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Message-ID: <175838399647.709179.13370572845425919919.tip-bot2@tip-bot2>
Date: Sat, 20 Sep 2025 15:59:56 -0000
From: "tip-bot2 for Pranav Tyagi" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Jann Horn <jann@...jh.net>, Pranav Tyagi <pranav.tyagi03@...il.com>,
Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject:
[tip: locking/futex] futex: Don't leak robust_list pointer on exec race
The following commit has been merged into the locking/futex branch of tip:
Commit-ID: 6b54082c3ed4dc9821cdf0edb17302355cc5bb45
Gitweb: https://git.kernel.org/tip/6b54082c3ed4dc9821cdf0edb17302355cc5bb45
Author: Pranav Tyagi <pranav.tyagi03@...il.com>
AuthorDate: Mon, 15 Sep 2025 23:51:54 +05:30
Committer: Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Sat, 20 Sep 2025 17:54:01 +02:00
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.
Suggested-by: Jann Horn <jann@...jh.net>
Signed-off-by: Pranav Tyagi <pranav.tyagi03@...il.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
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 | 106 ++++++++++++++++++++-------------------
1 file changed, 56 insertions(+), 50 deletions(-)
diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
index 4b6da91..880c9bf 100644
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -39,6 +39,56 @@ SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
return 0;
}
+static inline void __user *futex_task_robust_list(struct task_struct *p, bool compat)
+{
+#ifdef CONFIG_COMPAT
+ if (compat)
+ return p->compat_robust_list;
+#endif
+ return p->robust_list;
+}
+
+static void __user *futex_get_robust_list_common(int pid, bool compat)
+{
+ struct task_struct *p = current;
+ void __user *head;
+ int ret;
+
+ scoped_guard(rcu) {
+ if (pid) {
+ p = find_task_by_vpid(pid);
+ if (!p)
+ return (void __user *)ERR_PTR(-ESRCH);
+ }
+ get_task_struct(p);
+ }
+
+ /*
+ * 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 = futex_task_robust_list(p, compat);
+
+ up_read(&p->signal->exec_update_lock);
+ put_task_struct(p);
+
+ return head;
+
+err_unlock:
+ up_read(&p->signal->exec_update_lock);
+err_put:
+ put_task_struct(p);
+ return (void __user *)ERR_PTR(ret);
+}
+
/**
* sys_get_robust_list() - Get the robust-futex list head of a task
* @pid: pid of the process [zero for current task]
@@ -49,36 +99,14 @@ 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;
- unsigned long ret;
- struct task_struct *p;
-
- 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;
+ struct robust_list_head __user *head = futex_get_robust_list_common(pid, false);
- head = p->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(head, head_ptr);
-
-err_unlock:
- rcu_read_unlock();
-
- return ret;
}
long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
@@ -455,36 +483,14 @@ 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;
-
- 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;
+ struct compat_robust_list_head __user *head = futex_get_robust_list_common(pid, true);
- 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 */
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