[PATCH v7 17/26] x86/insn-eval: Handle 32-bit address encodings in virtual-8086 mode

It is possible to utilize 32-bit address encodings in virtual-8086 mode via
an address override instruction prefix. However, the range of address is
still limited to [0x-0xffff]. In such a case, return error.

Also, linear addresses in virtual-8086 mode are limited to 20 bits. Enforce
such limit by truncating the most significant bytes of the computed linear
address.

Cc: Dave Hansen <[email protected]>
Cc: Adam Buchbinder <[email protected]>
Cc: Colin Ian King <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Qiaowei Ren <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Thomas Garnier <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Ravi V. Shankar <[email protected]>
Cc: [email protected]
Signed-off-by: Ricardo Neri <[email protected]>
---
 arch/x86/lib/insn-eval.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index c7c1239..9822061 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -848,6 +848,12 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 		linear_addr &= 0xffffffff;
 
 	/*
+	 * Even though 32-bit address encodings are allowed in virtual-8086
+	 * mode, the address range is still limited to [0x-0xffff].
+	 */
+	if (v8086_mode(regs) && (linear_addr & ~0xffff))
+		goto out_err;
+	/*
 	 * 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.
@@ -857,6 +863,10 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 
 	linear_addr += seg_base_addr;
 
+	/* Limit linear address to 20 bits */
+	if (v8086_mode(regs))
+		linear_addr &= 0xfffff;
+
 	return (void __user *)linear_addr;
 out_err:
 	return (void __user *)-1;
-- 
2.9.3