[PATCH 4.9 01/42] proc: Track /proc/$pid/attr/ opener mm_struct

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Kees Cook <keescook@chromium.org>

commit 591a22c14d3f45cc38bd1931c593c221df2f1881 upstream.

Commit bfb819ea20ce ("proc: Check /proc/$pid/attr/ writes against file opener")
tried to make sure that there could not be a confusion between the opener of
a /proc/$pid/attr/ file and the writer. It used struct cred to make sure
the privileges didn't change. However, there were existing cases where a more
privileged thread was passing the opened fd to a differently privileged thread
(during container setup). Instead, use mm_struct to track whether the opener
and writer are still the same process. (This is what several other proc files
already do, though for different reasons.)

Reported-by: Christian Brauner <christian.brauner@ubuntu.com>
Reported-by: Andrea Righi <andrea.righi@canonical.com>
Tested-by: Andrea Righi <andrea.righi@canonical.com>
Fixes: bfb819ea20ce ("proc: Check /proc/$pid/attr/ writes against file opener")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/proc/base.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2493,6 +2493,11 @@ out:
 }
 
 #ifdef CONFIG_SECURITY
+static int proc_pid_attr_open(struct inode *inode, struct file *file)
+{
+	return __mem_open(inode, file, PTRACE_MODE_READ_FSCREDS);
+}
+
 static ssize_t proc_pid_attr_read(struct file * file, char __user * buf,
 				  size_t count, loff_t *ppos)
 {
@@ -2523,7 +2528,7 @@ static ssize_t proc_pid_attr_write(struc
 	struct task_struct *task = get_proc_task(inode);
 
 	/* A task may only write when it was the opener. */
-	if (file->f_cred != current_real_cred())
+	if (file->private_data != current->mm)
 		return -EPERM;
 
 	length = -ESRCH;
@@ -2561,9 +2566,11 @@ out_no_task:
 }
 
 static const struct file_operations proc_pid_attr_operations = {
+	.open		= proc_pid_attr_open,
 	.read		= proc_pid_attr_read,
 	.write		= proc_pid_attr_write,
 	.llseek		= generic_file_llseek,
+	.release	= mem_release,
 };
 
 static const struct pid_entry attr_dir_stuff[] = {