[PATCH 42/70] x86/sev-es: Support nested #VC exceptions

Joerg Roedel
2020-03-19T09:17:16Z

From: Joerg Roedel <jroedel@suse.de>

Handle #VC exceptions that happen while the GHCB is in use. This can
happen when an NMI happens in the #VC exception handler and the NMI
handler causes a #VC exception itself. Save the contents of the GHCB
when nesting is detected and restore it when the GHCB is no longer
used.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/kernel/sev-es.c | 63 +++++++++++++++++++++++++++++++++++++---
1 file changed, 59 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 97241d2f0f70..3b7bbc8d841e 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -32,9 +32,57 @@ struct ghcb boot_ghcb_page __bss_decrypted __aligned(PAGE_SIZE);
*/
struct ghcb __initdata *boot_ghcb;

+struct ghcb_state {
+ struct ghcb *ghcb;
+};
+
/* Runtime GHCB pointers */
static struct ghcb __percpu *ghcb_page;

+/*
+ * Mark the per-cpu GHCB as in-use to detect nested #VC exceptions.
+ * There is no need for it to be atomic, because nothing is written to the GHCB
+ * between the read and the write of ghcb_active. So it is safe to use it when a
+ * nested #VC exception happens before the write.
+ */
+static DEFINE_PER_CPU(bool, ghcb_active);
+
+static struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
+{
+ struct ghcb *ghcb = (struct ghcb *)this_cpu_ptr(ghcb_page);
+ bool *active = this_cpu_ptr(&ghcb_active);
+
+ if (unlikely(*active)) {
+ /* GHCB is already in use - save its contents */
+
+ state->ghcb = kzalloc(sizeof(struct ghcb), GFP_ATOMIC);
+ if (!state->ghcb)
+ return NULL;
+
+ *state->ghcb = *ghcb;
+ } else {
+ state->ghcb = NULL;
+ *active = true;
+ }
+
+ return ghcb;
+}
+
+static void sev_es_put_ghcb(struct ghcb_state *state)
+{
+ bool *active = this_cpu_ptr(&ghcb_active);
+ struct ghcb *ghcb = (struct ghcb *)this_cpu_ptr(ghcb_page);
+
+ if (state->ghcb) {
+ /* Restore saved state and free backup memory */
+ *ghcb = *state->ghcb;
+ kfree(state->ghcb);
+ state->ghcb = NULL;
+ } else {
+ *active = false;
+ }
+}
+
/* Needed in vc_early_vc_forward_exception */
extern void early_exception(struct pt_regs *regs, int trapnr);

@@ -272,6 +320,7 @@ static void vc_forward_exception(struct es_em_ctxt *ctxt)

dotraplinkage void do_vmm_communication(struct pt_regs *regs, unsigned long exit_code)
{
+ struct ghcb_state state;
struct es_em_ctxt ctxt;
enum es_result result;
struct ghcb *ghcb;
@@ -282,14 +331,20 @@ dotraplinkage void do_vmm_communication(struct pt_regs *regs, unsigned long exit
* keep the IRQs disabled to protect us against concurrent TLB flushes.
*/

- ghcb = (struct ghcb *)this_cpu_ptr(ghcb_page);
-
- vc_ghcb_invalidate(ghcb);
- result = vc_init_em_ctxt(&ctxt, regs, exit_code);
+ ghcb = sev_es_get_ghcb(&state);
+ if (!ghcb) {
+ /* This can only fail on an allocation error, so just retry */
+ result = ES_RETRY;
+ } else {
+ vc_ghcb_invalidate(ghcb);
+ result = vc_init_em_ctxt(&ctxt, regs, exit_code);
+ }

if (result == ES_OK)
result = vc_handle_exitcode(&ctxt, ghcb, exit_code);

+ sev_es_put_ghcb(&state);
+
/* Done - now check the result */
switch (result) {
case ES_OK:
--
2.17.1