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Re: snapshot 05/05: ssh segmentation fault within screen
- From: Doug Henderson <djndnbvg at gmail dot com>
- To: cygwin <cygwin at cygwin dot com>
- Date: Wed, 7 May 2014 12:49:08 -0600
- Subject: Re: snapshot 05/05: ssh segmentation fault within screen
- Authentication-results: sourceware.org; auth=none
- References: <5368525F dot 2070301 at shaddybaddah dot name> <20140506163936 dot GY30918 at calimero dot vinschen dot de> <536920BB dot 3080102 at redhat dot com> <20140506184915 dot GA30918 at calimero dot vinschen dot de> <20140507122218 dot GF30918 at calimero dot vinschen dot de> <20140507145736 dot GN30918 at calimero dot vinschen dot de>
On Wed, May 7, 2014 at 8:57 AM, Corinna Vinschen wrote:
> On May 7 14:22, Corinna Vinschen wrote:
>> On May 6 20:49, Corinna Vinschen wrote:
>> > 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.
>> I created a matching patch. Please give the today's developer snapshot
>> from http://cygwin.com/snapshots/ a try.
> I just created another snapshot. The former snapshot had an off
> by one bug so the passwd buffer given to the application was one
> byte short. Please make sure to try the snapshot with timestamp
> 14:55:24 (x86) or 14:55:06 (x86_64).
It sounds like the result of using the Microsoft Account connected to
a regular account instead of using the regular account directly has
both a static and dynamic component. This suggests that the behavior
could be modeled by a regular account log in, immediately followed by
a chgrp which is dependant on the type of regular account, or some
other variation of su and chgrp actions.
Doug Henderson, Calgary, Alberta, Canada
Problem reports: http://cygwin.com/problems.html
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