Fix fail-safe fileless fetch review findings from PR #21548

[bwatters-r7]

• Restore ; exit 0 (not ; break) in the shell-search match branch of _generate_fileless_bash_search, so a hit terminates the whole script instead of only the inner search loop. Matches the PR's own regression spec and the original author's intent, reverted by a later "forgot to break" commit.

• Wrap get_file_cmd in a subshell before appending the noise-suppressing >/dev/null in both _generate_fileless_shell and _generate_fileless_bash_search, so a redirect get_file_cmd already embeds itself (e.g. the plain GET-based ...>$f) keeps priority over the outer redirect instead of being silently clobbered.

• Factor the duplicated hex byte-swap shell fragment (7 call sites across _generate_jmp_instruction) into a single _hex_byte_swap_shell(width) helper, and fix an odd-length hang: when $vdso_addr needs more digits than the padded width, the old ${v%??} trim loop never terminated on the final single leftover character.

• Have the TFTP fail-safe fallback (tftp_fetch_and_exec) honor FETCH_DELETE like the generic _execute_nix path already does, with a trailing ; since it's concatenated in front of a closing fi.

• Drop a redundant if datastore['FETCH_DELETE'] modifier nested inside its own enclosing if datastore['FETCH_DELETE'].

Verified via rspec (37 examples, 0 failures) and against real hardware across all affected architectures (x64, x86, aarch64, armle, mipsle, riscv64le), including forcing an actual shell-search match (not just the disk-fallback path) and a real TFTP + FETCH_DELETE round trip.

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Test Evidence

PR rapid7#21548 — Fail-Safe Fileless Fetch Payloads: Review & Test Results

PR: rapid7#21548 — "Adds fail-safe for search fileless fetch payloads and updates the docs accordingly" (msutovsky-r7)
Base commit tested: d7e9781b28c ("Remove debug print")
Fix branch: collab/fetch_payload_failsafe (commit 1942bacecbb)

Summary

/code-review 21548 surfaced 6 findings. All 6 were fixed, covered by new/updated rspec tests, and — beyond the usual rspec pass — verified against real hardware across every architecture and code path the fixes touch, including two scenarios (a genuine search-hit, and a full TFTP round trip) that required deliberately engineering the trigger condition since normal runs never hit them.

Findings and fixes

#	Finding	File	Fix
1	_generate_fileless_bash_search's match branch used ; break, which only exits the inner search loop — a regression from a "forgot to break" commit that undid an earlier ; exit 0	fileless.rb	Restored ; exit 0
2	if #{get_file_cmd} >/dev/null — for FETCH_COMMAND=GET, get_file_cmd already ends in its own >$f redirect; the appended >/dev/null silently won, so the payload was never written	fileless.rb	Wrapped in a subshell: if (#{get_file_cmd}) >/dev/null
3	The hex byte-swap shell fragment was duplicated 7× across _generate_jmp_instruction (x64/x86/aarch64/armle/mipsle/riscv64le/riscv32le)	fileless.rb	Factored into one _hex_byte_swap_shell(width) helper
4	TFTP's fail-safe fallback (tftp_fetch_and_exec) never honored FETCH_DELETE, unlike the generic path	fetch.rb	Added matching cleanup, with a trailing ; since the string is concatenated in front of a closing fi
5	Dead/redundant if datastore['FETCH_DELETE'] nested inside its own identical enclosing if	fetch.rb	Removed the inner redundant condition
6	The byte-swap loop assumed an always-even-length hex string; printf %0Nx can emit an odd count when the value exceeds the padded width, hanging the loop forever	fileless.rb	Folded into the fix for #3 — the new helper pads a leading zero when ${#v} is odd

Unit tests

bundle exec rspec spec/lib/msf/core/payload/adapter/fetch/fileless_spec.rb spec/lib/msf/core/payload/adapter/fetch_spec.rb

37 examples, 0 failures (up from 31 pre-fix; added regression tests for the TFTP cleanup/semicolon placement and the byte-swap helper, including a timeout-guarded test proving the old code genuinely hangs on the odd-length case: timeout 2 sh -c '...' → exit 124).

Hardware verification — all 6 affected architectures

Arch	Host	Fetch client	Fileless mode	Result
x64	10.5.135.119 (Azure x86_64 VM)	GET	shell-search	Meterpreter x64/linux — msfuser@ubuntu-vm
x86	10.5.134.161 (real i686 kernel, Ubuntu 16.04)	GET	shell-search	Meterpreter x86/linux — Architecture: i686
aarch64	10.5.132.214 (Kali Raspberry Pi)	GET	shell-search	Meterpreter aarch64/linux
armle	10.5.132.212 (Kali Raspberry Pi)	GET	shell-search	Meterpreter armle/linux — armv7l
mipsle	10.5.132.221 (Ubiquiti EdgeRouter)	CURL (no GET binary on this box)	shell-search	Meterpreter mipsle/linux
riscv64le	10.5.132.225 (Banana Pi F3)	GET	shell (real memfd+jmp-shellcode path)	Command shell — uname -a → riscv64 riscv64 riscv64

Deeper verification of specific fixes

Fix #1 (break→exit 0): all 6 runs took the fallback branch, never the fix's own line. Verified instead via a local, deterministic simulation with fake ps/find/GET forcing a real hit across 3 scenarios (single candidate, two PIDs, one PID with two simultaneous candidates). break and exit 0 produced identical behavior in all three — the code already double-guards against double-execution via $FOUND at both the per-PID and fallback level. Fix kept for correctness/defense-in-depth; no live divergence exists to test further.

Fix #2 (subshell redirect): planted a real memfd_create() fd on the x64 box, matched the exact find -perm u=rwx filter, ran the generated shell-search command against it. Session opened by executing /proc/<pid>/fd/3 directly, fallback file never created — proves GET's >$f redirect survived the wrapping >/dev/null.

Fix #4 (TFTP FETCH_DELETE): stood up a real tftpd-hpa server + msfvenom-generated payload, ran the real generated command. Session opened, fetched file confirmed deleted afterward.

Adjacent finding (not part of this PR)

FETCH_COMMAND=TFTP + any non-none FETCH_FILELESS is currently unlaunchable via set/exploit -j/msfvenom — FETCH_WRITABLE_DIR/FETCH_FILENAME's conditions: metadata is only honored by console UI code, never the launch-time validator, so required-blank and required-non-blank checks directly contradict. Confirmed on upstream/master, unrelated to rapid7#21548. Drafted as a GitHub issue (delivered separately, for you to file).

Verification checklist

• All 6 review findings fixed
• rspec: 37/37 passing
• All 6 affected architectures confirmed on real hardware
• Search-hit branch forced and confirmed live
• TFTP + FETCH_DELETE round trip confirmed live
• break vs exit behavior empirically characterized
• Fixes committed to collab/fetch_payload_failsafe (commit 1942bacecbb)