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 eeb81d9648c672bfea4f4233fe7b51642b4b5a2c..75bc3405c2f7e64d11e8a10493cd31ad14249064 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2557,6 +2557,10 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
        ssize_t length;
        struct task_struct *task = get_proc_task(inode);
 
+       /* A task may only write when it was the opener. */
+       if (file->f_cred != current_real_cred())
+               return -EPERM;
+
        length = -ESRCH;
        if (!task)
                goto out_no_task;