[Qemu-devel] [PATCHv3 0/2] Preparing safe sigprocmask wrapper on qemu-user

[Alex Barcelo <abarcelo@ac.upc.edu>]

qemu-user needs SIGSEGV (at least) for some internal use. If the guest
application masks it and does unsafe sigprocmask, then the application
crashes. Problems happen in applications with self-modifying code (who
also change the signal mask). Other guest applications may have related
problems if they use the SIGSEGV.

A way to be more safe is adding a wrapper for all sigprocmask calls from
the guest. The wrapper proposed here is quite simple, but the code can
be improved, here I try to ensure that the wrapper is set up properly.

Changes in v3:
 - Wrapping also sigreturn's sigprocmask calls (on signal.c)

Here, a test case where qemu-user goes wrong:

////////////
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <malloc.h>
#include <signal.h>

unsigned char *testfun;

int main ( void )
{
    unsigned int ra;
    testfun=memalign(getpagesize(),1024);
    // We block the SIGSEGV signal, used by qemu-user
    sigset_t set;
    sigemptyset(&set);
    sigaddset(&set, 11);
    sigprocmask(SIG_BLOCK, &set, NULL);
    mprotect(testfun, 1024, PROT_READ|PROT_EXEC|PROT_WRITE);

    //400687: b8 0d 00 00 00          mov    $0xd,%eax
    //40068d: c3                      retq
    testfun[ 0]=0xb8;
    testfun[ 1]=0x0d;
    testfun[ 2]=0x00;
    testfun[ 3]=0x00;
    testfun[ 4]=0x00;
    testfun[ 5]=0xc3;
    printf ( "0x%02X\n",
             ((unsigned int (*)())testfun)() );

    //400687: b8 20 00 00 00          mov    $0x20,%eax
    //40068d: c3                      retq
    // This self-modifying code will break because of the sigsegv signal block
    testfun[ 1]=0x20;
    printf ( "0x%02X\n",
             ((unsigned int (*)())testfun)() );
}
////////////

On an i386 native host:
0x0D
0x20

On a non-patched qemu-i386:
0x0D
Segmentation fault

Alex Barcelo (2):
  signal: added a wrapper for sigprocmask function
  signal: sigsegv protection on do_sigprocmask

 linux-user/qemu.h    |    1 +
 linux-user/signal.c  |   27 +++++++++++++++++++++++++++
 linux-user/syscall.c |   14 +++++++-------
 3 files changed, 35 insertions(+), 7 deletions(-)

-- 
1.7.5.4