On 5/14/2020 7:56 AM, Eric W. Biederman wrote:
> Kees Cook <keescook@xxxxxxxxxxxx> writes:
>
>> On Tue, May 12, 2020 at 04:47:14PM -0700, Kees Cook wrote:
>>> And now I wonder if qemu actually uses the resulting AT_EXECFD ...
>> It does, though I'm not sure if this is to support crossing mount points,
>> dropping privileges, or something else, since it does fall back to just
>> trying to open the file.
>>
>> execfd = qemu_getauxval(AT_EXECFD);
>> if (execfd == 0) {
>> execfd = open(filename, O_RDONLY);
>> if (execfd < 0) {
>> printf("Error while loading %s: %s\n", filename, strerror(errno));
>> _exit(EXIT_FAILURE);
>> }
>> }
> My hunch is that the fallback exists from a time when the kernel did not
> implement AT_EXECFD, or so that qemu can run on kernels that don't
> implement AT_EXECFD. It doesn't really matter unless the executable is
> suid, or otherwise changes privileges.
>
>
> I looked into this a bit to remind myself why exec works the way it
> works, with changing privileges.
>
> The classic attack is pointing a symlink at a #! script that is suid or
> otherwise changes privileges. The kernel will open the script and set
> the privileges, read the interpreter from the first line, and proceed to
> exec the interpreter. The interpreter will then open the script using
> the pathname supplied by the kernel. The name of the symlink.
> Before the interpreter reopens the script the attack would replace
> the symlink with a script that does something else, but gets to run
> with the privileges of the script.
>
>
> Defending against that time of check vs time of use attack is why
> bprm_fill_uid, and cap_bprm_set_creds use the credentials derived from
> the interpreter instead of the credentials derived from the script.
>
>
> The other defense is to replace the pathname of the executable that the
> intepreter will open with /dev/fd/N.
>
> All of this predates Linux entirely. I do remember this was fixed at
> some point in Linux but I don't remember the details. I can just read
> the solution that was picked in the code.
>
>
>
> All of this makes me wonder how are the LSMs protected against this
> attack.
>
> Let's see the following LSMS implement brpm_set_creds:
> tomoyo - Abuses bprm_set_creds to call tomoyo_load_policy [ safe ]
> smack - Requires CAP_MAC_ADMIN to smack setxattrs [ vulnerable? ]
> Uses those xattrs in smack_bprm_set_creds
What is the concern? If the xattrs change after the check,
the behavior should still be consistent.
> apparmor - Everything is based on names so the symlink [ safe? ]
> attack won't work as it has the wrong name.
> As long as the trusted names can't be renamed
> apparmor appears good.
> selinux - Appears to let anyone set selinux xattrs [ safe? ]
> Requires permission for a sid transfer
> As the attack appears not to allow anything that
> would not be allowed anyway it looks like selinux
> is safe.
>
> LSM folks, especially Casey am I reading this correctly? Did I
> correctly infer how your LSMs deal with the time of check to time of use
> attack on the script name?
>
> Eric
>
