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Commit38f9dfc

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fix(agent/agentssh): pin random seed for RSA key generation
Change-Id: I8c7e3070324e5d558374fd6891eea9d48660e1e9Signed-off-by: Thomas Kosiewski <tk@coder.com>
1 parentdedc32f commit38f9dfc

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2 files changed

+59
-8
lines changed

2 files changed

+59
-8
lines changed

‎agent/agent.go

Lines changed: 22 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -6,6 +6,7 @@ import (
66
"encoding/json"
77
"errors"
88
"fmt"
9+
"hash/fnv"
910
"io"
1011
"net/http"
1112
"net/netip"
@@ -372,7 +373,6 @@ func (a *agent) collectMetadata(ctx context.Context, md codersdk.WorkspaceAgentM
372373
// Important: if the command times out, we may see a misleading error like
373374
// "exit status 1", so it's important to include the context error.
374375
err=errors.Join(err,ctx.Err())
375-
376376
iferr!=nil {
377377
result.Error=fmt.Sprintf("run cmd: %+v",err)
378378
}
@@ -906,6 +906,14 @@ func (a *agent) handleManifest(manifestOK *checkpoint) func(ctx context.Context,
906906
}
907907
a.client.RewriteDERPMap(manifest.DERPMap)
908908

909+
seed,err:=stringToInt64Hash(manifest.WorkspaceID)
910+
iferr!=nil {
911+
returnxerrors.Errorf("generate hash from workspace id: %w",err)
912+
}
913+
iferr:=a.sshServer.UpdateHostSigner(seed);err!=nil {
914+
returnxerrors.Errorf("update host signer: %w",err)
915+
}
916+
909917
// Expand the directory and send it back to coderd so external
910918
// applications that rely on the directory can use it.
911919
//
@@ -1850,3 +1858,16 @@ func PrometheusMetricsHandler(prometheusRegistry *prometheus.Registry, logger sl
18501858
}
18511859
})
18521860
}
1861+
1862+
// stringToInt64Hash converts a WorkspaceID UUID to an int64 hash.
1863+
// This uses the FNV-1a hash algorithm which provides decent distribution and collision
1864+
// resistance for string inputs.
1865+
funcstringToInt64Hash(workspaceID uuid.UUID) (int64,error) {
1866+
h:=fnv.New64a()
1867+
_,err:=h.Write(workspaceID[:])
1868+
iferr!=nil {
1869+
return42,err
1870+
}
1871+
1872+
returnint64(h.Sum64()),nil
1873+
}

‎agent/agentssh/agentssh.go

Lines changed: 37 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -3,11 +3,11 @@ package agentssh
33
import (
44
"bufio"
55
"context"
6-
"crypto/rand"
76
"crypto/rsa"
87
"errors"
98
"fmt"
109
"io"
10+
"math/rand"
1111
"net"
1212
"os"
1313
"os/exec"
@@ -131,14 +131,15 @@ func NewServer(ctx context.Context, logger slog.Logger, prometheusRegistry *prom
131131
// Clients' should ignore the host key when connecting.
132132
// The agent needs to authenticate with coderd to SSH,
133133
// so SSH authentication doesn't improve security.
134-
randomHostKey,err:=rsa.GenerateKey(rand.Reader,2048)
135-
iferr!=nil {
136-
returnnil,err
137-
}
138-
randomSigner,err:=gossh.NewSignerFromKey(randomHostKey)
134+
135+
// Create a fixed host key, as at least one host key has to
136+
// exist for the SSH server to start. We'll later replace
137+
// the host key with one that's based off the workspace uuid.
138+
coderSigner,err:=coderSigner(42)
139139
iferr!=nil {
140140
returnnil,err
141141
}
142+
142143
ifconfig==nil {
143144
config=&Config{}
144145
}
@@ -206,7 +207,7 @@ func NewServer(ctx context.Context, logger slog.Logger, prometheusRegistry *prom
206207
slog.Error(err))
207208
},
208209
Handler:s.sessionHandler,
209-
HostSigners: []ssh.Signer{randomSigner},
210+
HostSigners: []ssh.Signer{coderSigner},
210211
LocalPortForwardingCallback:func(ctx ssh.Context,destinationHoststring,destinationPortuint32)bool {
211212
// Allow local port forwarding all!
212213
s.logger.Debug(ctx,"local port forward",
@@ -1099,3 +1100,32 @@ func userHomeDir() (string, error) {
10991100
}
11001101
returnu.HomeDir,nil
11011102
}
1103+
1104+
// UpdateHostSigner updates the host signer with a new key generated from the provided seed.
1105+
// This function replaces the previously used host key with the newly generated one.
1106+
func (s*Server)UpdateHostSigner(seedint64)error {
1107+
key,err:=coderSigner(seed)
1108+
iferr!=nil {
1109+
returnerr
1110+
}
1111+
1112+
s.mu.Lock()
1113+
defers.mu.Unlock()
1114+
1115+
s.srv.AddHostKey(key)
1116+
1117+
returnnil
1118+
}
1119+
1120+
// coderSigner generates a deterministic SSH signer based on the provided seed.
1121+
// It uses RSA with a key size of 2048 bits.
1122+
funccoderSigner(seedint64) (gossh.Signer,error) {
1123+
// nolint: gosec
1124+
deterministicRand:=rand.New(rand.NewSource(seed))
1125+
coderHostKey,err:=rsa.GenerateKey(deterministicRand,2048)
1126+
iferr!=nil {
1127+
returnnil,err
1128+
}
1129+
coderSigner,err:=gossh.NewSignerFromKey(coderHostKey)
1130+
returncoderSigner,err
1131+
}

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