[PATCH v8 20/28] x86/insn-eval: Handle 32-bit address encodings in virtual-8086 mode

[Ricardo Neri]

It is possible to utilize 32-bit address encodings in virtual-8086 mode via
an address override instruction prefix. However, the range of the
effective 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 <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 | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 6537b613d0b3..93a6d1f57c2d 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -801,12 +801,23 @@ static void __user *get_addr_ref_32(struct insn *insn, struct pt_regs *regs)
 		goto out_err;
 
 	/*
+	 * 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) && (eff_addr & ~0xffff))
+		goto out_err;
+
+	/*
 	 * Data type long could be 64 bits in size. Ensure that our 32-bit
 	 * effective address is not sign-extended when computing the linear
 	 * address.
 	 */
 	linear_addr = (unsigned long)(eff_addr & 0xffffffff) + 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 *)-1L;
-- 
2.13.0