From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:54369) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1T7UUF-0006s0-NN for qemu-devel@nongnu.org; Fri, 31 Aug 2012 12:54:31 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1T7UUE-0008Q2-Fj for qemu-devel@nongnu.org; Fri, 31 Aug 2012 12:54:27 -0400 Received: from hub021-nj-4.exch021.serverdata.net ([206.225.164.219]:13868) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1T7UUE-0008Pw-BN for qemu-devel@nongnu.org; Fri, 31 Aug 2012 12:54:26 -0400 Message-ID: <5040EC40.90003@CloudSwitch.Com> Date: Fri, 31 Aug 2012 12:54:24 -0400 From: Don Slutz MIME-Version: 1.0 References: <502E63A3.1040304@sysgo.com> In-Reply-To: <502E63A3.1040304@sysgo.com> Content-Type: text/plain; charset="ISO-8859-15"; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] x86: enforce DPL checking on task gate switches invoked through IDT List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Alex ZUEPKE Cc: qemu-devel@nongnu.org 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. > > 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 > 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); > I think it makes sense to move the next 2 checks into the switch (no real code flow change). 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) {