[v5 07/20] x86/insn-eval: Add utility function to get segment descriptor

[Ricardo Neri]

The segment descriptor contains information that is relevant to how linear
address need to be computed. It contains the default size of addresses as
well as the base address of the segment. Thus, given a segment selector,
we ought look at segment descriptor to correctly calculate the linear
address.

In protected mode, the segment selector might indicate a segment
descriptor from either the global descriptor table or a local descriptor
table. Both cases are considered in this function.

This function is the initial implementation for subsequent functions that
will obtain the aforementioned attributes of the segment descriptor.

Cc: Dave Hansen <dave.hansen at linux.intel.com>
Cc: Adam Buchbinder <adam.buchbinder at gmail.com>
Cc: Colin Ian King <colin.king at canonical.com>
Cc: Lorenzo Stoakes <lstoakes at gmail.com>
Cc: Qiaowei Ren <qiaowei.ren at intel.com>
Cc: Arnaldo Carvalho de Melo <acme at redhat.com>
Cc: Masami Hiramatsu <mhiramat at kernel.org>
Cc: Adrian Hunter <adrian.hunter at intel.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Thomas Garnier <thgarnie at google.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Borislav Petkov <bp at suse.de>
Cc: Dmitry Vyukov <dvyukov at google.com>
Cc: Ravi V. Shankar <ravi.v.shankar at intel.com>
Cc: x86 at kernel.org
Signed-off-by: Ricardo Neri <ricardo.neri-calderon at linux.intel.com>
---
 arch/x86/lib/insn-eval.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 8d45df8..8608adf 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -5,9 +5,13 @@
  */
 #include <linux/kernel.h>
 #include <linux/string.h>
+#include <asm/desc_defs.h>
+#include <asm/desc.h>
 #include <asm/inat.h>
 #include <asm/insn.h>
 #include <asm/insn-eval.h>
+#include <asm/ldt.h>
+#include <linux/mmu_context.h>
 #include <asm/vm86.h>
 
 enum reg_type {
@@ -294,6 +298,63 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
 }
 
 /**
+ * get_desc() - Obtain address of segment descriptor
+ * @seg:	Segment selector
+ * @desc:	Pointer to the selected segment descriptor
+ *
+ * Given a segment selector, obtain a memory pointer to the segment
+ * descriptor. Both global and local descriptor tables are supported.
+ * desc will contain the address of the descriptor.
+ *
+ * Return: 0 if success, -EINVAL if failure
+ */
+static int get_desc(unsigned short seg, struct desc_struct **desc)
+{
+	struct desc_ptr gdt_desc = {0, 0};
+	unsigned long desc_base;
+
+	if (!desc)
+		return -EINVAL;
+
+	desc_base = seg & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK);
+
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+	if ((seg & SEGMENT_TI_MASK) == SEGMENT_LDT) {
+		seg >>= 3;
+
+		mutex_lock(&current->active_mm->context.lock);
+		if (unlikely(!current->active_mm->context.ldt ||
+			     seg >= current->active_mm->context.ldt->size)) {
+			*desc = NULL;
+			mutex_unlock(&current->active_mm->context.lock);
+			return -EINVAL;
+		}
+
+		*desc = &current->active_mm->context.ldt->entries[seg];
+		mutex_unlock(&current->active_mm->context.lock);
+		return 0;
+	}
+#endif
+	native_store_gdt(&gdt_desc);
+
+	/*
+	 * Bits [15:3] of the segment selector contain the index. Such
+	 * index needs to be multiplied by 8. However, as the index
+	 * least significant bit is already in bit 3, we don't have
+	 * to perform the multiplication.
+	 */
+	desc_base = seg & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK);
+
+	if (desc_base > gdt_desc.size) {
+		*desc = NULL;
+		return -EINVAL;
+	}
+
+	*desc = (struct desc_struct *)(gdt_desc.address + desc_base);
+	return 0;
+}
+
+/**
  * insn_get_reg_offset_modrm_rm - Obtain register in r/m part of ModRM byte
  * @insn:	Instruction structure containing the ModRM byte
  * @regs:	Set of registers indicated by the ModRM byte
-- 
2.9.3