Re: [PATCH] selftests/exec: Add binfmt_script regression test

Kees Cook keescook@chromium.org writes:

While working on commit b5372fe5dc84 ("exec: load_script: Do not exec truncated interpreter path"), I wrote a series of test scripts to verify corner cases. However, soon after, commit 6eb3c3d0a52d ("exec: increase BINPRM_BUF_SIZE to 256") landed, resulting in the tests needing to be refactored for the larger BINPRM_BUF_SIZE, which got lost on my TODO list. During the recent exec refactoring work[1], the need for these tests resurfaced, so I've finished them up for addition to the kernel selftests.

I have to say there is something poetic about testing binfmt_script with Parot. Parot is what you get when you combined perl and python isn't it?

I will apply this to my exec-next tree right after the patchset under discussion.

Eric

[1] https://lore.kernel.org/lkml/202005191144.E3112135@keescook/

Signed-off-by: Kees Cook keescook@chromium.org

tools/testing/selftests/exec/Makefile | 1 + tools/testing/selftests/exec/binfmt_script | 171 +++++++++++++++++++++ 2 files changed, 172 insertions(+) create mode 100755 tools/testing/selftests/exec/binfmt_script

diff --git a/tools/testing/selftests/exec/Makefile b/tools/testing/selftests/exec/Makefile index 33339e31e365..7f4527f897c4 100644 --- a/tools/testing/selftests/exec/Makefile +++ b/tools/testing/selftests/exec/Makefile @@ -3,6 +3,7 @@ CFLAGS = -Wall CFLAGS += -Wno-nonnull CFLAGS += -D_GNU_SOURCE +TEST_PROGS := binfmt_script TEST_GEN_PROGS := execveat TEST_GEN_FILES := execveat.symlink execveat.denatured script subdir # Makefile is a run-time dependency, since it's accessed by the execveat test diff --git a/tools/testing/selftests/exec/binfmt_script b/tools/testing/selftests/exec/binfmt_script new file mode 100755 index 000000000000..05f94a741c7a --- /dev/null +++ b/tools/testing/selftests/exec/binfmt_script @@ -0,0 +1,171 @@ +#!/usr/bin/env python3 +# SPDX-License-Identifier: GPL-2.0 +# +# Test that truncation of bprm->buf doesn't cause unexpected execs paths, along +# with various other pathological cases. +import os, subprocess

+# Relevant commits +# +# b5372fe5dc84 ("exec: load_script: Do not exec truncated interpreter path") +# 6eb3c3d0a52d ("exec: increase BINPRM_BUF_SIZE to 256")

+# BINPRM_BUF_SIZE +SIZE=256

+NAME_MAX=int(subprocess.check_output(["getconf", "NAME_MAX", "."]))

+test_num=0

+code='''#!/usr/bin/perl +print "Executed interpreter! Args:\n"; +print "0 : '$0'\n"; +$counter = 1; +foreach my $a (@ARGV) {

• print "$counter : '$a'\n";
• $counter++;

+} +'''

+## +# test - produce a binfmt_script hashbang line for testing +# +# @size: bytes for bprm->buf line, including hashbang but not newline +# @good: whether this script is expected to execute correctly +# @hashbang: the special 2 bytes for running binfmt_script +# @leading: any leading whitespace before the executable path +# @root: start of executable pathname +# @target: end of executable pathname +# @arg: bytes following the executable pathname +# @fill: character to fill between @root and @target to reach @size bytes +# @newline: character to use as newline, not counted towards @size +# ... +def test(name, size, good=True, leading="", root="./", target="/perl",

•                 fill="A", arg="", newline="\n", hashbang="#!"):
  

• global test_num, tests, NAME_MAX
• test_num += 1
• if test_num > tests:

•    raise ValueError("more binfmt_script tests than expected! (want %d, expected %d)"
  

•                     % (test_num, tests))
  

• middle = ""
• remaining = size - len(hashbang) - len(leading) - len(root) - len(target) - len(arg)
• # The middle of the pathname must not exceed NAME_MAX
• while remaining >= NAME_MAX:

•    middle += fill * (NAME_MAX - 1)
  

•    middle += '/'
  

•    remaining -= NAME_MAX
  

• middle += fill * remaining
• dirpath = root + middle
• binary = dirpath + target
• if len(target):

•    os.makedirs(dirpath, mode=0o755, exist_ok=True)
  

•    open(binary, "w").write(code)
  

•    os.chmod(binary, 0o755)
  

• buf=hashbang + leading + root + middle + target + arg + newline
• if len(newline) > 0:

•    buf += 'echo this is not really perl\n'
  

• script = "binfmt_script-%s" % (name)
• open(script, "w").write(buf)
• os.chmod(script, 0o755)
• proc = subprocess.Popen(["./%s" % (script)], shell=True,

•                        stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
  

• stdout = proc.communicate()[0]
• if proc.returncode == 0 and b'Executed interpreter' in stdout:

•    if good:
  

•        print("ok %d - binfmt_script %s (successful good exec)"
  

•              % (test_num, name))
  

•    else:
  

•        print("not ok %d - binfmt_script %s succeeded when it should have failed"
  

•              % (test_num, name))
  

• else:

•    if good:
  

•        print("not ok %d - binfmt_script %s failed when it should have succeeded (rc:%d)"
  

•              % (test_num, name, proc.returncode))
  

•    else:
  

•        print("ok %d - binfmt_script %s (correctly failed bad exec)"
  

•              % (test_num, name))
  

• # Clean up crazy binaries
• os.unlink(script)
• if len(target):

•    elements = binary.split('/')
  

•    os.unlink(binary)
  

•    elements.pop()
  

•    while len(elements) > 1:
  

•        os.rmdir("/".join(elements))
  

•        elements.pop()
  

+tests=27 +print("TAP version 1.3") +print("1..%d" % (tests))

+### FAIL (8 tests)

+# Entire path is well past the BINFMT_BUF_SIZE. +test(name="too-big", size=SIZE+80, good=False) +# Path is right at max size, making it impossible to tell if it was truncated. +test(name="exact", size=SIZE, good=False) +# Same as above, but with leading whitespace. +test(name="exact-space", size=SIZE, good=False, leading=" ") +# Huge buffer of only whitespace. +test(name="whitespace-too-big", size=SIZE+71, good=False, root="",

•                                          fill=" ", target="")
  

+# A good path, but it gets truncated due to leading whitespace. +test(name="truncated", size=SIZE+17, good=False, leading=" " * 19) +# Entirely empty except for #! +test(name="empty", size=2, good=False, root="",

•                                      fill="", target="", newline="")
  

+# Within size, but entirely spaces +test(name="spaces", size=SIZE-1, good=False, root="", fill=" ",

•                                      target="", newline="")
  

+# Newline before binary. +test(name="newline-prefix", size=SIZE-1, good=False, leading="\n",

•                                      root="", fill=" ", target="")
  

+### ok (19 tests)

+# The original test case that was broken by commit: +# 8099b047ecc4 ("exec: load_script: don't blindly truncate shebang string") +test(name="test.pl", size=439, leading=" ",

• root="./nix/store/bwav8kz8b3y471wjsybgzw84mrh4js9-perl-5.28.1/bin",
  

• arg=" -I/nix/store/x6yyav38jgr924nkna62q3pkp0dgmzlx-perl5.28.1-File-Slurp-9999.25/lib/perl5/site_perl -I/nix/store/ha8v67sl8dac92r9z07vzr4gv1y9nwqz-perl5.28.1-Net-DBus-1.1.0/lib/perl5/site_perl -I/nix/store/dcrkvnjmwh69ljsvpbdjjdnqgwx90a9d-perl5.28.1-XML-Parser-2.44/lib/perl5/site_perl -I/nix/store/rmji88k2zz7h4zg97385bygcydrf2q8h-perl5.28.1-XML-Twig-3.52/lib/perl5/site_perl")
  

+# One byte under size, leaving newline visible. +test(name="one-under", size=SIZE-1) +# Two bytes under size, leaving newline visible. +test(name="two-under", size=SIZE-2) +# Exact size, but trailing whitespace visible instead of newline +test(name="exact-trunc-whitespace", size=SIZE, arg=" ") +# Exact size, but trailing space and first arg char visible instead of newline. +test(name="exact-trunc-arg", size=SIZE, arg=" f") +# One bute under, with confirmed non-truncated arg since newline now visible. +test(name="one-under-full-arg", size=SIZE-1, arg=" f") +# Short read buffer by one byte. +test(name="one-under-no-nl", size=SIZE-1, newline="") +# Short read buffer by half buffer size. +test(name="half-under-no-nl", size=int(SIZE/2), newline="") +# One byte under with whitespace arg. leaving wenline visible. +test(name="one-under-trunc-arg", size=SIZE-1, arg=" ") +# One byte under with whitespace leading. leaving wenline visible. +test(name="one-under-leading", size=SIZE-1, leading=" ") +# One byte under with whitespace leading and as arg. leaving newline visible. +test(name="one-under-leading-trunc-arg", size=SIZE-1, leading=" ", arg=" ") +# Same as above, but with 2 bytes under +test(name="two-under-no-nl", size=SIZE-2, newline="") +test(name="two-under-trunc-arg", size=SIZE-2, arg=" ") +test(name="two-under-leading", size=SIZE-2, leading=" ") +test(name="two-under-leading-trunc-arg", size=SIZE-2, leading=" ", arg=" ") +# Same as above, but with buffer half filled +test(name="two-under-no-nl", size=int(SIZE/2), newline="") +test(name="two-under-trunc-arg", size=int(SIZE/2), arg=" ") +test(name="two-under-leading", size=int(SIZE/2), leading=" ") +test(name="two-under-lead-trunc-arg", size=int(SIZE/2), leading=" ", arg=" ")

+if test_num != tests:

• raise ValueError("fewer binfmt_script tests than expected! (ran %d, expected %d"

•                 % (test_num, tests))
  

-- 2.20.1