Re: [PATCH v7] fs: clear file privilege bits when mmap writing

[Jann Horn]

On Mon, Jan 11, 2016 at 02:57:50PM -0800, Kees Cook wrote:
> Normally, when a user can modify a file that has setuid or setgid bits,
> those bits are cleared when they are not the file owner or a member
> of the group. This is enforced when using write and truncate but not
> when writing to a shared mmap on the file. This could allow the file
> writer to gain privileges by changing a binary without losing the
> setuid/setgid/caps bits.
> 
> Changing the bits requires holding inode->i_mutex, so it cannot be done
> during the page fault (due to mmap_sem being held during the fault). We
> could do this during vm_mmap_pgoff, but that would need coverage in
> mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem
> again. We could clear at open() time, but it's possible things are
> accidentally opening with O_RDWR and only reading. Better to clear on
> close and error failures (i.e. an improvement over now, which is not
> clearing at all).
> 
> Instead, detect the need to clear the bits during the page fault, and
> actually remove the bits during final fput. Since the file was open for
> writing, it wouldn't have been possible to execute it yet (ETXTBSY).
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
[...]
> diff --git a/fs/file_table.c b/fs/file_table.c
> index ad17e05ebf95..ca11b86613cf 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -191,6 +191,21 @@ static void __fput(struct file *file)
>  
>  	might_sleep();
>  
> +	/*
> +	 * XXX: This is a delayed removal of privs (we've already been
> +	 * written to), since we must avoid mmap_sem. But a race shouldn't
> +	 * be possible since when open for writing, execve() will fail
> +	 * with ETXTBSY (via deny_write_access()). A remaining problem
> +	 * is that since we've already been written to, we must ignore the
> +	 * return value of file_remove_privs(), since we can't reject the
> +	 * writes of the past.
> +	 */
> +	if (unlikely(file->f_flags & O_REMOVEPRIV)) {
> +		mutex_lock(&inode->i_mutex);
> +		file_remove_privs(file);
> +		mutex_unlock(&inode->i_mutex);
> +	}
> +

If there is any other setuid file I can run, can't I just do this?

pid_t child = fork();
if (child == 0) {
  /* fd will be 3 or so */
  int fd = open("setuid-file-with-bad-privs", O_WRONLY);
  char *ptr = mmap(..., fd, 0);
  memcpy(ptr, my_evil_code, sizeof(my_evil_code));
  /* su --bad-option just prints usage and exits, without touching
   * the fd - but since su has the last reference to the fd, __fput
   * will run with its privileges */
  execlp("su", "su", "--bad-option", NULL);
}
int status;
wait(&status);
execlp("setuid-file-with-bad-privs", "setuid-file-with-bad-privs", NULL);

I think that file_remove_privs() really needs to be changed to use f_cred
instead of current_cred(). That would also fix the known bypass where
you pass the fd to a setuid process as fd 1, causing the setuid process
to write more-or-less controlled data to a chosen offset, or similar
stuff (see
http://www.halfdog.net/Security/2015/SetgidDirectoryPrivilegeEscalation/).

Or was there already another patch that does this that I didn't see?

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