Re: Fetch/push lets a malicious server steal the targets of "have" lines

[Matt McCutchen <matt@mattmccutchen.net>]

On Fri, 2016-10-28 at 15:00 -0700, Junio C Hamano wrote:
> Let me see if I understood your scenario correctly.
> 
> Suppose we start from this history where 'O' are common, your victim
> has a 'Y' branch with two commits that are private to it, as well as
> a 'X' branch on which it has X1 that it previously obtained from the
> server.  On the other hand, the server does not know about Y1 or Y2,
> and it added one commit X2 to the branch 'x' the victim is
> following:
> 
>            victim                server
> 
>              Y1---Y2               
>             /                      
>     ---O---O---X1           ---O---O---X1---X2
> 
> Then when victim wants to fetch 'x' from the server, it would say
> 
>     have X1, have Y2, have Y1, have O
> 
> and gets told to shut up by the server who heard enough.  The
> histories on these two parties will then become like this:
> 
> 
>            victim                server
> 
>              Y1---Y2               
>             /                      
>     ---O---O---X1---X2      ---O---O---X1---X2

Then the server generates a commit X3 that lists Y2 as a parent, even
though it doesn't have Y2, and advances 'x' to X3.  The victim fetches
'x':

           victim                  server

             Y1---Y2----                      (Y2)
            /           \                         \ 
    ---O---O---X1---X2---X3   ---O---O---X1---X2---X3

Then the server rolls back 'x' to X2:

           victim                  server

             Y1---Y2----
            /           \
    ---O---O---X1---X2---X3   ---O---O---X1---X2

And the victim pushes:

           victim                  server

             Y1---Y2----               Y1---Y2----
            /           \             /           \
    ---O---O---X1---X2---X3   ---O---O---X1---X2---X3

Now the server has the content of Y2.

If the victim is fetching and pulling a whole "directory" of refs, e.g:

fetch: refs/heads/*:refs/remotes/server1/*
push: refs/heads/for-server1/*:refs/heads/*

then instead of generating a merge commit, the server can just generate
another ref 'xx' pointing to Y2, assuming it can entice the victim to
set up a corresponding local branch refs/heads/for-server1/xx and push
it back.  Or if the victim is for some reason just mirroring back and
forth:

fetch: refs/heads/*:refs/heads/for-server1/*
push: refs/heads/for-
server1/*:refs/heads/*

then it doesn't have to set up a local branch as separate step.

Matt