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Subject[PATCH v3 5/5] x86/sev-es: Do not support MMIO to/from encrypted memory
Date
From: Joerg Roedel <jroedel@suse.de>

MMIO memory is usually not mapped encrypted, so there is no reason to
support emulated MMIO when it is mapped encrypted.

This prevents a possible hypervisor attack where it maps a RAM page as
an MMIO page in the nested page-table, so that any guest access to it
will trigger a #VC exception and leak the data on that page to the
hypervisor or allows the hypervisor to inject data into the guest.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/kernel/sev-es.c | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
index 4a96726fbaf8..0bd1a0fc587e 100644
--- a/arch/x86/kernel/sev-es.c
+++ b/arch/x86/kernel/sev-es.c
@@ -374,8 +374,8 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
return ES_EXCEPTION;
}

-static bool vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
- unsigned long vaddr, phys_addr_t *paddr)
+static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
+ unsigned long vaddr, phys_addr_t *paddr)
{
unsigned long va = (unsigned long)vaddr;
unsigned int level;
@@ -394,15 +394,19 @@ static bool vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
if (user_mode(ctxt->regs))
ctxt->fi.error_code |= X86_PF_USER;

- return false;
+ return ES_EXCEPTION;
}

+ if (WARN_ON_ONCE(pte_val(*pte) & _PAGE_ENC))
+ /* Emulated MMIO to/from encrypted memory not supported */
+ return ES_UNSUPPORTED;
+
pa = (phys_addr_t)pte_pfn(*pte) << PAGE_SHIFT;
pa |= va & ~page_level_mask(level);

*paddr = pa;

- return true;
+ return ES_OK;
}

/* Include code shared with pre-decompression boot stage */
@@ -731,6 +735,7 @@ static enum es_result vc_do_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt,
{
u64 exit_code, exit_info_1, exit_info_2;
unsigned long ghcb_pa = __pa(ghcb);
+ enum es_result res;
phys_addr_t paddr;
void __user *ref;

@@ -740,11 +745,12 @@ static enum es_result vc_do_mmio(struct ghcb *ghcb, struct es_em_ctxt *ctxt,

exit_code = read ? SVM_VMGEXIT_MMIO_READ : SVM_VMGEXIT_MMIO_WRITE;

- if (!vc_slow_virt_to_phys(ghcb, ctxt, (unsigned long)ref, &paddr)) {
- if (!read)
+ res = vc_slow_virt_to_phys(ghcb, ctxt, (unsigned long)ref, &paddr);
+ if (res != ES_OK) {
+ if (res == ES_EXCEPTION && !read)
ctxt->fi.error_code |= X86_PF_WRITE;

- return ES_EXCEPTION;
+ return res;
}

exit_info_1 = paddr;
--
2.28.0
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