| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <20250804115533.14186-1-pranav.tyagi03@gmail.com>
Date: Mon, 4 Aug 2025 17:25:33 +0530
From: Pranav Tyagi <pranav.tyagi03@...il.com>
To: tglx@...utronix.de,
mingo@...hat.com,
peterz@...radead.org,
dvhart@...radead.org,
dave@...olabs.net,
andrealmeid@...lia.com,
linux-kernel@...r.kernel.org
Cc: jann@...jh.net,
keescook@...omium.org,
skhan@...uxfoundation.org,
linux-kernel-mentees@...ts.linux.dev,
Pranav Tyagi <pranav.tyagi03@...il.com>
Subject: [PATCH v2] 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.
changed in v2:
- improved changelog
- helper function for common part of the compat and native syscalls
Signed-off-by: Pranav Tyagi <pranav.tyagi03@...il.com>
Suggested-by: Jann Horn <jann@...jh.net>
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
Powered by blists - more mailing lists