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ssh/tailssh: add logic for matching against AcceptEnv patterns (#13466)

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Add logic for parsing and matching against our planned format for
AcceptEnv values. Namely, this supports direct matches against string
values and matching where * and ? are treated as wildcard characters
which match against an arbitrary number of characters and a single
character respectively.

Actually using this logic in non-test code will come in subsequent
changes.

Updates https://github.com/tailscale/corp/issues/22775

Signed-off-by: Mario Minardi <mario@tailscale.com>
This commit is contained in:
Mario Minardi 2024-09-25 21:09:05 -06:00 committed by GitHub
parent 9eb59c72c1
commit c90c9938c8
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110
ssh/tailssh/accept_env.go Normal file
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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package tailssh
import (
"slices"
"strings"
)
// filterEnv filters a passed in environ string slice (a slice with strings
// representing environment variables in the form "key=value") based on
// the supplied slice of acceptEnv values.
//
// acceptEnv is a slice of environment variable names that are allowlisted
// for the SSH rule in the policy file.
//
// acceptEnv values may contain * and ? wildcard characters which match against
// zero or one or more characters and a single character respectively.
func filterEnv(acceptEnv []string, environ []string) []string {
var acceptedPairs []string
for _, envPair := range environ {
envVar := strings.Split(envPair, "=")[0]
// Short circuit if we have a direct match between the environment
// variable and an AcceptEnv value.
if slices.Contains(acceptEnv, envVar) {
acceptedPairs = append(acceptedPairs, envPair)
continue
}
// Otherwise check if we have a wildcard pattern that matches.
if matchAcceptEnv(acceptEnv, envVar) {
acceptedPairs = append(acceptedPairs, envPair)
continue
}
}
return acceptedPairs
}
// matchAcceptEnv is a convenience function that wraps calling matchAcceptEnvPattern
// with every value in acceptEnv for a given env that is being matched against.
func matchAcceptEnv(acceptEnv []string, env string) bool {
for _, pattern := range acceptEnv {
if matchAcceptEnvPattern(pattern, env) {
return true
}
}
return false
}
// matchAcceptEnvPattern returns true if the pattern matches against the target string.
// Patterns may include * and ? wildcard characters which match against zero or one or
// more characters and a single character respectively.
func matchAcceptEnvPattern(pattern string, target string) bool {
patternIdx := 0
targetIdx := 0
for {
// If we are at the end of the pattern we can only have a match if we
// are also at the end of the target.
if patternIdx >= len(pattern) {
return targetIdx >= len(target)
}
if pattern[patternIdx] == '*' {
// Optimization to skip through any repeated asterisks as they
// have the same net effect on our search.
for patternIdx < len(pattern) {
if pattern[patternIdx] != '*' {
break
}
patternIdx++
}
// We are at the end of the pattern after matching the asterisk,
// implying a match.
if patternIdx >= len(pattern) {
return true
}
// Search through the target sequentially for the next character
// from the pattern string, recursing into matchAcceptEnvPattern
// to try and find a match.
for ; targetIdx < len(target); targetIdx++ {
if matchAcceptEnvPattern(pattern[patternIdx:], target[targetIdx:]) {
return true
}
}
// No match after searching through the entire target.
return false
}
if targetIdx >= len(target) {
return false
}
if pattern[patternIdx] != '?' && pattern[patternIdx] != target[targetIdx] {
return false
}
patternIdx++
targetIdx++
}
}

140
ssh/tailssh/accept_env_test.go Normal file
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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
package tailssh
import (
"fmt"
"testing"
"github.com/google/go-cmp/cmp"
)
func TestMatchAcceptEnvPattern(t *testing.T) {
testCases := []struct {
pattern string
target string
match bool
}{
{pattern: "*", target: "EXAMPLE_ENV", match: true},
{pattern: "***", target: "123456", match: true},
{pattern: "?", target: "A", match: true},
{pattern: "?", target: "123", match: false},
{pattern: "?*", target: "EXAMPLE_2", match: true},
{pattern: "?*", target: "", match: false},
{pattern: "*?", target: "A", match: true},
{pattern: "*?", target: "", match: false},
{pattern: "??", target: "CC", match: true},
{pattern: "??", target: "123", match: false},
{pattern: "*?*", target: "ABCDEFG", match: true},
{pattern: "*?*", target: "C", match: true},
{pattern: "*?*", target: "", match: false},
{pattern: "?*?", target: "ABCDEFG", match: true},
{pattern: "?*?", target: "A", match: false},
{pattern: "**?TEST", target: "_TEST", match: true},
{pattern: "**?TEST", target: "_TESTING", match: false},
{pattern: "TEST**?", target: "TEST_", match: true},
{pattern: "TEST**?", target: "A_TEST_", match: false},
{pattern: "TEST_*", target: "TEST_A", match: true},
{pattern: "TEST_*", target: "TEST_A_LONG_ENVIRONMENT_VARIABLE_NAME", match: true},
{pattern: "TEST_*", target: "TEST", match: false},
{pattern: "EXAMPLE_?_ENV", target: "EXAMPLE_A_ENV", match: true},
{pattern: "EXAMPLE_?_ENV", target: "EXAMPLE_ENV", match: false},
{pattern: "EXAMPLE_*_ENV", target: "EXAMPLE_aBcd2231---_ENV", match: true},
{pattern: "EXAMPLE_*_ENV", target: "EXAMPLEENV", match: false},
{pattern: "COMPLICA?ED_PATTERN*", target: "COMPLICATED_PATTERN_REST", match: true},
{pattern: "COMPLICA?ED_PATTERN*", target: "COMPLICATED_PATT", match: false},
{pattern: "COMPLICAT???ED_PATT??ERN", target: "COMPLICAT123ED_PATTggERN", match: true},
{pattern: "COMPLICAT???ED_PATT??ERN", target: "COMPLICATED_PATTERN", match: false},
{pattern: "DIRECT_MATCH", target: "DIRECT_MATCH", match: true},
{pattern: "DIRECT_MATCH", target: "MISS", match: false},
// OpenSSH compatibility cases
// See https://github.com/openssh/openssh-portable/blob/master/regress/unittests/match/tests.c
{pattern: "", target: "", match: true},
{pattern: "aaa", target: "", match: false},
{pattern: "", target: "aaa", match: false},
{pattern: "aaaa", target: "aaa", match: false},
{pattern: "aaa", target: "aaaa", match: false},
{pattern: "*", target: "", match: true},
{pattern: "?", target: "a", match: true},
{pattern: "a?", target: "aa", match: true},
{pattern: "*", target: "a", match: true},
{pattern: "a*", target: "aa", match: true},
{pattern: "?*", target: "aa", match: true},
{pattern: "**", target: "aa", match: true},
{pattern: "?a", target: "aa", match: true},
{pattern: "*a", target: "aa", match: true},
{pattern: "a?", target: "ba", match: false},
{pattern: "a*", target: "ba", match: false},
{pattern: "?a", target: "ab", match: false},
{pattern: "*a", target: "ab", match: false},
}
for _, tc := range testCases {
name := fmt.Sprintf("pattern_%s_target_%s", tc.pattern, tc.target)
if tc.match {
name += "_should_match"
} else {
name += "_should_not_match"
}
t.Run(name, func(t *testing.T) {
match := matchAcceptEnvPattern(tc.pattern, tc.target)
if match != tc.match {
t.Errorf("got %v, want %v", match, tc.match)
}
})
}
}
func TestFilterEnv(t *testing.T) {
testCases := []struct {
name string
acceptEnv []string
environ []string
expectedFiltered []string
}{
{
name: "simple direct matches",
acceptEnv: []string{"FOO", "FOO2", "FOO_3"},
environ: []string{"FOO=BAR", "FOO2=BAZ", "FOO_3=123", "FOOOO4-2=AbCdEfG"},
expectedFiltered: []string{"FOO=BAR", "FOO2=BAZ", "FOO_3=123"},
},
{
name: "bare wildcard",
acceptEnv: []string{"*"},
environ: []string{"FOO=BAR", "FOO2=BAZ", "FOO_3=123", "FOOOO4-2=AbCdEfG"},
expectedFiltered: []string{"FOO=BAR", "FOO2=BAZ", "FOO_3=123", "FOOOO4-2=AbCdEfG"},
},
{
name: "complex matches",
acceptEnv: []string{"FO?", "FOOO*", "FO*5?7"},
environ: []string{"FOO=BAR", "FOO2=BAZ", "FOO_3=123", "FOOOO4-2=AbCdEfG", "FO1-kmndGamc79567=ABC", "FO57=BAR2"},
expectedFiltered: []string{"FOO=BAR", "FOOOO4-2=AbCdEfG", "FO1-kmndGamc79567=ABC"},
},
}
for _, tc := range testCases {
t.Run(tc.name, func(t *testing.T) {
filtered := filterEnv(tc.acceptEnv, tc.environ)
if diff := cmp.Diff(tc.expectedFiltered, filtered); diff != "" {
t.Errorf("unexpected filter result (-got,+want): \n%s", diff)
}
})
}
}