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

[Kees Cook <keescook@chromium.org>]

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>
---
 fs/proc/base.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 58bbf334265b..7118ebe38fa6 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2674,6 +2674,11 @@ static int proc_pident_readdir(struct file *file, struct dir_context *ctx,
 }
 
 #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)
 {
@@ -2704,7 +2709,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 	int rv;
 
 	/* 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;
 
 	rcu_read_lock();
@@ -2754,9 +2759,11 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
 }
 
 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,
 };
 
 #define LSM_DIR_OPS(LSM) \
-- 
2.25.1