Re: [REVIEW][PATCH 2/3] exec: Don't allow ptracing an exec of an unreadable file

[Andy Lutomirski]

On Thu, Nov 17, 2016 at 9:08 AM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
>
> It is the reasonable expectation that if an executable file is not
> readable there will be no way for a user without special privileges to
> read the file.  This is enforced in ptrace_attach but if we are
> already attached there is no enforcement if a readonly executable
> is exec'd.
>
> Therefore do the simple thing and if there is a non-dumpable
> executable that we are tracing without privilege fail to exec it.
>
> Fixes: v1.0
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
>  fs/exec.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index fdec760bfac3..de107f74e055 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1230,6 +1230,11 @@ int flush_old_exec(struct linux_binprm * bprm)
>  {
>         int retval;
>
> +       /* Fail if the tracer can't read the executable */
> +       if ((bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP) &&
> +           !ptracer_capable(current, bprm->mm->user_ns))
> +               return -EPERM;
> +

At the very least, I think that BINPRM_FLAGS_ENFORCE_NONDUMP needs to
check capable_wrt_inode_uidgid too.  Otherwise we risk breaking:

$ gcc whatever.c
$ chmod 400 a.out
$ strace a.out