[PATCH 5.4 01/84] proc: Track /proc/$pid/attr/ opener mm_struct

Greg Kroah-Hartman
2021-06-14T10:53:21Z

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
@@ -2527,6 +2527,11 @@ 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)
{
@@ -2557,7 +2562,7 @@ static ssize_t proc_pid_attr_write(struc
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();
@@ -2607,9 +2612,11 @@ out:
}

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) \