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 653c2d8aa1cd75b587abbe591a22f69e81382dac..35114624fb036de9a071b1dc481603509db412d2 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2556,6 +2556,10 @@ 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) {