Re: [FFmpeg-devel] rebasing security

[Michael Niedermayer <michael@niedermayer.cc>]

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Hi Alexander

On Mon, Aug 04, 2025 at 10:15:53PM +0200, Alexander Strasser via ffmpeg-devel wrote:
> Hi Michael,
> hi all!
> 
> I think it's a good time to bring stuff like this up for discussion.
> 
> On 2025-08-03 21:02 +0200, Michael Niedermayer wrote:
> > 
> > On Sun, Aug 03, 2025 at 05:31:39PM +0200, Michael Niedermayer wrote:
> > [...]
> > > The solutions are obvious:
> > > 1. ignore security and supply chain attacks
> > > 2. use merges not rebases on the server
> > > 3. rebase locally, use fast forward only
> > > 4. verify on server rebases
> > 
> > Maybe not everyone understood the problem. So let me try a different
> > explanation. Without any signatures.
> > 
> > In the ML workflow: (for simplicity we assume reviewer and commiter is the same person)
> > 1. someone posts a patch
> > 2. patch is locally applied or rebased
> > 3. commit is reviewed
> > 4. commit is tested
> > 5. commit is pushed
> > 
> > Here the only way to get bad code in, is through the reviewer
> > If the reviewer doesnt miss anything and his setup is not compromised
> > then what he pushes is teh reviewed code
> > 
> > if its manipulated after its pushed git should light up like a christmess tree
> > on the next "git pull --rebase"
> > 
> > 
> > With the rebase on webapp (gitlab or forgejo) workflow
> > 1. someone posts a pull request
> > 2. pr is reviewed
> > 3. pr is approved
> > 4. pr is rebased
> > 5. pr is tested
> > 6, pr is pushed
> > 
> > now here of course the same reviewer trust or compromised scenarios exist
> > but we also have an extra one and that is the server
> > because the server strips the signatures during rebase it can modify the
> > commit. And this happens after review and because a rebase was litterally
> > requested by the reviewer its not likely to be noticed as something out of
> > place
> 
> If I understand the original point you wanted to discuss correctly,
> than this is not a question of rebase or merge but one of letting
> **commits happen on the forge**. If it happens it bears the
> possibility of modification on the server the forge is running on.

It is a question of rebase vs merge because
if the forge generates a merge A+B and lets assume it tampers with it
this is trivially detectable from nothing than just the git checkout

To detect it:
just redo every merge that is not signed or that is signed by the forgejo key
the tree after it, either matches or it was very likely tampered with

With rebases, detection is possible but more complex
First you need not just the git checkout but every single pull request
and exactly the last pushed one before the rebase and they need to have been
signed.
Then you can redo all the rebases and verify that they have not been tampered with

With the merge case the last pull requests are part of the git checkout and
signing is not critical because when something is part of a git checkout
its just hard to tamper with it, the author might notice it mismatches

thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Some Animals are More Equal Than Others. - George Orwell's book Animal Farm

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