a primitive in a larger exploit chain. Consequently, the race can lead to unauthorized disclosure of information across privilege boundaries and poses a potential security risk. Take a read lock on signal->exec_update_lock prior to invoking ptrace_may_access() and accessing the robust_list/compat_robust_list. This ensures that the target task's exec state remains stable during the check, allowing for consistent and synchronized validation of credentials.