Re: [PATCH v7 5/6] seccomp: add a way to pass FDs via a notification fd

On Thu, Sep 27, 2018 at 06:39:02PM +0200, Jann Horn wrote:

On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen [off-list ref] wrote:

quoted

This patch adds a way to insert FDs into the tracee's process (also
close/overwrite fds for the tracee). This functionality is necessary to
mock things like socketpair() or dup2() or similar, but since it depends on
external (vfs) patches, I've left it as a separate patch as before so the
core functionality can still be merged while we argue about this. Except
this time it doesn't add any ugliness to the API :)

[...]

quoted

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 17685803a2af..07a05ad59731 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c

@@ -41,6 +41,8 @@
 #include <linux/tracehook.h>
 #include <linux/uaccess.h>
 #include <linux/anon_inodes.h>
+#include <linux/fdtable.h>
+#include <net/cls_cgroup.h>

 enum notify_state {
        SECCOMP_NOTIFY_INIT,

@@ -1684,6 +1686,56 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
        return ret;
 }

+static long seccomp_notify_put_fd(struct seccomp_filter *filter,
+                                 unsigned long arg)
+{
+       struct seccomp_notif_put_fd req;
+       void __user *buf = (void __user *)arg;
+       struct seccomp_knotif *knotif = NULL;
+       long ret;
+
+       if (copy_from_user(&req, buf, sizeof(req)))
+               return -EFAULT;
+
+       if (req.fd < 0 && req.to_replace < 0)
+               return -EINVAL;
+
+       ret = mutex_lock_interruptible(&filter->notify_lock);
+       if (ret < 0)
+               return ret;
+
+       ret = -ENOENT;
+       list_for_each_entry(knotif, &filter->notif->notifications, list) {
+               struct file *file = NULL;
+
+               if (knotif->id != req.id)
+                       continue;
+
+               if (req.fd >= 0)
+                       file = fget(req.fd);

So here we take a reference on `file`.

quoted

+               if (req.to_replace >= 0) {
+                       ret = replace_fd_task(knotif->task, req.to_replace,
+                                             file, req.fd_flags);

Then here we try to place the file in knotif->task's file descriptor
table. This can either fail (e.g. due to exceeded rlimit), in which
case nothing happens, or it can do do_dup2(), which first takes an
extra reference to the file, then places it in the task's fd table.

Either way, afterwards, we still hold a reference to the file.

quoted

+               } else {
+                       unsigned long max_files;
+
+                       max_files = task_rlimit(knotif->task, RLIMIT_NOFILE);
+                       ret = __alloc_fd(knotif->task->files, 0, max_files,
+                                        req.fd_flags);
+                       if (ret < 0)
+                               break;

If we bail out here, we still hold a reference to `file`.

Suggestion: Change this to "if (ret >= 0) {" and make the following
code conditional instead of breaking.

quoted

+                       __fd_install(knotif->task->files, ret, file);

But if we reach this point, __fd_install() consumes the file pointer,
so `file` is a dangling pointer now.

Suggestion: Add "break;" here.

quoted

+               }

Suggestion: Add "if (file != NULL) fput(file);" here.

Ugh, yes, thanks.

Tycho