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[Qemu-devel] [PATCH 1/1] x86: enforce DPL checking on task gate switches
From: |
Alex Zuepke |
Subject: |
[Qemu-devel] [PATCH 1/1] x86: enforce DPL checking on task gate switches |
Date: |
Tue, 13 May 2014 23:12:53 +0200 |
x86 software emulation (non-KVM mode) does not check privilege levels on task
gate switches.
An "int $8" in user mode panics any OS kernel by a forbidden direct call into
the double fault handler.
This testcase crashes a Linux kernel with a double fault panic:
$ cat test.c
int main(void)
{
__asm__ volatile ("int $8");
}
$ gcc test.c
$ ./a.out
panic ...
Signed-off-by: Alex Zuepke <address@hidden>
---
target-i386/seg_helper.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/target-i386/seg_helper.c b/target-i386/seg_helper.c
index 8c3f92c..d7f3d49 100644
--- a/target-i386/seg_helper.c
+++ b/target-i386/seg_helper.c
@@ -582,12 +582,18 @@ static void do_interrupt_protected(CPUX86State *env, int
intno, int is_int,
e2 = cpu_ldl_kernel(env, ptr + 4);
/* check gate type */
type = (e2 >> DESC_TYPE_SHIFT) & 0x1f;
+ dpl = (e2 >> DESC_DPL_SHIFT) & 3;
+ cpl = env->hflags & HF_CPL_MASK;
switch (type) {
case 5: /* task gate */
/* must do that check here to return the correct error code */
if (!(e2 & DESC_P_MASK)) {
raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2);
}
+ /* check privilege if software int */
+ if (is_int && (dpl < cpl)) {
+ raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
+ }
switch_tss(env, intno * 8, e1, e2, SWITCH_TSS_CALL, old_eip);
if (has_error_code) {
int type;
@@ -620,8 +626,6 @@ static void do_interrupt_protected(CPUX86State *env, int
intno, int is_int,
raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
break;
}
- dpl = (e2 >> DESC_DPL_SHIFT) & 3;
- cpl = env->hflags & HF_CPL_MASK;
/* check privilege if software int */
if (is_int && dpl < cpl) {
raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
--
1.7.9.5
- [Qemu-devel] [PATCH 1/1] x86: enforce DPL checking on task gate switches,
Alex Zuepke <=