From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from fanzine2.igalia.com (fanzine2.igalia.com [213.97.179.56]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2A36E272E71; Thu, 4 Sep 2025 19:52:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=213.97.179.56 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757015561; cv=none; b=MAtG3uE0tT5taMNL8kfqRtC8H/v45hRYDA73RTtUiuStdMapLwTZteARQ7dh5XO9UFHuwxtFyZWoFyIJHnUaMYMko62fMuYoGh+6K8dvdCN7keN+LdyTI/VZ7X7syvcB7MvsAv2i6Jmda10E2QkB0RqXVyQBR8id1OBzHkXLd0Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757015561; c=relaxed/simple; bh=clZg3DRP1WK628VKHxpm33A1dty/3Fut7NCYFv+NR+I=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GH6I2NBWhfrIwS5TlKBXu5phGsmdUk7oka9Y10Ac4THUXHxcSn9YtPuU69brzEfl6hJtTy0wbe2EgbShVs6xBTfKOY8MzuOgZwi1Gz9OwQCo9ETgcCRyV39xG48Ficl18W0Ehp0t5pWLGPhgi0Wpz4dKNsQlpmdyDcWFaePW/Eg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=igalia.com; spf=pass smtp.mailfrom=igalia.com; dkim=pass (2048-bit key) header.d=igalia.com header.i=@igalia.com header.b=bjhmky/C; arc=none smtp.client-ip=213.97.179.56 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=igalia.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=igalia.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=igalia.com header.i=@igalia.com header.b="bjhmky/C" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=igalia.com; s=20170329; h=Content-Transfer-Encoding:Content-Type:In-Reply-To:From: References:Cc:To:Subject:MIME-Version:Date:Message-ID:Sender:Reply-To: Content-ID:Content-Description:Resent-Date:Resent-From:Resent-Sender: Resent-To:Resent-Cc:Resent-Message-ID:List-Id:List-Help:List-Unsubscribe: List-Subscribe:List-Post:List-Owner:List-Archive; bh=90WWo3ngxb5YnOOH42tlOsV1yM7SHg+xd8fsX6IodEM=; b=bjhmky/C/K2jRa5b3zE06gWSXU 6tKmzrhW2LUrjJa1VEiQauciIwOqHJVt36j19JdOlyek2tliV3LcMOmRtBfN/j3a1BIUhSfYduakv vOBI9aSZrU0qI5iUm18eL5WAhUMsaahG+LRegSn61APROOL3rJa7omiv4zVXge0XO3zCUpinDOHrt qhfy+Ycl/7mYf2khR2RQQVqbjOsU6k5gKsYBXyFfioFMUWBXCq57L+u1mIWNhF56v3sKjJDuztNCt CK2xb5xgtbeyPoP0gDRwNVm3L4mbWJlfayhGM4nXaNBjA58pTpPh3h65dd5VQ37m92ZTIs5GtUr96 pnNFvGcg==; Received: from [177.139.20.31] (helo=[192.168.15.100]) by fanzine2.igalia.com with esmtpsa (Cipher TLS1.3:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_128_GCM:128) (Exim) id 1uuG0U-006ySr-N4; Thu, 04 Sep 2025 21:52:30 +0200 Message-ID: Date: Thu, 4 Sep 2025 16:52:26 -0300 Precedence: bulk X-Mailing-List: linux-kernel-mentees@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] futex: don't leak robust_list pointer on exec race To: Pranav Tyagi Cc: jann@thejh.net, keescook@chromium.org, linux-kernel@vger.kernel.org, dvhart@infradead.org, peterz@infradead.org, mingo@redhat.com, tglx@linutronix.de, skhan@linuxfoundation.org, dave@stgolabs.net, linux-kernel-mentees@lists.linux.dev References: <20250813074201.6253-1-pranav.tyagi03@gmail.com> Content-Language: en-US From: =?UTF-8?Q?Andr=C3=A9_Almeida?= In-Reply-To: <20250813074201.6253-1-pranav.tyagi03@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi Pranav, Thanks for your patch! Some feedback bellow. Em 13/08/2025 04:42, Pranav Tyagi escreveu: > 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 > 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 > --- > 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) Function names inside of kernel/futex/ have the futex_ prefix > +{ > +#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) Same here > +{ > + struct task_struct *p; > + void __user *head; > + unsigned long ret; down_read_killable() returns a int, but you are storing the return value in an unsigned long. > + > + p = current; Could this be initialized in the declaration? > + > + 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 */ >