x86/sev-es: Use __copy_from_user_inatomic()

The #VC handler must run in atomic context and cannot sleep. This is a
problem when it tries to fetch instruction bytes from user-space via
copy_from_user().

Introduce a insn_fetch_from_user_inatomic() helper which uses
__copy_from_user_inatomic() to safely copy the instruction bytes to
kernel memory in the #VC handler.

Fixes: 5e3427a7bc ("x86/sev-es: Handle instruction fetches from user-space")
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org # v5.10+
Link: https://lkml.kernel.org/r/20210303141716.29223-6-joro@8bytes.org

arch/x86/include/asm/insn-eval.h

@@ -23,6 +23,8 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx);
 int insn_get_code_seg_params(struct pt_regs *regs);
 int insn_fetch_from_user(struct pt_regs *regs,
 			 unsigned char buf[MAX_INSN_SIZE]);
+int insn_fetch_from_user_inatomic(struct pt_regs *regs,
+				  unsigned char buf[MAX_INSN_SIZE]);
 bool insn_decode(struct insn *insn, struct pt_regs *regs,
 		 unsigned char buf[MAX_INSN_SIZE], int buf_size);
 

arch/x86/kernel/sev-es.c

@@ -258,7 +258,7 @@ static enum es_result vc_decode_insn(struct es_em_ctxt *ctxt)
 	int res;
 
 	if (user_mode(ctxt->regs)) {
-		res = insn_fetch_from_user(ctxt->regs, buffer);
+		res = insn_fetch_from_user_inatomic(ctxt->regs, buffer);
 		if (!res) {
 			ctxt->fi.vector     = X86_TRAP_PF;
 			ctxt->fi.error_code = X86_PF_INSTR | X86_PF_USER;

arch/x86/lib/insn-eval.c

@@ -1415,6 +1415,25 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 	}
 }
 
+static unsigned long insn_get_effective_ip(struct pt_regs *regs)
+{
+	unsigned long seg_base = 0;
+
+	/*
+	 * If not in user-space long mode, a custom code segment could be in
+	 * use. This is true in protected mode (if the process defined a local
+	 * descriptor table), or virtual-8086 mode. In most of the cases
+	 * seg_base will be zero as in USER_CS.
+	 */
+	if (!user_64bit_mode(regs)) {
+		seg_base = insn_get_seg_base(regs, INAT_SEG_REG_CS);
+		if (seg_base == -1L)
+			return 0;
+	}
+
+	return seg_base + regs->ip;
+}
+
 /**
  * insn_fetch_from_user() - Copy instruction bytes from user-space memory
  * @regs:	Structure with register values as seen when entering kernel mode
@@ -1431,24 +1450,43 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
  */
 int insn_fetch_from_user(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE])
 {
-	unsigned long seg_base = 0;
+	unsigned long ip;
 	int not_copied;
 
-	/*
+	ip = insn_get_effective_ip(regs);
-	 * If not in user-space long mode, a custom code segment could be in
+	if (!ip)
-	 * use. This is true in protected mode (if the process defined a local
+		return 0;
-	 * descriptor table), or virtual-8086 mode. In most of the cases
-	 * seg_base will be zero as in USER_CS.
-	 */
-	if (!user_64bit_mode(regs)) {
-		seg_base = insn_get_seg_base(regs, INAT_SEG_REG_CS);
-		if (seg_base == -1L)
-			return 0;
-	}
 
+	not_copied = copy_from_user(buf, (void __user *)ip, MAX_INSN_SIZE);
 
-	not_copied = copy_from_user(buf, (void __user *)(seg_base + regs->ip),
+	return MAX_INSN_SIZE - not_copied;
-				    MAX_INSN_SIZE);
+}
+
+/**
+ * insn_fetch_from_user_inatomic() - Copy instruction bytes from user-space memory
+ *                                   while in atomic code
+ * @regs:	Structure with register values as seen when entering kernel mode
+ * @buf:	Array to store the fetched instruction
+ *
+ * Gets the linear address of the instruction and copies the instruction bytes
+ * to the buf. This function must be used in atomic context.
+ *
+ * Returns:
+ *
+ * Number of instruction bytes copied.
+ *
+ * 0 if nothing was copied.
+ */
+int insn_fetch_from_user_inatomic(struct pt_regs *regs, unsigned char buf[MAX_INSN_SIZE])
+{
+	unsigned long ip;
+	int not_copied;
+
+	ip = insn_get_effective_ip(regs);
+	if (!ip)
+		return 0;
+
+	not_copied = __copy_from_user_inatomic(buf, (void __user *)ip, MAX_INSN_SIZE);
 
 	return MAX_INSN_SIZE - not_copied;
 }