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

From: Ricardo Neri
Date: Fri May 05 2017 - 14:17:58 EST

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

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 | 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;
return (void __user *)-1;