Politique IAM et de bucket pour l'accès S3

Soutien Installation
1

J’ai remarqué que tous les tutoriels que j’ai trouvés pour l’accès S3 de Discourse accordaient à l’utilisateur une autorité absolue sur le bucket — ils permettent l’autorité ‘s3:*’.

C’est une politique extrêmement imprudente, car elle permet un contrôle bien plus important sur le bucket que ce qui est raisonnable. Si vous utilisez S3 pour le stockage des sauvegardes de Discourse, un attaquer enragé pourrait supprimer votre bucket et vos sauvegardes en partant.

Il existe deux façons de contrer cela : d’abord, une politique plus stricte…

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": [
                "s3:List*",
                "s3:Get*",
                "s3:AbortMultipartUpload",
                "s3:DeleteObject",
                "s3:PutObject"
            ],
            "Resource": [
                "arn:aws:s3:::whatever-bucket",
                "arn:aws:s3:::whatever-bucket/*"
            ]
        },
        {
            "Sid": "VisualEditor1",
            "Effect": "Allow",
            "Action": [
                "s3:ListAllMyBuckets",
                "s3:HeadBucket"
            ],
            "Resource": "*"
        }
    ]
}

… et deuxièmement, appliquer des précautions raisonnables à la politique du bucket. (Ceci va en fait bien au-delà du strict minimum nécessaire, mais j’étais pressé et n’avais pas le temps d’expérimenter, et c’est mieux que ce que j’ai trouvé.) Je recommande d’activer la gestion des versions, puis de définir une règle de cycle de vie pour les « versions précédentes » afin qu’elles soient supprimées après un délai raisonnable, comme 21 jours. La politique donnée permettrait la rotation des journaux et la suppression de fichiers, mais elle n’autoriserait pas l’utilisateur disposant des identifiants S3 à restaurer ou purger les versions précédentes. Cela signifie que, bien qu’ils puissent supprimer une sauvegarde, un attaquant enragé ne pourrait pas l’effacer de l’historique des versions avant qu’un administrateur disposant des identifiants root ne puisse la récupérer.

Merci !

2

This is good advice, certainly, but I’m unsure Discourse should really be considered responsible for giving out advice about S3 best practices?

We could put a note / reminder in the help text for the field, if it can be kept short.

3

Oh, I’m sorry – I wasn’t trying to suggest you /should/ be; certainly I don’t know where you’d put it. I just found that when I Googled “Discourse S3 IAM” all of the example policies were the same awful wide-open one, so I’m reporting what I did instead of that.

(That’s why I seperated this thread from the other.)

4

Were those examples here on meta.discourse.org? If they’re in a howto you might be able to edit them, or you could draft your own and get it moved into #howto:sysadmin.

5

My take on this is that our responsibility for S3 advice is about the same as our advice on things like TLS configs (which we do update on occasion). We should try to stay “safe by default”, because we know that just about everyone’s going to blindly use whatever we suggest, because very few people know what any of this magic actually does. Our as-close-to-official-as-we-get guide on setting up S3 does suggest using the wide-open policy, so I’ll fix that up to be more sensible.

@Asher_Densmore-Lynn: if you find any other examples of problematic IAM policies floating around anywhere we can control (here on meta, git repos under the discourse GitHub user, that sort of thing), feel free to let us (me) know (with a specific reference to what’s problematic; everyone’s Google search results are different), and I’ll get it fixed.

6

Sure, and thanks! I hope you add the part about the bucket versioning – keeping your backups safe from catastrophe is hard to do when you have to allow rotation and deletion. If you need me to go into more detail or explain it better I’ll be happy to oblige.

7

One thing to bear in mind is that the howto I linked above is about asset upload, not backups. The policy required for that is likely to be somewhat different to one required for backups. Also, that howto assumes that Discourse will be creating the bucket, and adding instructions on manually adding versioning and rotation would significantly complicate what is already a bit of a bear of a process. If the versioning/rotation settings can be set at creation time (without opening up the IAM policy to allow an attacker to remove those attributes later), then a PR to Discourse to add that ability (even by default) wouldn’t be a bad idea. Otherwise, I think it’s best if you write a separate “Discourse S3 201: Securitay!” topic, that can be linked from the main howto.

8

I looked at that setup guide – be aware that that bucket creation will fail with those permissions. You’ll want to add s3:CreateBucket if that’s something you want to keep.

9

I already added that action.