[v6 PATCH 07/21] 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@xxxxxxxxxxxxxxx>
Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
 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