remove is_root check from linux_hashdump module

[pingport80]

Summary

This PR fixes a logic bug in module /post/linux/gather/hashdump which is used to dump hashes from shadow file.

The Bug

unless is_root?
  fail_with Failure::NoAccess, 'You must run this module as root!'
end

The module checks if the user is root or not. If the user is not root it exits. But there can be a case when a non-root user might have read access to the shadow file because of weak permissions. I think it's better to check if the shadow file is readable or not.

Verification Steps

Fire up Metasploit and get a non-root user session

msf6 > sessions -c "id -u" -i 1
[*] Running 'id -u' on shell session 1 (127.0.0.1)
1000

msf6 > use post/linux/gather/hashdump
msf6 post(linux/gather/hashdump) > set session 1
msf6 post(linux/gather/hashdump) > run 
[*] Reloading module...

[-] Post aborted due to failure: no-access: Shadow file must be readable in order to dump hash
es
[*] Post module execution completed

As root do chmod o+r /etc/shadow

msf6 post(linux/gather/hashdump) > run
[+] root:[hash]:0:0:root:/root:/bin/bash
[+] user:[hash]:1000:1000:User Name,,,:/home/user:/bin/bash
[+] Unshadowed Password File: /home/pingport80/.msf4/loot/20210421104310_default_127.0.0.1_lin
ux.hashes_337278.txt
[*] Post module execution completed
msf6 post(linux/gather/hashdump) >

[pingport80]

Sorry I closed this PR by mistake, reopening it.

[gwillcox-r7]

Looks good to me, and from tests I can see this now allows one to dump the hashes if the configuration on the /etc/shadow file is incorrect. Nice work @pingport80! Will land this now.

[gwillcox-r7]

Release Notes

Improved the post/linux/gather/hashdump module such that, instead of checking if the user is root, it will now check if the user has access to the /etc/shadow file prior to attempting to dump the hashes from the shadow file. This allows users to dump password hashes in the case where the permissions of the /etc/shadow file may be set up incorrectly, even if they are not the root user.