Re: [Qemu-devel] bug report : kqemu and self-writing code

[G Portokalidis]

I had a similar problem, but only when not using kqemu.

When using a stack overflow exploit, the shellcode provided only
executes when using kqemu. I can attribute this to either the
shellcode being in a different location (maybe someone can clarify
this, is qemu using a different memory layout e.g. stack is located in
a different virtual address), or qemu does not translate the shellcode
located in the stack and instead causes a memory fault (again i have
no idea why this should be the case).

When using kqemu the shellcode executes normally.
I did not have any time to investigate the reasons, but i have a hunch
it is the probably the translation.
If anyone knows what the problem is, i would be glad to write a patch.


On 02/05/06, Kevin F. Quinn <address@hidden> wrote:
> Looks like SELinux to me.  Even - you should raise it with whoever
> writes your policy.
>
> On Mon, 01 May 2006 23:29:54 +0200
> Fabrice Bellard <address@hidden> wrote:
>
> > Are you sure that the bug is really in kqemu ? It is possible that
> > your guest kernel implements a security system which prevents self
> > modifying code using segment limits which QEMU does not check (but
> > kqemu checks them !).
> >
> > Regards,
> >
> > Fabrice.
> >
> > Even Rouault wrote:
> > > Guest OS : Linux 2.6.15-1.2054_FC5 i686 (Fedora Core 5 i386)
> > > Host OS: Linux 2.6.12-10-amd64-k8 #1 x86_64 (Ubuntu 5.10 amd64)
> > > QEMU Version : today CVS compiled with kqemu support
> > > KQEMU : 1.3.0pre6
> > > Binary used : qemu-system-x86-64 (so kqemu user-mode is used)
> > >
> > > I'm running the simple C code attached. With kqemu user-mode, this
> > > fails (sigsegv) with the following warning in dmesg :
> > >
> > > audit(1146505373.813:12): avc:  denied { execheap } for pid=1860
> > > comm="selfmodifying scontext=user_u:system_r:unconfined_t:s0
> > > tcontext=user_u:system_r:unconfined_t:s0 tclass=process
> > > Erreur de segmentation
> > >
> > > Without kqemu enabled, it runs fine.
> > >
> > >
> > >
> > > ------------------------------------------------------------------------
> > >
> > > #define _XOPEN_SOURCE 600
> > > #include <sys/mman.h>
> > > #include <unistd.h>
> > > #include <stdlib.h>
> > > #include <stdio.h>
> > >
> > > int main(int argc, char** argv)
> > > {
> > >   int pagesize = getpagesize();
> > >   unsigned char* addr = NULL;
> > >   posix_memalign((void**)&addr, pagesize, pagesize);
> > >   mprotect(addr, pagesize, PROT_WRITE | PROT_READ | PROT_EXEC);
> > >   addr[0] = 0x8b; addr[1] = 0x44; addr[2] = 0x24; addr[3] =
> > > 0x04; /* mov    0x4(%esp),%eax */ addr[4] = 0x83; addr[5] = 0xc0;
> > > addr[6] = 0x01; /* add    $0x1,%eax */ addr[7] = 0xc3; /* ret */
> > >
> > >   printf("10+1=%d\n", ((int (*)(int))addr)(10));
> > >   free(addr);
> > >   return 0;
> > > }
> > >
> > >
> > > ------------------------------------------------------------------------
> > >
> > > _______________________________________________
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> > > http://lists.nongnu.org/mailman/listinfo/qemu-devel
> >
> >
> >
> > _______________________________________________
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>
>
> --
> Kevin F. Quinn
>
>
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