proc: Check /proc/$pid/attr/ writes against file opener

commit bfb819ea20ce8bbeeba17e1a6418bf8bda91fc28 upstream.

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 <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 5e705fa9a913db3cc94f4c25e7e93ca01e603897..bc736ea1192ada89e5505190620eb639bd310be0 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2564,6 +2564,10 @@ 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;
+
        rcu_read_lock();
        task = pid_task(proc_pid(inode), PIDTYPE_PID);
        if (!task) {