linux-kernel - Re: [PATCH v2 00/39] Shadowstacks for userspace

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Message-ID: <202210041224.8B7071AAD@keescook>
Date:   Tue, 4 Oct 2022 12:28:09 -0700
From:   Kees Cook <keescook@...omium.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Jann Horn <jannh@...gle.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "joao.moreira@...el.com" <joao.moreira@...el.com>,
        John Allen <john.allen@....com>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "jamorris@...ux.microsoft.com" <jamorris@...ux.microsoft.com>,
        "dethoma@...rosoft.com" <dethoma@...rosoft.com>
Subject: Re: [PATCH v2 00/39] Shadowstacks for userspace

On Tue, Oct 04, 2022 at 09:57:48AM +0000, David Laight wrote:
> From: Kees Cook <keescook@...omium.org>
> ...
> > >
> > > If you don't want /proc/$pid/mem to be able to do stuff like that,
> > > then IMO the way to go is to change when /proc/$pid/mem uses
> > > FOLL_FORCE, or to limit overall write access to /proc/$pid/mem.
> > 
> > Yeah, all reasonable. I just wish we could ditch FOLL_FORCE; it continues
> > to weird me out how powerful that fd's side-effects are.
> 
> Could you remove FOLL_FORCE from /proc/$pid/mem and add a
> /proc/$pid/mem_force that enable FOLL_FORCE but requires root
> (or similar) access.
> 
> Although I suspect gdb may like to have write access to
> code?

As Jann has reminded me out of band, while FOLL_FORCE is still worrisome,
it's really /proc/$pid/mem access at all without an active ptrace
attachment (and to self).

Here's my totally untested idea to require access to /proc/$pid/mem
having an established ptrace relationship:

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index c952c5ba8fab..0393741eeabb 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -64,6 +64,7 @@ extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead);
 #define PTRACE_MODE_NOAUDIT	0x04
 #define PTRACE_MODE_FSCREDS	0x08
 #define PTRACE_MODE_REALCREDS	0x10
+#define PTRACE_MODE_ATTACHED	0x20
 
 /* shorthands for READ/ATTACH and FSCREDS/REALCREDS combinations */
 #define PTRACE_MODE_READ_FSCREDS (PTRACE_MODE_READ | PTRACE_MODE_FSCREDS)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 93f7e3d971e4..fadec587d133 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -826,7 +826,7 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
 
 static int mem_open(struct inode *inode, struct file *file)
 {
-	int ret = __mem_open(inode, file, PTRACE_MODE_ATTACH);
+	int ret = __mem_open(inode, file, PTRACE_MODE_ATTACHED);
 
 	/* OK to pass negative loff_t, we can catch out-of-range */
 	file->f_mode |= FMODE_UNSIGNED_OFFSET;
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 1893d909e45c..c97e6d734ae5 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -304,6 +304,12 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 	 * or halting the specified task is impossible.
 	 */
 
+	/*
+	 * If an existing ptrace relationship is required, not even
+	 * introspection is allowed.
+	 */
+	if ((mode & PTRACE_MODE_ATTACHED) && ptrace_parent(task) != current)
+		return -EPERM;
 	/* Don't let security modules deny introspection */
 	if (same_thread_group(task, current))
 		return 0;

-- 
Kees Cook