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 bd8c26a409a76ebb927778bbadae3a6d7ec62bc9..2166f24af37e4012b7bb5c5f070e0a2d008f0836 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2413,6 +2413,10 @@ 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;