New subject: [tip: x86/urgent] x86/sev: Unroll string mmio with CC_ATTR_GUEST_UNROLL_STRING_IO

21 Mar 2022

From: Joerg Roedel jroedel@suse.de
The io specific memcpy/memset functions use string mmio accesses to do
their work. Under SEV the hypervisor can't emulate these instructions,
because they read/write directly from/to encrypted memory.
KVM will inject a page fault exception into the guest when it is asked
to emulate string mmio instructions for an SEV guest:
BUG: unable to handle page fault for address: ffffc90000065068
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    PGD 8000100000067 P4D 8000100000067 PUD 80001000fb067 PMD 80001000fc067 PTE 80000000fed40173
    Oops: 0000 [#1] PREEMPT SMP NOPTI
    CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.17.0-rc7 #3
As string mmio for an SEV guest can not be supported by the
hypervisor, unroll the instructions for CC_ATTR_GUEST_UNROLL_STRING_IO
enabled kernels.
This issue appears when kernels are launched in recent libvirt-managed
SEV virtual machines, because libvirt started to add a tpm-crb device
to the guest by default.
The kernel driver for tpm-crb uses memcpy_to/from_io() functions to
access MMIO memory, resulting in a page-fault injected by KVM and
crashing the kernel at boot.
Cc: stable@vger.kernel.org #4.15+
Fixes: d8aa7eea78a1 ('x86/mm: Add Secure Encrypted Virtualization (SEV) support')
Signed-off-by: Joerg Roedel jroedel@suse.de
---
Changes: v1->v2:
    - Addressed review comments
      - Call memset directly and remove the string-wrapper
      - Add section about tpm-crb to the commit message
arch/x86/lib/iomem.c | 65 ++++++++++++++++++++++++++++++++++++++------
 1 file changed, 57 insertions(+), 8 deletions(-)

diff --git a/arch/x86/lib/iomem.c b/arch/x86/lib/iomem.c
index df50451d94ef..695419356fe3 100644
--- a/arch/x86/lib/iomem.c
+++ b/arch/x86/lib/iomem.c
@@ -22,7 +22,7 @@ static __always_inline void rep_movs(void *to, const void *from, size_t n)
    	     : "memory");
 }
-void memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
+static void string_memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
 {
    if (unlikely(!n))
    	return;
@@ -38,9 +38,8 @@ void memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
    }
    rep_movs(to, (const void *)from, n);
 }
-EXPORT_SYMBOL(memcpy_fromio);
-void memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
+static void string_memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
 {
    if (unlikely(!n))
    	return;
@@ -56,14 +55,64 @@ void memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
    }
    rep_movs((void *)to, (const void *) from, n);
 }
+
+static void unrolled_memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
+{
+	const volatile char __iomem *in = from;
+	char *out = to;
+	int i;
+
+	for (i = 0; i < n; ++i)
+		out[i] = in[i];
+}
+
+static void unrolled_memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
+{
+	volatile char __iomem *out = to;
+	const char *in = from;
+	int i;
+
+	for (i = 0; i < n; ++i)
+		out[i] = in[i];
+}
+
+static void unrolled_memset_io(volatile void __iomem *a, int b, size_t c)
+{
+	volatile char __iomem *mem = a;
+	int i;
+
+	for (i = 0; i < c; ++i)
+		mem[i] = b;
+}
+
+void memcpy_fromio(void *to, const volatile void __iomem *from, size_t n)
+{
+	if (cc_platform_has(CC_ATTR_GUEST_UNROLL_STRING_IO))
+		unrolled_memcpy_fromio(to, from, n);
+	else
+		string_memcpy_fromio(to, from, n);
+}
+EXPORT_SYMBOL(memcpy_fromio);
+
+void memcpy_toio(volatile void __iomem *to, const void *from, size_t n)
+{
+	if (cc_platform_has(CC_ATTR_GUEST_UNROLL_STRING_IO))
+		unrolled_memcpy_toio(to, from, n);
+	else
+		string_memcpy_toio(to, from, n);
+}
 EXPORT_SYMBOL(memcpy_toio);
void memset_io(volatile void __iomem *a, int b, size_t c)
 {
-	/*
-	 * TODO: memset can mangle the IO patterns quite a bit.
-	 * perhaps it would be better to use a dumb one:
-	 */
-	memset((void *)a, b, c);
+	if (cc_platform_has(CC_ATTR_GUEST_UNROLL_STRING_IO)) {
+		unrolled_memset_io(a, b, c);
+	} else {
+		/*
+		 * TODO: memset can mangle the IO patterns quite a bit.
+		 * perhaps it would be better to use a dumb one:
+		 */
+		memset((void *)a, b, c);
+	}
 }
 EXPORT_SYMBOL(memset_io);
-- 
2.35.1


    

[PATCH v2] x86/sev: Unroll string mmio with CC_ATTR_GUEST_UNROLL_STRING_IO