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 b9e41832315a6ade8c0c0177c83b4d1c862e4340..294fb8ee2ff46ca968b282f317508f2ade557d1c 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2522,6 +2522,10 @@ 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;