From: Dionna Glaze dionnaglaze@google.com
commit 72f7754dcf31c87c92c0c353dcf747814cc5ce10 upstream.
A potentially malicious SEV guest can constantly hammer the hypervisor using this driver to send down requests and thus prevent or at least considerably hinder other guests from issuing requests to the secure processor which is a shared platform resource.
Therefore, the host is permitted and encouraged to throttle such guest requests.
Add the capability to handle the case when the hypervisor throttles excessive numbers of requests issued by the guest. Otherwise, the VM platform communication key will be disabled, preventing the guest from attesting itself.
Realistically speaking, a well-behaved guest should not even care about throttling. During its lifetime, it would end up issuing a handful of requests which the hardware can easily handle.
This is more to address the case of a malicious guest. Such guest should get throttled and if its VMPCK gets disabled, then that's its own wrongdoing and perhaps that guest even deserves it.
To the implementation: the hypervisor signals with SNP_GUEST_REQ_ERR_BUSY that the guest requests should be throttled. That error code is returned in the upper 32-bit half of exitinfo2 and this is part of the GHCB spec v2.
So the guest is given a throttling period of 1 minute in which it retries the request every 2 seconds. This is a good default but if it turns out to not pan out in practice, it can be tweaked later.
For safety, since the encryption algorithm in GHCBv2 is AES_GCM, control must remain in the kernel to complete the request with the current sequence number. Returning without finishing the request allows the guest to make another request but with different message contents. This is IV reuse, and breaks cryptographic protections.
[ bp: - Rewrite commit message and do a simplified version. - The stable tags are supposed to denote that a cleanup should go upfront before backporting this so that any future fixes to this can preserve the sanity of the backporter(s). ]
Fixes: d5af44dde546 ("x86/sev: Provide support for SNP guest request NAEs") Signed-off-by: Dionna Glaze dionnaglaze@google.com Co-developed-by: Borislav Petkov (AMD) bp@alien8.de Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Reviewed-by: Tom Lendacky thomas.lendacky@amd.com Cc: stable@kernel.org # d6fd48eff750 ("virt/coco/sev-guest: Check SEV_SNP attribute at probe time") Cc: stable@kernel.org # 970ab823743f (" virt/coco/sev-guest: Simplify extended guest request handling") Cc: stable@kernel.org # c5a338274bdb ("virt/coco/sev-guest: Remove the disable_vmpck label in handle_guest_request()") Cc: stable@kernel.org # 0fdb6cc7c89c ("virt/coco/sev-guest: Carve out the request issuing logic into a helper") Cc: stable@kernel.org # d25bae7dc7b0 ("virt/coco/sev-guest: Do some code style cleanups") Cc: stable@kernel.org # fa4ae42cc60a ("virt/coco/sev-guest: Convert the sw_exit_info_2 checking to a switch-case") Link: https://lore.kernel.org/r/20230214164638.1189804-2-dionnaglaze@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/include/asm/sev-common.h | 3 ++- arch/x86/kernel/sev.c | 4 ++++ drivers/virt/coco/sev-guest/sev-guest.c | 19 ++++++++++++++++++- 3 files changed, 24 insertions(+), 2 deletions(-)
--- a/arch/x86/include/asm/sev-common.h +++ b/arch/x86/include/asm/sev-common.h @@ -128,8 +128,9 @@ struct snp_psc_desc { struct psc_entry entries[VMGEXIT_PSC_MAX_ENTRY]; } __packed;
-/* Guest message request error code */ +/* Guest message request error codes */ #define SNP_GUEST_REQ_INVALID_LEN BIT_ULL(32) +#define SNP_GUEST_REQ_ERR_BUSY BIT_ULL(33)
#define GHCB_MSR_TERM_REQ 0x100 #define GHCB_MSR_TERM_REASON_SET_POS 12 --- a/arch/x86/kernel/sev.c +++ b/arch/x86/kernel/sev.c @@ -2214,6 +2214,10 @@ int snp_issue_guest_request(u64 exit_cod case 0: break;
+ case SNP_GUEST_REQ_ERR_BUSY: + ret = -EAGAIN; + break; + case SNP_GUEST_REQ_INVALID_LEN: /* Number of expected pages are returned in RBX */ if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST) { --- a/drivers/virt/coco/sev-guest/sev-guest.c +++ b/drivers/virt/coco/sev-guest/sev-guest.c @@ -31,6 +31,9 @@ #define AAD_LEN 48 #define MSG_HDR_VER 1
+#define SNP_REQ_MAX_RETRY_DURATION (60*HZ) +#define SNP_REQ_RETRY_DELAY (2*HZ) + struct snp_guest_crypto { struct crypto_aead *tfm; u8 *iv, *authtag; @@ -322,7 +325,8 @@ static int enc_payload(struct snp_guest_
static int __handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, __u64 *fw_err) { - unsigned long err, override_err = 0; + unsigned long err = 0xff, override_err = 0; + unsigned long req_start = jiffies; unsigned int override_npages = 0; int rc;
@@ -362,6 +366,19 @@ retry_request: * user as an ioctl() return code. */ goto retry_request; + + /* + * The host may return SNP_GUEST_REQ_ERR_EBUSY if the request has been + * throttled. Retry in the driver to avoid returning and reusing the + * message sequence number on a different message. + */ + case -EAGAIN: + if (jiffies - req_start > SNP_REQ_MAX_RETRY_DURATION) { + rc = -ETIMEDOUT; + break; + } + schedule_timeout_killable(SNP_REQ_RETRY_DELAY); + goto retry_request; }
/*