[PATCH v7 15/26] x86/insn-eval: Incorporate segment base and limit in linear address computation

[Ricardo Neri]

insn_get_addr_ref() returns the effective address as defined by the
section 3.7.5.1 Vol 1 of the Intel 64 and IA-32 Architectures Software
Developer's Manual. In order to compute the linear address, we must add
to the effective address the segment base address as set in the segment
descriptor. Furthermore, the segment descriptor to use depends on the
register that is used as the base of the effective address. The effective
base address varies depending on whether the operand is a register or a
memory address and on whether a SiB byte is used.

In most cases, the segment base address will be 0 if the USER_DS/USER32_DS
segment is used or if segmentation is not used. However, the base address
is not necessarily zero if a user programs defines its own segments. This
is possible by using a local descriptor table.

Since the effective address is a signed quantity, the unsigned segment
base address is saved in a separate variable and added to the final
effective address.

Before returning the linear address, we check if the computed effective
address is within the segment limit. In protected mode segment limits are
not enforced. We can keep the check as get_seg_limit() return -1L in this
case.

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 | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 4f600de..1a5f5a6 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -695,7 +695,7 @@ int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs)
  */
 void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 {
-	unsigned long linear_addr;
+	unsigned long linear_addr, seg_base_addr, seg_limit;
 	long eff_addr, base, indx;
 	int addr_offset, base_offset, indx_offset;
 	insn_byte_t sib;
@@ -709,6 +709,10 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 		if (addr_offset < 0)
 			goto out_err;
 		eff_addr = regs_get_register(regs, addr_offset);
+		seg_base_addr = insn_get_seg_base(regs, insn, addr_offset);
+		if (seg_base_addr == -1L)
+			goto out_err;
+		seg_limit = get_seg_limit(regs, insn, addr_offset);
 	} else {
 		if (insn->sib.nbytes) {
 			/*
@@ -734,6 +738,11 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 				indx = regs_get_register(regs, indx_offset);
 
 			eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
+			seg_base_addr = insn_get_seg_base(regs, insn,
+							  base_offset);
+			if (seg_base_addr == -1L)
+				goto out_err;
+			seg_limit = get_seg_limit(regs, insn, base_offset);
 		} else {
 			addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
 			/*
@@ -751,10 +760,25 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 			} else {
 				eff_addr = regs_get_register(regs, addr_offset);
 			}
+			seg_base_addr = insn_get_seg_base(regs, insn,
+							  addr_offset);
+			if (seg_base_addr == -1L)
+				goto out_err;
+			seg_limit = get_seg_limit(regs, insn, addr_offset);
 		}
 		eff_addr += insn->displacement.value;
 	}
+
 	linear_addr = (unsigned long)eff_addr;
+	/*
+	 * Make sure the effective address is within the limits of the
+	 * segment. In long mode, the limit is -1L. Thus, the second part
+	 * of the check always succeeds.
+	 */
+	if (linear_addr > seg_limit)
+		goto out_err;
+
+	linear_addr += seg_base_addr;
 
 	return (void __user *)linear_addr;
 out_err:
-- 
2.9.3