| 1 | package objstore |
| 2 | |
| 3 | import ( |
| 4 | "crypto/hmac" |
| 5 | "crypto/sha256" |
| 6 | "encoding/binary" |
| 7 | "encoding/hex" |
| 8 | "errors" |
| 9 | "strings" |
| 10 | ) |
| 11 | |
| 12 | // Signed verdict tags: why the security property is authenticity and not |
| 13 | // write-permission. |
| 14 | // |
| 15 | // The design began with "the app cannot write the carlos-scan tag", enforced |
| 16 | // by keeping s3:PutObjectTagging out of the app's IAM policy. That rule was |
| 17 | // necessary, correctly enforced, and — three times over — not the property. |
| 18 | // There are several ways to attach a tag to an object, and the IAM condition |
| 19 | // key that covers them (s3:RequestObjectTagKeys) is not populated by every |
| 20 | // gateway for every route. A browser-direct POST-policy upload against MinIO |
| 21 | // sets carlos-scan freely with the Deny in place; no condition key describes |
| 22 | // "this request is a form upload", so IAM cannot be made to reach it. |
| 23 | // |
| 24 | // SO WE STOP TRYING TO MAKE THE TAG UNWRITABLE AND MAKE IT UNFORGEABLE. Every |
| 25 | // verdict carries an HMAC over the facts it asserts, keyed by a secret only |
| 26 | // the console holds. A tag that does not verify is not a verdict — it reads as |
| 27 | // unknown, and the object is scanned. An app can still write the bytes; it |
| 28 | // cannot make them mean anything. |
| 29 | // |
| 30 | // This is gateway-independent, which is the point: it closes the routes we |
| 31 | // found, the ones we did not, and the ones a future S3 implementation adds. |
| 32 | // The IAM Denys stay, demoted to defence in depth, which is the job they were |
| 33 | // always suited to. |
| 34 | // |
| 35 | // WHAT IT DOES NOT DO. An app can still forge `infected` on its own object and |
| 36 | // have `scan enforce on` deny reads of it. That is a tenant harming only |
| 37 | // itself with a credential it already holds, and it is not worth a second |
| 38 | // mechanism. |
| 39 | |
| 40 | // TagSignature is the fourth tag: the one that makes the other three mean |
| 41 | // something. It is in isScanTag, so the scanner owns and replaces it. |
| 42 | const TagSignature = "carlos-scan-sig" |
| 43 | |
| 44 | // sigFormat is the version prefix on the tag value. A format change gets a new |
| 45 | // prefix rather than a new parser guess: `v2:...` read by this build is an |
| 46 | // unknown signature, which reads as no verdict, which rescans. Safe direction. |
| 47 | const sigFormat = "v1" |
| 48 | |
| 49 | // scanFacts is exactly what a verdict asserts, and therefore exactly what the |
| 50 | // signature covers. Anything a reader will later trust has to be in here — a |
| 51 | // field left out is a field an attacker may vary freely while keeping a |
| 52 | // legitimately issued signature. |
| 53 | type scanFacts struct { |
| 54 | bucket string |
| 55 | key string |
| 56 | version string |
| 57 | verdict string |
| 58 | at string |
| 59 | } |
| 60 | |
| 61 | // TagSigner signs and verifies verdict tags. It holds the deployment's tag |
| 62 | // key, and optionally the previous one so a rotation does not invalidate every |
| 63 | // stored verdict at once. |
| 64 | type TagSigner struct { |
| 65 | current tagKey |
| 66 | previous *tagKey |
| 67 | } |
| 68 | |
| 69 | type tagKey struct { |
| 70 | secret []byte |
| 71 | id string |
| 72 | } |
| 73 | |
| 74 | // ErrNoTagKey is what a deployment gets for asking to scan without a key to |
| 75 | // sign verdicts with. It is a refusal and not a fallback: signing with nothing |
| 76 | // would verify every forgery ever written. |
| 77 | var ErrNoTagKey = errors.New("objstore: no scan tag key (CARLOS_SCAN_TAG_KEY)") |
| 78 | |
| 79 | // NewTagSigner builds the signer from the deployment's current key and, |
| 80 | // optionally, the previous one. Both come from the environment and neither |
| 81 | // ever leaves the console — unlike the callback key, which is derived per app |
| 82 | // and delivered, this one is never given to anybody. |
| 83 | func NewTagSigner(current, previous string) (*TagSigner, error) { |
| 84 | if strings.TrimSpace(current) == "" { |
| 85 | return nil, ErrNoTagKey |
| 86 | } |
| 87 | if previous != "" && previous == current { |
| 88 | // A rotation that rotated to itself. Accepting it would leave an |
| 89 | // operator believing a key had been retired while every signature |
| 90 | // still verifies under it. |
| 91 | return nil, errors.New("objstore: the previous scan tag key is identical to the current one — that is not a rotation") |
| 92 | } |
| 93 | s := &TagSigner{current: newTagKey(current)} |
| 94 | if previous != "" { |
| 95 | p := newTagKey(previous) |
| 96 | s.previous = &p |
| 97 | } |
| 98 | return s, nil |
| 99 | } |
| 100 | |
| 101 | // newTagKey derives the public identifier for a key. It is a domain-separated |
| 102 | // hash rather than any part of the key itself, so a tag can name which key |
| 103 | // signed it without the tag becoming a place the key leaks. |
| 104 | func newTagKey(secret string) tagKey { |
| 105 | h := hmac.New(sha256.New, []byte("carlos-scan-tag-key-id")) |
| 106 | h.Write([]byte(secret)) |
| 107 | return tagKey{secret: []byte(secret), id: hex.EncodeToString(h.Sum(nil))[:16]} |
| 108 | } |
| 109 | |
| 110 | // Sign returns the value for the TagSignature tag: v1:<key-id>:<mac>. |
| 111 | func (s *TagSigner) Sign(f scanFacts) string { |
| 112 | return sigFormat + ":" + s.current.id + ":" + hex.EncodeToString(mac(s.current.secret, f)) |
| 113 | } |
| 114 | |
| 115 | // Verify reports whether sig is a signature this deployment issued over f, and |
| 116 | // whether it was issued under the PREVIOUS key — which is valid, and wants |
| 117 | // re-signing so the old key can eventually be dropped. |
| 118 | // |
| 119 | // Every failure mode is the same answer: false. A malformed value, an unknown |
| 120 | // key id, a truncated MAC and a deliberate forgery are indistinguishable to a |
| 121 | // caller and should be, because the response to all of them is identical — |
| 122 | // treat it as no verdict and scan the object. |
| 123 | func (s *TagSigner) Verify(f scanFacts, sig string) (ok bool, stale bool) { |
| 124 | parts := strings.Split(sig, ":") |
| 125 | if len(parts) != 3 || parts[0] != sigFormat { |
| 126 | return false, false |
| 127 | } |
| 128 | got, err := hex.DecodeString(parts[2]) |
| 129 | if err != nil || len(got) != sha256.Size { |
| 130 | return false, false |
| 131 | } |
| 132 | switch parts[1] { |
| 133 | case s.current.id: |
| 134 | return hmac.Equal(got, mac(s.current.secret, f)), false |
| 135 | } |
| 136 | if s.previous != nil && parts[1] == s.previous.id { |
| 137 | return hmac.Equal(got, mac(s.previous.secret, f)), true |
| 138 | } |
| 139 | return false, false |
| 140 | } |
| 141 | |
| 142 | // mac is the signature over the facts, under one key. |
| 143 | // |
| 144 | // THE ENCODING IS LENGTH-PREFIXED, NOT DELIMITED, and that is not fastidiousness. |
| 145 | // An S3 object key may contain any byte a delimiter could be — `|`, `:`, a |
| 146 | // newline, a NUL. With a delimiter, an app that controls its own object keys |
| 147 | // can arrange for two different (bucket, key, version, verdict, at) tuples to |
| 148 | // serialise identically, and then a signature legitimately issued for one |
| 149 | // object is a valid signature for another. Length prefixes make that |
| 150 | // impossible rather than unlikely. |
| 151 | // |
| 152 | // The format label goes in first, so a future encoding cannot collide with |
| 153 | // this one either. |
| 154 | func mac(secret []byte, f scanFacts) []byte { |
| 155 | h := hmac.New(sha256.New, secret) |
| 156 | for _, s := range []string{sigFormat, f.bucket, f.key, f.version, f.verdict, f.at} { |
| 157 | var n [4]byte |
| 158 | binary.BigEndian.PutUint32(n[:], uint32(len(s))) |
| 159 | h.Write(n[:]) |
| 160 | h.Write([]byte(s)) |
| 161 | } |
| 162 | return h.Sum(nil) |
| 163 | } |
| 164 | |
amadan