From 74e870b7bd1585b4b509da47e0e75db66336e576 Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Wed, 23 Feb 2022 12:35:04 -0800 Subject: Strictly validate metadata file ownership by default The metadata validation checks introduced by the previous commits are good, but to reduce the attack surface it would be much better to avoid reading and parsing files owned by other users in the first place. There are some possible use cases for users sharing fscrypt metadata files, but I think that for the vast majority of users it is unneeded and just opens up attack surface. Thus, make fscrypt (and pam_fscrypt) not process policies or protectors owned by other users by default. Specifically, * If fscrypt or pam_fscrypt is running as a non-root user, only policies and protectors owned by the user or by root can be used. * If fscrypt is running as root, any policy or protector can be used. (This is to match user expectations -- starting a sudo session should gain rights, not remove rights.) * If pam_fscrypt is running as root, only policies and protectors owned by root can be used. Note that this only applies when the root user themselves has an fscrypt login protector, which is rare. Add an option 'allow_cross_user_metadata' to /etc/fscrypt.conf which allows restoring the old behavior for anyone who really needs it. --- pam_fscrypt/run_fscrypt.go | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) (limited to 'pam_fscrypt/run_fscrypt.go') diff --git a/pam_fscrypt/run_fscrypt.go b/pam_fscrypt/run_fscrypt.go index ef7ff92..8c640ce 100644 --- a/pam_fscrypt/run_fscrypt.go +++ b/pam_fscrypt/run_fscrypt.go @@ -137,6 +137,13 @@ func loginProtector(handle *pam.Handle) (*actions.Protector, error) { if err != nil { return nil, err } + // Ensure that pam_fscrypt only processes metadata files owned by the + // user or root, even if the user is root themselves. (Normally, when + // fscrypt is run as root it is allowed to process all metadata files. + // This implements stricter behavior for pam_fscrypt.) + if !ctx.Config.GetAllowCrossUserMetadata() { + ctx.TrustedUser = handle.PamUser + } // Find the user's PAM protector. options, err := ctx.ProtectorOptions() @@ -164,10 +171,11 @@ func policiesUsingProtector(protector *actions.Protector) []*actions.Policy { var policies []*actions.Policy for _, mount := range mounts { // Skip mountpoints that do not use the protector. - if _, _, err := mount.GetProtector(protector.Descriptor()); err != nil { + if _, _, err := mount.GetProtector(protector.Descriptor(), + protector.Context.TrustedUser); err != nil { continue } - policyDescriptors, err := mount.ListPolicies() + policyDescriptors, err := mount.ListPolicies(protector.Context.TrustedUser) if err != nil { log.Printf("listing policies: %s", err) continue -- cgit v1.2.3