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Date: Wed, 25 Sep 2013 21:14:44 +0100
From: Djalal Harouni <tixxdz@...ndz.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	<kernel-hardening@...ts.openwall.com>
Cc: tixxdz@...il.com,
	Djalal Harouni <tixxdz@...ndz.org>
Subject: [PATCH 11/12] procfs: improve permission checks on /proc/*/syscall

Permission checks need to happen during each system call. Therefore we
need to switch the /proc/*/syscall entry from an INF node to a REG node,
to avoid breaking shared INF file operations. This way it will have its
own file operations to implement the appropriate checks.

Add the syscall_open() to check if the file's opener has enough
permission to ptrace the target task.

Add the syscall_read() to check permissions and to read target syscall
information. If the classic ptrace_may_access() check is passed, then
check if current's cred have changed between ->open() and ->read(), if
so, call proc_allow_access() to check if the original file's opener had
enough permissions to read the task syscall info. This will block
passing the file descriptor to a more privileged process.

For readability split code into another __task_syscall_read() function
which is used to get the syscall entries of the task.

This patch also makes /proc/*/syscall 0400 so that the VFS will block
any unprivilged access right away.

Cc: Kees Cook <keescook@...omium.org>
Cc: Eric W. Biederman <ebiederm@...ssion.com>
Signed-off-by: Djalal Harouni <tixxdz@...ndz.org>
---
 fs/proc/base.c | 87 +++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 80 insertions(+), 7 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index fe02ee4..e82d0a4 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -630,13 +630,33 @@ static int proc_pid_limits(struct task_struct *task, char *buffer)
 }
 
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
-static int proc_pid_syscall(struct task_struct *task, char *buffer)
+static int syscall_open(struct inode *inode, struct file *filp)
 {
+	int err = -ESRCH;
+	struct task_struct *task = get_proc_task(file_inode(filp));
+
+	if (!task)
+		return err;
+
+	err = mutex_lock_killable(&task->signal->cred_guard_mutex);
+	if (err)
+		goto out;
+
+	if (!ptrace_may_access(task, PTRACE_MODE_ATTACH))
+		err = -EPERM;
+
+	mutex_unlock(&task->signal->cred_guard_mutex);
+
+out:
+	put_task_struct(task);
+	return err;
+}
+
+static int __task_syscall_read(struct task_struct *task, char *buffer)
+{
+	int res;
 	long nr;
 	unsigned long args[6], sp, pc;
-	int res = lock_trace(task);
-	if (res)
-		return res;
 
 	if (task_current_syscall(task, &nr, args, 6, &sp, &pc))
 		res = sprintf(buffer, "running\n");
@@ -648,9 +668,62 @@ static int proc_pid_syscall(struct task_struct *task, char *buffer)
 		       nr,
 		       args[0], args[1], args[2], args[3], args[4], args[5],
 		       sp, pc);
-	unlock_trace(task);
+
 	return res;
 }
+
+static ssize_t syscall_read(struct file *file, char __user *buf,
+			    size_t count, loff_t *ppos)
+{
+	ssize_t length;
+	unsigned long page;
+	struct inode *inode = file_inode(file);
+	struct task_struct *task = get_proc_task(inode);
+	int same_cred = proc_same_open_cred(file->f_cred);
+
+	length = -ESRCH;
+	if (!task)
+		return length;
+
+	if (count > PROC_BLOCK_SIZE)
+		count = PROC_BLOCK_SIZE;
+
+	length = -ENOMEM;
+	page = __get_free_page(GFP_TEMPORARY);
+	if (!page)
+		goto out;
+
+	length = lock_trace(task);
+	if (length)
+		goto out_free;
+
+	if (!same_cred &&
+	    !proc_allow_access(file->f_cred, task, PTRACE_MODE_ATTACH)) {
+	    length = -EPERM;
+	    unlock_trace(task);
+	    goto out_free;
+	}
+
+	length = __task_syscall_read(task, (char *)page);
+	unlock_trace(task);
+
+	if (length >= 0)
+		length = simple_read_from_buffer(buf, count, ppos,
+						 (char *)page, length);
+
+out_free:
+	free_page(page);
+out:
+	put_task_struct(task);
+	return length;
+}
+
+static const struct file_operations proc_pid_syscall_operations = {
+	.open		= syscall_open,
+	.read		= syscall_read,
+	.llseek         = generic_file_llseek,
+};
+
 #endif /* CONFIG_HAVE_ARCH_TRACEHOOK */
 
 /************************************************************************/
@@ -2706,7 +2779,7 @@ static const struct pid_entry tgid_base_stuff[] = {
 #endif
 	REG("comm",      S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
-	INF("syscall",    S_IRUGO, proc_pid_syscall),
+	REG("syscall",    S_IRUSR, proc_pid_syscall_operations),
 #endif
 	INF("cmdline",    S_IRUGO, proc_pid_cmdline),
 	ONE("stat",       S_IRUGO, proc_tgid_stat),
@@ -3042,7 +3115,7 @@ static const struct pid_entry tid_base_stuff[] = {
 #endif
 	REG("comm",      S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
-	INF("syscall",   S_IRUGO, proc_pid_syscall),
+	REG("syscall",   S_IRUSR, proc_pid_syscall_operations),
 #endif
 	INF("cmdline",   S_IRUGO, proc_pid_cmdline),
 	ONE("stat",      S_IRUGO, proc_tid_stat),
-- 
1.7.11.7

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