diff mbox series

[SRU,v2,j/gcp,2/2] virt/coco/sev-guest: Add throttling awareness

Message ID 20230511072357.1165970-3-khalid.elmously@canonical.com
State New
Headers show
Series sev-guest vulnerability fix + follow-up | expand

Commit Message

Khalid Elmously May 11, 2023, 7:23 a.m. UTC 
From: Dionna Glaze <dionnaglaze@google.com>

BugLink: https://bugs.launchpad.net/bugs/2013198

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
(backported from commit 72f7754dcf31c87c92c0c353dcf747814cc5ce10)
[ kmously: 72f7754dcf3 was initially written for 6.0 before finally
 landing in 6.3. The initial proposal can be found at
 https://lore.kernel.org/lkml/Y9emVjoTBrM2+Y5P@zn.tnic/T/
 I used that first implementation as the inspiration for this patch as
 it is closer to the Ubuntu 5.19 code. This was discussed
 with the patch author (Dionna) and confirmed this is the right approach ]
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
diff mbox series

Patch

diff --git a/arch/x86/include/asm/sev-common.h b/arch/x86/include/asm/sev-common.h
index e6e75f19f6fd9..5c8319798954d 100644
--- a/arch/x86/include/asm/sev-common.h
+++ b/arch/x86/include/asm/sev-common.h
@@ -126,8 +126,9 @@  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
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 2a99145fd3aee..db4f84b1f8423 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -2208,15 +2208,13 @@  int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, unsigned
 	if (ret)
 		goto e_put;
 
+	*fw_err = ghcb->save.sw_exit_info_2;
 	if (ghcb->save.sw_exit_info_2) {
 		/* Number of expected pages are returned in RBX */
 		if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST &&
 		    ghcb->save.sw_exit_info_2 == SNP_GUEST_REQ_INVALID_LEN)
 			input->data_npages = ghcb_get_rbx(ghcb);
 
-		if (fw_err)
-			*fw_err = ghcb->save.sw_exit_info_2;
-
 		ret = -EIO;
 	}
 
diff --git a/drivers/virt/coco/sevguest/sevguest.c b/drivers/virt/coco/sevguest/sevguest.c
index f7991bc920e4d..5f0db4a28a25c 100644
--- a/drivers/virt/coco/sevguest/sevguest.c
+++ b/drivers/virt/coco/sevguest/sevguest.c
@@ -324,7 +324,7 @@  static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 				u8 type, void *req_buf, size_t req_sz, void *resp_buf,
 				u32 resp_sz, __u64 *fw_err)
 {
-	unsigned long err;
+	unsigned long err = 0xff;
 	u64 seqno;
 	int rc;
 
@@ -340,6 +340,7 @@  static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 	if (rc)
 		return rc;
 
+retry:
 	/*
 	 * Call firmware to process the request. In this function the encrypted
 	 * message enters shared memory with the host. So after this call the
@@ -348,6 +349,14 @@  static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 	 */
 	rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
 
+	/*
+	 * 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.
+	 */
+	if (err == SNP_GUEST_REQ_ERR_BUSY)
+		goto retry;
+
 	/*
 	 * If the extended guest request fails due to having too small of a
 	 * certificate data buffer, retry the same guest request without the