Message ID | 502E63A3.1040304@sysgo.com |
---|---|
State | New |
Headers | show |
Ping, no response so far ... Thanks, Alex 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) > > > 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 <azu@sysgo.com> > 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); > >
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 <azu@sysgo.com> > 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) {
On 31 August 2012 17:54, Don Slutz <Don@cloudswitch.com> wrote: > I think it makes sense to move the next 2 checks into the switch (no real > code flow change). I agree (for symmetry). If you do that then I think the combination of those two patches means that in the task gate case we do the !(e2 & DESC_P_MASK) check first and then the dpl<cpl check; whereas in the other cases we do them the other way around. Is that actually correct behaviour? If so I think it probably deserves a comment (perhaps just a clarification/expansion of the one currently in the 'case 5' code) to the effect that the error handling on the task gate case is genuinely different. -- PMM
On Fri, Aug 17, 2012 at 3:30 PM, Alex ZUEPKE <azuepke@sysgo.com> 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) > > > Best Regards, > Alex > This would become the commit message and the text below would be cut by git am. > --- > 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. "Fix by adding the privilege checks, raise GPF when fails"? > > Signed-off-by: Alex Zuepke <azu@sysgo.com> > 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) Missing braces, please read CODING_STYLE. > + 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); > >
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);
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) 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 <azu@sysgo.com>