selftests/exec: Add binfmt_script regression test

While working on commit b5372fe5 ("exec: load_script: Do not exec
truncated interpreter path"), I wrote a series of test scripts to verify
corner cases. However, soon after, commit 6eb3c3d0 ("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.

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

Link: https://lkml.kernel.org/r/202005200204.D07DF079@keescookSigned-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>

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

tools/testing/selftests/exec/binfmt_script

+#!/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))