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Message-ID: <CAH4c4jLuhe-S+2BtKyn0+uQzzAwvVXoCSkQKZ-tqv5Ww9xU5sw@mail.gmail.com>
Date: Thu, 21 Aug 2025 17:17:53 +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
Subject: Re: [PATCH v4] futex: don't leak robust_list pointer on exec race
On Wed, Aug 13, 2025 at 1:12 PM Pranav Tyagi <pranav.tyagi03@...il.com> wrote:
>
> 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.
>
> 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
> ---
> changed in v4:
> - added task_robust_list() function
> changed in v3:
> - replaced RCU with scoped_guard(rcu)
> - corrected error return type cast
> - added IS_ENABLED(CONFIG_COMPAT) for ensuring compatability
> - removed stray newlines and unnecessary line breaks
> changed in v2:
> - improved changelog
> - helper function for common part of compat and native syscalls
>
> kernel/futex/syscalls.c | 108 +++++++++++++++++++++-------------------
> 1 file changed, 58 insertions(+), 50 deletions(-)
>
> diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
> index 4b6da9116aa6..0da33abc2f17 100644
> --- a/kernel/futex/syscalls.c
> +++ b/kernel/futex/syscalls.c
> @@ -39,6 +39,58 @@ SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
> return 0;
> }
>
> +static inline void __user *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 *get_robust_list_common(int pid, bool compat)
> +{
> + struct task_struct *p;
> + void __user *head;
> + unsigned long ret;
> +
> + p = current;
> +
> + 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 = 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 +101,14 @@ 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;
> - }
> + struct robust_list_head __user *head = get_robust_list_common(pid, false);
>
> - ret = -EPERM;
> - if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
> - goto err_unlock;
> -
> - 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 +485,14 @@ 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 = 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 */
>
> --
> 2.49.0
>
Hi,
This is a gentle follow-up on this patch. Please let me know if any further
changes are required.
Regards
Pranav Tyagi
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