[PATCH v1 4/4] x86/tdx: Implement movs for MMIO

[Alexey Gladkov (Intel)]

Adapt AMD's implementation of the MOVS instruction. Since the
implementations are similar, it is possible to reuse the code.

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>
---


I don't really understand the reasoning behind AMD's approach of returning to
userspace after every read/write operation in vc_handle_mmio_movs(). I didn't
change this so as not to break their implementation.

But if this can be changed then the whole vc_handle_mmio_movs() could be used as
a common helper.


 arch/x86/coco/sev/core.c  | 133 ++++----------------------------------
 arch/x86/coco/tdx/tdx.c   |  56 ++++++++++++++--
 arch/x86/include/asm/io.h |   3 +
 arch/x86/lib/iomem.c      | 132 +++++++++++++++++++++++++++++++++++++
 4 files changed, 199 insertions(+), 125 deletions(-)

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 082d61d85dfc..3135c89802e9 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -369,72 +369,17 @@ static enum es_result vc_decode_insn(struct es_em_ctxt *ctxt)
 static enum es_result vc_write_mem(struct es_em_ctxt *ctxt,
 				   char *dst, char *buf, size_t size)
 {
-	unsigned long error_code = X86_PF_PROT | X86_PF_WRITE;
+	unsigned long error_code;
+	int ret = __put_iomem(dst, buf, size);
 
-	/*
-	 * This function uses __put_user() independent of whether kernel or user
-	 * memory is accessed. This works fine because __put_user() does no
-	 * sanity checks of the pointer being accessed. All that it does is
-	 * to report when the access failed.
-	 *
-	 * Also, this function runs in atomic context, so __put_user() is not
-	 * allowed to sleep. The page-fault handler detects that it is running
-	 * in atomic context and will not try to take mmap_sem and handle the
-	 * fault, so additional pagefault_enable()/disable() calls are not
-	 * needed.
-	 *
-	 * The access can't be done via copy_to_user() here because
-	 * vc_write_mem() must not use string instructions to access unsafe
-	 * memory. The reason is that MOVS is emulated by the #VC handler by
-	 * splitting the move up into a read and a write and taking a nested #VC
-	 * exception on whatever of them is the MMIO access. Using string
-	 * instructions here would cause infinite nesting.
-	 */
-	switch (size) {
-	case 1: {
-		u8 d1;
-		u8 __user *target = (u8 __user *)dst;
-
-		memcpy(&d1, buf, 1);
-		if (__put_user(d1, target))
-			goto fault;
-		break;
-	}
-	case 2: {
-		u16 d2;
-		u16 __user *target = (u16 __user *)dst;
-
-		memcpy(&d2, buf, 2);
-		if (__put_user(d2, target))
-			goto fault;
-		break;
-	}
-	case 4: {
-		u32 d4;
-		u32 __user *target = (u32 __user *)dst;
-
-		memcpy(&d4, buf, 4);
-		if (__put_user(d4, target))
-			goto fault;
-		break;
-	}
-	case 8: {
-		u64 d8;
-		u64 __user *target = (u64 __user *)dst;
+	if (!ret)
+		return ES_OK;
 
-		memcpy(&d8, buf, 8);
-		if (__put_user(d8, target))
-			goto fault;
-		break;
-	}
-	default:
-		WARN_ONCE(1, "%s: Invalid size: %zu\n", __func__, size);
+	if (ret == -EIO)
 		return ES_UNSUPPORTED;
-	}
 
-	return ES_OK;
+	error_code = X86_PF_PROT | X86_PF_WRITE;
 
-fault:
 	if (user_mode(ctxt->regs))
 		error_code |= X86_PF_USER;
 
@@ -448,71 +393,17 @@ static enum es_result vc_write_mem(struct es_em_ctxt *ctxt,
 static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
 				  char *src, char *buf, size_t size)
 {
-	unsigned long error_code = X86_PF_PROT;
+	unsigned long error_code;
+	int ret = __get_iomem(src, buf, size);
 
-	/*
-	 * This function uses __get_user() independent of whether kernel or user
-	 * memory is accessed. This works fine because __get_user() does no
-	 * sanity checks of the pointer being accessed. All that it does is
-	 * to report when the access failed.
-	 *
-	 * Also, this function runs in atomic context, so __get_user() is not
-	 * allowed to sleep. The page-fault handler detects that it is running
-	 * in atomic context and will not try to take mmap_sem and handle the
-	 * fault, so additional pagefault_enable()/disable() calls are not
-	 * needed.
-	 *
-	 * The access can't be done via copy_from_user() here because
-	 * vc_read_mem() must not use string instructions to access unsafe
-	 * memory. The reason is that MOVS is emulated by the #VC handler by
-	 * splitting the move up into a read and a write and taking a nested #VC
-	 * exception on whatever of them is the MMIO access. Using string
-	 * instructions here would cause infinite nesting.
-	 */
-	switch (size) {
-	case 1: {
-		u8 d1;
-		u8 __user *s = (u8 __user *)src;
-
-		if (__get_user(d1, s))
-			goto fault;
-		memcpy(buf, &d1, 1);
-		break;
-	}
-	case 2: {
-		u16 d2;
-		u16 __user *s = (u16 __user *)src;
-
-		if (__get_user(d2, s))
-			goto fault;
-		memcpy(buf, &d2, 2);
-		break;
-	}
-	case 4: {
-		u32 d4;
-		u32 __user *s = (u32 __user *)src;
+	if (!ret)
+		return ES_OK;
 
-		if (__get_user(d4, s))
-			goto fault;
-		memcpy(buf, &d4, 4);
-		break;
-	}
-	case 8: {
-		u64 d8;
-		u64 __user *s = (u64 __user *)src;
-		if (__get_user(d8, s))
-			goto fault;
-		memcpy(buf, &d8, 8);
-		break;
-	}
-	default:
-		WARN_ONCE(1, "%s: Invalid size: %zu\n", __func__, size);
+	if (ret == -EIO)
 		return ES_UNSUPPORTED;
-	}
 
-	return ES_OK;
+	error_code = X86_PF_PROT;
 
-fault:
 	if (user_mode(ctxt->regs))
 		error_code |= X86_PF_USER;
 
diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index 26b2e52457be..cf209381d63b 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -499,6 +499,53 @@ 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.
+	 */
+	WARN_ON_ONCE(!user_mode(regs));
+
+	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;
+
+		ret = __get_iomem(src, buffer, size);
+		if (ret)
+			return ret;
+
+		ret = __put_iomem(dst, buffer, size);
+		if (ret)
+			return ret;
+
+		off = (regs->flags & X86_EFLAGS_DF) ? -size : size;
+
+		regs->si += off;
+		regs->di += off;
+
+		if (rep)
+			regs->cx -= 1;
+	} while (rep || regs->cx > 0);
+
+	return insn->length;
+}
+
 static int handle_mmio_write(struct insn *insn, enum insn_mmio_type mmio, int size,
 		struct pt_regs *regs, struct ve_info *ve)
 {
@@ -520,9 +567,8 @@ 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:
@@ -591,6 +637,9 @@ 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);
+
 	vaddr = (unsigned long)insn_get_addr_ref(&insn, regs);
 
 	if (user_mode(regs)) {
@@ -619,7 +668,6 @@ 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:
diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index 1d60427379c9..ac01d53466cb 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -402,4 +402,7 @@ static inline void iosubmit_cmds512(void __iomem *dst, const void *src,
 	}
 }
 
+int __get_iomem(char *src, char *buf, size_t size);
+int __put_iomem(char *src, char *buf, size_t size);
+
 #endif /* _ASM_X86_IO_H */
diff --git a/arch/x86/lib/iomem.c b/arch/x86/lib/iomem.c
index 5eecb45d05d5..e4919ad22206 100644
--- a/arch/x86/lib/iomem.c
+++ b/arch/x86/lib/iomem.c
@@ -2,6 +2,7 @@
 #include <linux/module.h>
 #include <linux/io.h>
 #include <linux/kmsan-checks.h>
+#include <asm/uaccess.h>
 
 #define movs(type,to,from) \
 	asm volatile("movs" type:"=&D" (to), "=&S" (from):"0" (to), "1" (from):"memory")
@@ -124,3 +125,134 @@ void memset_io(volatile void __iomem *a, int b, size_t c)
 	}
 }
 EXPORT_SYMBOL(memset_io);
+
+int __get_iomem(char *src, char *buf, size_t size)
+{
+	/*
+	 * This function uses __get_user() independent of whether kernel or user
+	 * memory is accessed. This works fine because __get_user() does no
+	 * sanity checks of the pointer being accessed. All that it does is
+	 * to report when the access failed.
+	 *
+	 * Also, this function runs in atomic context, so __get_user() is not
+	 * allowed to sleep. The page-fault handler detects that it is running
+	 * in atomic context and will not try to take mmap_sem and handle the
+	 * fault, so additional pagefault_enable()/disable() calls are not
+	 * needed.
+	 *
+	 * The access can't be done via copy_from_user() here because
+	 * mmio_read_mem() must not use string instructions to access unsafe
+	 * memory. The reason is that MOVS is emulated by the #VC handler by
+	 * splitting the move up into a read and a write and taking a nested #VC
+	 * exception on whatever of them is the MMIO access. Using string
+	 * instructions here would cause infinite nesting.
+	 */
+	switch (size) {
+	case 1: {
+		u8 d1;
+		u8 __user *s = (u8 __user *)src;
+
+		if (__get_user(d1, s))
+			return -EFAULT;
+		memcpy(buf, &d1, 1);
+		break;
+	}
+	case 2: {
+		u16 d2;
+		u16 __user *s = (u16 __user *)src;
+
+		if (__get_user(d2, s))
+			return -EFAULT;
+		memcpy(buf, &d2, 2);
+		break;
+	}
+	case 4: {
+		u32 d4;
+		u32 __user *s = (u32 __user *)src;
+
+		if (__get_user(d4, s))
+			return -EFAULT;
+		memcpy(buf, &d4, 4);
+		break;
+	}
+	case 8: {
+		u64 d8;
+		u64 __user *s = (u64 __user *)src;
+		if (__get_user(d8, s))
+			return -EFAULT;
+		memcpy(buf, &d8, 8);
+		break;
+	}
+	default:
+		WARN_ONCE(1, "%s: Invalid size: %zu\n", __func__, size);
+		return -EIO;
+	}
+
+	return 0;
+}
+
+int __put_iomem(char *dst, char *buf, size_t size)
+{
+	/*
+	 * This function uses __put_user() independent of whether kernel or user
+	 * memory is accessed. This works fine because __put_user() does no
+	 * sanity checks of the pointer being accessed. All that it does is
+	 * to report when the access failed.
+	 *
+	 * Also, this function runs in atomic context, so __put_user() is not
+	 * allowed to sleep. The page-fault handler detects that it is running
+	 * in atomic context and will not try to take mmap_sem and handle the
+	 * fault, so additional pagefault_enable()/disable() calls are not
+	 * needed.
+	 *
+	 * The access can't be done via copy_to_user() here because
+	 * put_iomem() must not use string instructions to access unsafe
+	 * memory. The reason is that MOVS is emulated by the #VC handler by
+	 * splitting the move up into a read and a write and taking a nested #VC
+	 * exception on whatever of them is the MMIO access. Using string
+	 * instructions here would cause infinite nesting.
+	 */
+	switch (size) {
+	case 1: {
+		u8 d1;
+		u8 __user *target = (u8 __user *)dst;
+
+		memcpy(&d1, buf, 1);
+		if (__put_user(d1, target))
+			return -EFAULT;
+		break;
+	}
+	case 2: {
+		u16 d2;
+		u16 __user *target = (u16 __user *)dst;
+
+		memcpy(&d2, buf, 2);
+		if (__put_user(d2, target))
+			return -EFAULT;
+		break;
+	}
+	case 4: {
+		u32 d4;
+		u32 __user *target = (u32 __user *)dst;
+
+		memcpy(&d4, buf, 4);
+		if (__put_user(d4, target))
+			return -EFAULT;
+		break;
+	}
+	case 8: {
+		u64 d8;
+		u64 __user *target = (u64 __user *)dst;
+
+		memcpy(&d8, buf, 8);
+		if (__put_user(d8, target))
+			return -EFAULT;
+		break;
+	}
+	default:
+		WARN_ONCE(1, "%s: Invalid size: %zu\n", __func__, size);
+		return -EIO;
+	}
+
+	return 0;
+}
-- 
2.45.2