[PATCH v4 6/6] x86/tdx: Implement movs for MMIO

[Alexey Gladkov]

From: "Alexey Gladkov (Intel)" <legion@xxxxxxxxxx>

Add emulation of the MOVS instruction on MMIO regions. MOVS emulation
consists of dividing it into a series of read and write operations,
which in turn will be validated separately.

Signed-off-by: Alexey Gladkov (Intel) <legion@xxxxxxxxxx>
---
 arch/x86/coco/tdx/tdx.c          | 84 +++++++++++++++++++++++++++++---
 arch/x86/include/asm/processor.h |  4 ++
 2 files changed, 80 insertions(+), 8 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 65f65015238a..d4bec84de034 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -518,6 +518,62 @@ static int decode_insn_struct(struct insn *insn, struct pt_regs *regs)
 	return 0;
 }
 
+static int handle_mmio_movs(struct insn *insn, struct pt_regs *regs, int size, struct ve_info *ve)
+{
+	unsigned long ds_base, es_base;
+	unsigned char *src, *dst;
+	unsigned char buffer[8];
+	int off, ret;
+	bool rep;
+
+	/*
+	 * The in-kernel code must use a special API that does not use MOVS.
+	 * If the MOVS instruction is received from in-kernel, then something
+	 * is broken.
+	 */
+	if (WARN_ON_ONCE(!user_mode(regs)))
+		return -EFAULT;
+
+	ds_base = insn_get_seg_base(regs, INAT_SEG_REG_DS);
+	es_base = insn_get_seg_base(regs, INAT_SEG_REG_ES);
+
+	if (ds_base == -1L || es_base == -1L)
+		return -EINVAL;
+
+	rep = insn_has_rep_prefix(insn);
+
+	do {
+		src = ds_base + (unsigned char *) regs->si;
+		dst = es_base + (unsigned char *) regs->di;
+
+		current->thread.mmio_emul = (unsigned long) src;
+
+		ret = __get_iomem(src, buffer, size);
+		if (ret)
+			goto out;
+
+		current->thread.mmio_emul = (unsigned long) dst;
+
+		ret = __put_iomem(dst, buffer, size);
+		if (ret)
+			goto out;
+
+		off = (regs->flags & X86_EFLAGS_DF) ? -size : size;
+
+		regs->si += off;
+		regs->di += off;
+
+		if (rep)
+			regs->cx -= 1;
+	} while (rep || regs->cx > 0);
+
+	ret = insn->length;
+out:
+	current->thread.mmio_emul = 0;
+
+	return ret;
+}
+
 static int handle_mmio_write(struct insn *insn, enum insn_mmio_type mmio, int size,
 			     struct pt_regs *regs, struct ve_info *ve)
 {
@@ -539,9 +595,8 @@ static int handle_mmio_write(struct insn *insn, enum insn_mmio_type mmio, int si
 		return insn->length;
 	case INSN_MMIO_MOVS:
 		/*
-		 * MMIO was accessed with an instruction that could not be
-		 * decoded or handled properly. It was likely not using io.h
-		 * helpers or accessed MMIO accidentally.
+		 * MOVS is processed through higher level emulation which breaks
+		 * this instruction into a sequence of reads and writes.
 		 */
 		return -EINVAL;
 	default:
@@ -600,6 +655,7 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 {
 	enum insn_mmio_type mmio;
 	struct insn insn = {};
+	int need_validation;
 	unsigned long vaddr;
 	int size, ret;
 
@@ -611,14 +667,27 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 	if (WARN_ON_ONCE(mmio == INSN_MMIO_DECODE_FAILED))
 		return -EINVAL;
 
+	if (mmio == INSN_MMIO_MOVS)
+		return handle_mmio_movs(&insn, regs, size, ve);
+
+	need_validation = user_mode(regs);
+
 	if (!user_mode(regs) && !is_kernel_addr(ve->gla)) {
-		WARN_ONCE(1, "Access to userspace address is not supported");
-		return -EINVAL;
+		/*
+		 * Access from kernel to userspace addresses is not allowed
+		 * unless it is a nested exception during MOVS emulation.
+		 */
+		if (current->thread.mmio_emul != ve->gla || !current->mm) {
+			WARN_ONCE(1, "Access to userspace address is not supported");
+			return -EINVAL;
+		}
+
+		need_validation = 1;
 	}
 
 	vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
 
-	if (user_mode(regs)) {
+	if (need_validation) {
 		if (mmap_read_lock_killable(current->mm))
 			return -EINTR;
 
@@ -644,7 +713,6 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 	switch (mmio) {
 	case INSN_MMIO_WRITE:
 	case INSN_MMIO_WRITE_IMM:
-	case INSN_MMIO_MOVS:
 		ret = handle_mmio_write(&insn, mmio, size, regs, ve);
 		break;
 	case INSN_MMIO_READ:
@@ -665,7 +733,7 @@ static int handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 		ret = -EINVAL;
 	}
 unlock:
-	if (user_mode(regs))
+	if (need_validation)
 		mmap_read_unlock(current->mm);
 
 	return ret;
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index a75a07f4931f..45136b1b02cc 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -503,6 +503,10 @@ struct thread_struct {
 	struct thread_shstk	shstk;
 #endif
 
+#ifdef CONFIG_INTEL_TDX_GUEST
+	unsigned long		mmio_emul;
+#endif
+
 	/* Floating point and extended processor state */
 	struct fpu		fpu;
 	/*
-- 
2.45.2