Re: [PATCH] proc: Check /proc/$pid/attr/ writes against file opener

Jann Horn [off-list ref] writes:

On Tue, May 25, 2021 at 9:37 PM Kees Cook [off-list ref] wrote:

quoted

Fix another "confused deputy" weakness[1]. Writes to /proc/$pid/attr/
files need to check the opener credentials, since these fds do not
transition state across execve(). Without this, it is possible to
trick another process (which may have different credentials) to write
to its own /proc/$pid/attr/ files, leading to unexpected and possibly
exploitable behaviors.

[1] https://www.kernel.org/doc/html/latest/security/credentials.html?highlight=confused#open-file-credentials

Fixes: 1da177e4c3f41 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <redacted>
---
 fs/proc/base.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 3851bfcdba56..58bbf334265b 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c

@@ -2703,6 +2703,10 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
        void *page;
        int rv;

+       /* A task may only write when it was the opener. */
+       if (file->f_cred != current_real_cred())
+               return -EPERM;

With this, if a task forks, the child will then still be able to open
its parent's /proc/$pid/attr/current and trick the parent into writing
to that, right? Is that acceptable? If not, the ->open handler should
probably also check for "current->thread_pid == proc_pid(inode)", or
something like that?

Currently exec always allocates a new cred.  So you can only ``trick''
another process that was forked from you.  I don't think it counts as
tricking or any kind of danger if you are simply confusing yourself.

Eric