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From: | Don Slutz |
Subject: | Re: [Qemu-devel] [PATCH] x86: enforce DPL checking on task gate switches invoked through IDT |
Date: | Fri, 31 Aug 2012 12:54:24 -0400 |
User-agent: | Mozilla/5.0 (X11; Linux i686 on x86_64; rv:15.0) Gecko/20120824 Thunderbird/15.0 |
On 08/17/12 11:30, Alex ZUEPKE wrote:
Hi, x86 software emulation (non-KVM mode) does not check privilege levels on task gate switches ... so one can invoke a kernel's double fault handler from user space -- very bad. Expected behaviour (testcase works with any linux distribution + gcc): $ cat test.c int main(void) { __asm__ volatile ("int $8"); } $ gcc test.c $ ./a.out Segmentation fault $ ... and not a kernel panic (double fault)
Some where you should say that this is a 32 bit only issue.
I think it makes sense to move the next 2 checks into the switch (no real code flow change).Best Regards, Alex --- x86 software emulation (non-KVM mode) does not check privilege levels on task gate switches ... so one can invoke a kernel's double fault handler from user space.Signed-off-by: Alex Zuepke <address@hidden>diff --git a/target-i386/seg_helper.c b/target-i386/seg_helper.c index 5fff8d5..23c5542 100644 --- a/target-i386/seg_helper.c +++ b/target-i386/seg_helper.c @@ -578,12 +578,17 @@ 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; @@ -616,8 +621,6 @@ 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);
Doing this I get:@@ -611,21 +617,19 @@ static void do_interrupt_protected(CPUX86State *env, int intno, int is_int,
case 7: /* 286 trap gate */ case 14: /* 386 interrupt gate */ case 15: /* 386 trap gate */ + /* check privilege if software int */ + if (is_int && dpl < cpl) { + raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2); + } + /* check valid bit */ + if (!(e2 & DESC_P_MASK)) { + raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2); + } break; default: 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); - } - /* check valid bit */ - if (!(e2 & DESC_P_MASK)) { - raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2); - } selector = e1 >> 16; offset = (e2 & 0xffff0000) | (e1 & 0x0000ffff); if ((selector & 0xfffc) == 0) {
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