Re: [Qemu-devel] [PATCH v2] linux-user: fix is_proc_myself to check the paths via realpath

[Zach Riggle]

The symlink was just an easy test case.  Doing cd proc && cat ./self/maps
will achieve the same thing.

One instance where this matters is for testing IoT device firmware
exploits, where one might do a GET request against
../../../../../../proc/self/maps in order to bypass ASLR.  Currently, this
will return the memory layout of QEMU, not the emulated memory layout of
the software under test.


*Zach Riggle*

On Fri, Oct 27, 2017 at 4:36 AM, Riku Voipio <address@hidden> wrote:

> On Thu, Oct 26, 2017 at 04:06:22PM -0500, Zach Riggle wrote:
> > Friendly ping :)
> >
> > I've updated the patch with v2 which addresses the style issue
>
> I'll have a look at it soon.
>
> >
> > *Zach Riggle*
> >
> > On Tue, Oct 24, 2017 at 10:34 PM, Zach Riggle <address@hidden>
> wrote:
> >
> > > Previously, it was possible to get a handle to the "real"
> /proc/self/mem
> > > by creating a symlink to it and opening the symlink, or opening e.g.
> > > "./mem" after chdir'ing to "/proc/self"
>
> When is this a problem? Symlinking to /proc/self seems to be a quite weird
> usecase.
>
> > >
> > >     $ ln -s /proc/self self
> > >     $ cat self/maps
> > >     60000000-602bc000 r-xp 00000000 fc:01 270375
> > >    /usr/bin/qemu-arm-static
> > >     604bc000-6050f000 rw-p 002bc000 fc:01 270375
> > >    /usr/bin/qemu-arm-static
> > >     ...
> > >
> > > Signed-off-by: Zach Riggle <address@hidden>
> > > ---
> > >  linux-user/syscall.c | 47 ++++++++++++++++++++++++++++--
> -----------------
> > >  1 file changed, 28 insertions(+), 19 deletions(-)
> > >
> > > diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> > > index 9bf901fa11..6c1f28a1f7 100644
> > > --- a/linux-user/syscall.c
> > > +++ b/linux-user/syscall.c
> > > @@ -7496,26 +7496,35 @@ static int open_self_auxv(void *cpu_env, int
> fd)
> > >
> > >  static int is_proc_myself(const char *filename, const char *entry)
> > >  {
> > > -    if (!strncmp(filename, "/proc/", strlen("/proc/"))) {
> > > -        filename += strlen("/proc/");
> > > -        if (!strncmp(filename, "self/", strlen("self/"))) {
> > > -            filename += strlen("self/");
> > > -        } else if (*filename >= '1' && *filename <= '9') {
> > > -            char myself[80];
> > > -            snprintf(myself, sizeof(myself), "%d/", getpid());
> > > -            if (!strncmp(filename, myself, strlen(myself))) {
> > > -                filename += strlen(myself);
> > > -            } else {
> > > -                return 0;
> > > -            }
> > > -        } else {
> > > -            return 0;
> > > -        }
> > > -        if (!strcmp(filename, entry)) {
> > > -            return 1;
> > > -        }
> > > +    char proc_self_entry[PATH_MAX + 1];
> > > +    char proc_self_entry_realpath[PATH_MAX + 1];
> > > +    char filename_realpath[PATH_MAX + 1];
> > > +
> > > +    if (PATH_MAX < snprintf(proc_self_entry,
> > > +                            sizeof(proc_self_entry),
> > > +                            "/proc/self/%s",
> > > +                            entry)) {
> > > +        /* Full path to "entry" is too long to fit in the buffer */
> > > +        return 0;
> > >      }
> > > -    return 0;
> > > +
> > > +    if (!realpath(filename, filename_realpath)) {
> > > +        /* File does not exist, or can't be canonicalized */
> > > +        return 0;
> > > +    }
> > > +
> > > +    if (!realpath(proc_self_entry, proc_self_entry_realpath)) {
> > > +        /* Procfs entry does not exist */
> > > +        return 0;
> > > +    }
> > > +
> > > +    if (strcmp(filename_realpath, proc_self_entry_realpath) != 0) {
> > > +        /* Paths are different */
> > > +        return 0;
> > > +    }
> > > +
> > > +    /* filename refers to /proc/self/<entry> */
> > > +    return 1;
> > >  }
> > >
> > >  #if defined(HOST_WORDS_BIGENDIAN) != defined(TARGET_WORDS_BIGENDIAN)
> > > --
> > > 2.14.3
> > >
> > >
>