[PATCH v9 15/29] x86/insn-eval: Add utility functions to get segment descriptor base address and limit

[Ricardo Neri]

With segmentation, the base address of the segment is needed to compute a
linear address. This base address is obtained from the applicable segment
descriptor. Such segment descriptor is referenced from a segment selector.
These new functions obtain the segment base and limit of the segment
selector indicated by segment register index given as argument. This index
is any of the INAT_SEG_REG_* family of #define's.

The logic to obtain the segment selector is wrapped in the function
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. 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.

In protected mode, segment limits are enforced. Thus, a function to
determine the limit of the segment is added. Segment limits are not
enforced in long or virtual-8086. For the latter, addresses are limited
to 20 bits; address size will be handled when computing the linear
address.

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 |   1 +
 arch/x86/lib/insn-eval.c         | 117 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 118 insertions(+)

diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
index 7e8c963..25d6e44 100644
--- a/arch/x86/include/asm/insn-eval.h
+++ b/arch/x86/include/asm/insn-eval.h
@@ -13,5 +13,6 @@
 
 void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
 int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs);
+unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx);
 
 #endif /* _ASM_X86_INSN_EVAL_H */
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index d599dc3..02c4498 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -507,6 +507,123 @@ static struct desc_struct *get_desc(unsigned short sel)
 }
 
 /**
+ * insn_get_seg_base() - Obtain base address of segment descriptor.
+ * @regs:		Register values as seen when entering kernel mode
+ * @seg_reg_idx:	Index of the segment register pointing to seg descriptor
+ *
+ * Obtain the base address of the segment as indicated by the segment descriptor
+ * pointed by the segment selector. The segment selector is obtained from the
+ * input segment register index seg_reg_idx.
+ *
+ * Returns:
+ *
+ * In protected mode, base address of the segment. Zero in long mode,
+ * except when FS or GS are used. In virtual-8086 mode, the segment
+ * selector shifted 4 positions to the right.
+ *
+ * -1L in case of error.
+ */
+unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx)
+{
+	struct desc_struct *desc;
+	short sel;
+
+	sel = get_segment_selector(regs, seg_reg_idx);
+	if (sel < 0)
+		return -1L;
+
+	if (v8086_mode(regs))
+		/*
+		 * Base is simply the segment selector shifted 4
+		 * positions to the right.
+		 */
+		return (unsigned long)(sel << 4);
+
+	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_reg_idx == INAT_SEG_REG_FS)
+			rdmsrl(MSR_FS_BASE, base);
+		else if (seg_reg_idx == INAT_SEG_REG_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 if (seg_reg_idx != INAT_SEG_REG_IGNORE)
+			/* We should ignore the rest of segment registers. */
+			base = -1L;
+		else
+			base = 0;
+		return base;
+	}
+
+	/* In protected mode the segment selector cannot be null. */
+	if (!sel)
+		return -1L;
+
+	desc = get_desc(sel);
+	if (!desc)
+		return -1L;
+
+	return get_desc_base(desc);
+}
+
+/**
+ * get_seg_limit() - Obtain the limit of a segment descriptor
+ * @regs:		Register values as seen when entering kernel mode
+ * @seg_reg_idx:	Index of the segment register pointing to seg descriptor
+ *
+ * Obtain the limit of the segment as indicated by the segment descriptor
+ * pointed by the segment selector. The segment selector is obtained from the
+ * input segment register index seg_reg_idx.
+ *
+ * Returns:
+ *
+ * In protected mode, the limit of the segment descriptor in bytes.
+ * In long mode and virtual-8086 mode, segment limits are not enforced. Thus,
+ * limit is returned as -1L to imply a limit-less segment.
+ *
+ * Zero is returned on error.
+ */
+static unsigned long get_seg_limit(struct pt_regs *regs, int seg_reg_idx)
+{
+	struct desc_struct *desc;
+	unsigned long limit;
+	short sel;
+
+	sel = get_segment_selector(regs, seg_reg_idx);
+	if (sel < 0)
+		return 0;
+
+	if (user_64bit_mode(regs) || v8086_mode(regs))
+		return -1L;
+
+	if (!sel)
+		return 0;
+
+	desc = get_desc(sel);
+	if (!desc)
+		return 0;
+
+	/*
+	 * If the granularity bit is set, the limit is given in multiples
+	 * of 4096. This also means that the 12 least significant bits are
+	 * not tested when checking the segment limits. In practice,
+	 * this means that the segment ends in (limit << 12) + 0xfff.
+	 */
+	limit = get_desc_limit(desc);
+	if (desc->g)
+		limit = (limit << 12) + 0xfff;
+
+	return limit;
+}
+
+/**
  * insn_get_modrm_rm_off() - Obtain register in r/m part of the ModRM byte
  * @insn:	Instruction containing the ModRM byte
  * @regs:	Register values as seen when entering kernel mode
-- 
2.7.4