[v6 PATCH 08/21] x86/insn-eval: Add utility function to get segment descriptor base address

[Ricardo Neri]

With segmentation, the base address of the segment descriptor is needed
to compute a linear address. The segment descriptor used in the address
computation depends on either any segment override prefixes in the in the
instruction or the default segment determined by the registers involved
in the address computation. Thus, both the instruction as well as the
register (specified as the offset from the base of pt_regs) are given as
inputs, along with a boolean variable to select between override and
default.

The segment selector is determined by get_seg_selector with the inputs
described above. Once the selector is known the base address is
determined. In protected mode, the selector is used to obtain the segment
descriptor and then its base address. If in 64-bit user mode, the segment =
base address is zero except when FS or GS are used. In virtual-8086 mode,
the base address is computed as the value of the segment selector shifted 4
positions to the left.

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/include/asm/insn-eval.h |  2 ++
 arch/x86/lib/insn-eval.c         | 66 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
index 754211b..b201742 100644
--- a/arch/x86/include/asm/insn-eval.h
+++ b/arch/x86/include/asm/insn-eval.h
@@ -15,5 +15,7 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
 int insn_get_reg_offset_modrm_rm(struct insn *insn, struct pt_regs *regs);
 int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs);
 int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs);
+unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
+				int regoff, bool use_default_seg);
 
 #endif /* _ASM_X86_INSN_EVAL_H */
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 8608adf..383ca83 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -355,6 +355,72 @@ static int get_desc(unsigned short seg, struct desc_struct **desc)
 }
 
 /**
+ * insn_get_seg_base() - Obtain base address contained in descriptor
+ * @regs:	Set of registers containing the segment selector
+ * @insn:	Instruction structure with selector override prefixes
+ * @regoff:	Operand offset, in pt_regs, of which the selector is needed
+ * @use_default_seg: Use the default segment instead of prefix overrides
+ *
+ * Obtain the base address of the segment descriptor as indicated by either
+ * any segment override prefixes contained in insn or the default segment
+ * applicable to the register indicated by regoff. regoff is specified as the
+ * offset in bytes from the base of pt_regs.
+ *
+ * Return: In protected mode, base address of the segment. It may be zero in
+ * certain cases for 64-bit builds and/or 64-bit applications. In virtual-8086
+ * mode, the segment selector shifed 4 positions to the right. -1L in case of
+ * error.
+ */
+unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
+				int regoff, bool use_default_seg)
+{
+	struct desc_struct *desc;
+	unsigned short seg;
+	enum segment seg_type;
+	int ret;
+
+	seg_type = resolve_seg_selector(insn, regoff, use_default_seg);
+
+	seg = get_segment_selector(regs, seg_type);
+	if (seg < 0)
+		return -1L;
+
+	if (v8086_mode(regs))
+		/*
+		 * Base is simply the segment selector shifted 4
+		 * positions to the right.
+		 */
+		return (unsigned long)(seg << 4);
+
+#ifdef CONFIG_X86_64
+	if (user_64bit_mode(regs)) {
+		/*
+		 * Only FS or GS will have a base address, the rest of
+		 * the segments' bases are forced to 0.
+		 */
+		unsigned long base;
+
+		if (seg_type == SEG_FS)
+			rdmsrl(MSR_FS_BASE, base);
+		else if (seg_type == SEG_GS)
+			/*
+			 * swapgs was called at the kernel entry point. Thus,
+			 * MSR_KERNEL_GS_BASE will have the user-space GS base.
+			 */
+			rdmsrl(MSR_KERNEL_GS_BASE, base);
+		else
+			base = 0;
+		return base;
+	}
+#endif
+	ret = get_desc(seg, &desc);
+	if (ret)
+		return -1L;
+
+	return get_desc_base(desc);
+}
+
+/**
  * 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