Re: snapshot 05/05: ssh segmentation fault within screen

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On May  6 11:49, Eric Blake wrote:
> On 05/06/2014 10:39 AM, Corinna Vinschen wrote:
> 
> > The problem, which I totally not realized since I started implementing
> > this stuff is, that by propagating this cache to child processes, said
> > child processes suffer from what the parent process does to the passwd
> > structures in the cache.
> > 
> > Screen seems to call getpwuid and then sets some of the pointers in the
> > passwd structure it got from the call to NULL, apparently for some sort
> > of security, this way overwriting the cached passwd struct for the
> 
> Bug in screen.  POSIX states:
> 
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/getpwuid.html
> 
> The application shall not modify the structure to which the return value
> points, nor any storage areas pointed to by pointers within the
> structure. The returned pointer, and pointers within the structure,
> might be invalidated or the structure or the storage areas might be
> overwritten by a subsequent call to getpwent(), getpwnam(), or getpwuid().

Oh, wow.  However, what if screen (thinks it) never calls getpwuid or
getpwnam again.  In that case it may do whatever it wants with the
pointers inside the returned passwd structure, doesn't it?  It certainly
doesn't have to expect sharing with another process.

> > current user.  Ssh on the other hand tries to copy the passwd structure,
> > but it never checks for NULL pointers because, well, the passwd
> > structure never contains NULL pointers.
> > 
> > This annihilates every advantage the cygheap caching has.
> 
> Caching still sounds correct, let's fix the bug in screen instead of
> bloating cygwin to work around it.  Or maybe find a way to cause a SEGV
> in any process that tries to write into the pointer returned by getpwuid
> and friends, to help them realize their bug, rather than the current
> state of propagating the broken memory to other processes.

Hmm, I'd have to allocate a full 4K page for this.  Also, ssh called
from screen works fine on Linux, even if the above behaviour is buggy...

> Maybe you
> just memcpy the result out of the cache into local memory, instead of
> returning a pointer into the actual cygheap cache.

Yes, that's what I was coming to realize, too.  I'm going to copy the
entire entry to local storage and return a pointer to that.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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