target/i386: fix interrupt CPL error when using ist in x86-64

In do_interrupt64(), when interrupt stack table(ist) is enabled
and the the target code segment is conforming(e2 & DESC_C_MASK), the
old implementation always set new CPL to 0, and SS.RPL to 0.

This is incorrect for when CPL3 code access a CPL0 conforming code
segment, the CPL should remain unchanged. Otherwise higher privileged
code can be compromised.

The patch fix this for always set dpl = cpl when the target code segment
is conforming, and modify the last parameter `flags`, which contains
correct new CPL, in cpu_x86_load_seg_cache().

Signed-off-by: Wu Xiang <willx8@gmail.com>
Message-Id: <20170621142152.GA18094@wxdeubuntu.ipads-lab.se.sjtu.edu.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/target/i386/seg_helper.c b/target/i386/seg_helper.c
index 0374031ea2..9af69c23e0 100644
--- a/target/i386/seg_helper.c
+++ b/target/i386/seg_helper.c
@@ -931,12 +931,14 @@ static void do_interrupt64(CPUX86State *env, int intno, int is_int,
         }
         new_stack = 0;
         esp = env->regs[R_ESP];
-        dpl = cpl;
     } else {
         raise_exception_err(env, EXCP0D_GPF, selector & 0xfffc);
         new_stack = 0; /* avoid warning */
         esp = 0; /* avoid warning */
     }
+    if (e2 & DESC_C_MASK) {
+        dpl = cpl;
+    }
     esp &= ~0xfLL; /* align stack */
 
     PUSHQ(esp, env->segs[R_SS].selector);
@@ -956,7 +958,7 @@ static void do_interrupt64(CPUX86State *env, int intno, int is_int,
 
     if (new_stack) {
         ss = 0 | dpl;
-        cpu_x86_load_seg_cache(env, R_SS, ss, 0, 0, 0);
+        cpu_x86_load_seg_cache(env, R_SS, ss, 0, 0, dpl << DESC_DPL_SHIFT);
     }
     env->regs[R_ESP] = esp;