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Mon, 04 Aug 2025 04:55:58 -0700 (PDT) Received: from manjaro.domain.name ([2401:4900:1c31:41da:bdc2:5f7d:c803:6cd1]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-241d1ef6aa4sm107374325ad.35.2025.08.04.04.55.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 04 Aug 2025 04:55:58 -0700 (PDT) From: Pranav Tyagi To: tglx@linutronix.de, mingo@redhat.com, peterz@infradead.org, dvhart@infradead.org, dave@stgolabs.net, andrealmeid@igalia.com, linux-kernel@vger.kernel.org Cc: jann@thejh.net, keescook@chromium.org, skhan@linuxfoundation.org, linux-kernel-mentees@lists.linux.dev, Pranav Tyagi Subject: [PATCH v2] futex: don't leak robust_list pointer on exec race Date: Mon, 4 Aug 2025 17:25:33 +0530 Message-ID: <20250804115533.14186-1-pranav.tyagi03@gmail.com> X-Mailer: git-send-email 2.49.0 Precedence: bulk X-Mailing-List: linux-kernel-mentees@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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. changed in v2: - improved changelog - helper function for common part of the compat and native syscalls Signed-off-by: Pranav Tyagi Suggested-by: Jann Horn Link: https://lore.kernel.org/linux-fsdevel/1477863998-3298-5-git-send-email-jann@thejh.net/ Link: https://github.com/KSPP/linux/issues/119 --- kernel/futex/syscalls.c | 110 ++++++++++++++++++++++------------------ 1 file changed, 62 insertions(+), 48 deletions(-) diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c index 4b6da9116aa6..3278d91d95ce 100644 --- a/kernel/futex/syscalls.c +++ b/kernel/futex/syscalls.c @@ -39,46 +39,81 @@ SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head, return 0; } -/** - * sys_get_robust_list() - Get the robust-futex list head of a task - * @pid: pid of the process [zero for current task] - * @head_ptr: pointer to a list-head pointer, the kernel fills it in - * @len_ptr: pointer to a length field, the kernel fills in the header size - */ -SYSCALL_DEFINE3(get_robust_list, int, pid, - struct robust_list_head __user * __user *, head_ptr, - size_t __user *, len_ptr) +static void __user *get_robust_list_common(int pid, + bool compat) { - struct robust_list_head __user *head; + void __user *head; unsigned long ret; - struct task_struct *p; - rcu_read_lock(); + struct task_struct *p; - ret = -ESRCH; - if (!pid) + if (!pid) { p = current; - else { + get_task_struct(p); + } else { + rcu_read_lock(); p = find_task_by_vpid(pid); + /* + * pin the task to permit dropping the RCU read lock before + * acquiring the semaphore + */ + if (p) + get_task_struct(p); + rcu_read_unlock(); if (!p) - goto err_unlock; + return ERR_PTR(-ESRCH); } + /* + * Hold exec_update_lock to serialize with concurrent exec() + * so ptrace_may_access() is checked against stable credentials + */ + + ret = down_read_killable(&p->signal->exec_update_lock); + if (ret) + goto err_put; + ret = -EPERM; if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS)) goto err_unlock; - head = p->robust_list; - rcu_read_unlock(); + if (compat) + head = p->compat_robust_list; + else + head = p->robust_list; - if (put_user(sizeof(*head), len_ptr)) - return -EFAULT; - return put_user(head, head_ptr); + up_read(&p->signal->exec_update_lock); + put_task_struct(p); + + return head; err_unlock: - rcu_read_unlock(); + up_read(&p->signal->exec_update_lock); +err_put: + put_task_struct(p); + return ERR_PTR(ret); +} - return ret; + +/** + * sys_get_robust_list() - Get the robust-futex list head of a task + * @pid: pid of the process [zero for current task] + * @head_ptr: pointer to a list-head pointer, the kernel fills it in + * @len_ptr: pointer to a length field, the kernel fills in the header size + */ +SYSCALL_DEFINE3(get_robust_list, int, pid, + struct robust_list_head __user * __user *, head_ptr, + size_t __user *, len_ptr) +{ + struct robust_list_head __user *head = + get_robust_list_common(pid, false); + + if (IS_ERR(head)) + return PTR_ERR(head); + + if (put_user(sizeof(*head), len_ptr)) + return -EFAULT; + return put_user(head, head_ptr); } long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, @@ -455,36 +490,15 @@ COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid, compat_uptr_t __user *, head_ptr, compat_size_t __user *, len_ptr) { - struct compat_robust_list_head __user *head; - unsigned long ret; - struct task_struct *p; + struct compat_robust_list_head __user *head = + get_robust_list_common(pid, true); - rcu_read_lock(); - - ret = -ESRCH; - if (!pid) - p = current; - else { - p = find_task_by_vpid(pid); - if (!p) - goto err_unlock; - } - - ret = -EPERM; - if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS)) - goto err_unlock; - - head = p->compat_robust_list; - rcu_read_unlock(); + if (IS_ERR(head)) + return PTR_ERR(head); if (put_user(sizeof(*head), len_ptr)) return -EFAULT; return put_user(ptr_to_compat(head), head_ptr); - -err_unlock: - rcu_read_unlock(); - - return ret; } #endif /* CONFIG_COMPAT */ -- 2.49.0