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. --- actions/context.go | 20 ++++++++++++++++++-- 1 file changed, 18 insertions(+), 2 deletions(-) (limited to 'actions/context.go') diff --git a/actions/context.go b/actions/context.go index 26295ec..1ee0d60 100644 --- a/actions/context.go +++ b/actions/context.go @@ -58,6 +58,12 @@ type Context struct { // the user for whom the keys are claimed in the filesystem keyring when // v2 policies are provisioned. TargetUser *user.User + // TrustedUser is the user for whom policies and protectors are allowed + // to be read. Specifically, if TrustedUser is set, then only + // policies and protectors owned by TrustedUser or by root will be + // allowed to be read. If it's nil, then all policies and protectors + // the process has filesystem-level read access to will be allowed. + TrustedUser *user.User } // NewContextFromPath makes a context for the filesystem containing the @@ -112,6 +118,16 @@ func newContextFromUser(targetUser *user.User) (*Context, error) { return nil, err } + // By default, when running as a non-root user we only read policies and + // protectors owned by the user or root. When running as root, we allow + // reading all policies and protectors. + if !ctx.Config.GetAllowCrossUserMetadata() && !util.IsUserRoot() { + ctx.TrustedUser, err = util.EffectiveUser() + if err != nil { + return nil, err + } + } + log.Printf("creating context for user %q", targetUser.Username) return ctx, nil } @@ -136,7 +152,7 @@ func (ctx *Context) getKeyringOptions() *keyring.Options { // getProtectorOption returns the ProtectorOption for the protector on the // context's mountpoint with the specified descriptor. func (ctx *Context) getProtectorOption(protectorDescriptor string) *ProtectorOption { - mnt, data, err := ctx.Mount.GetProtector(protectorDescriptor) + mnt, data, err := ctx.Mount.GetProtector(protectorDescriptor, ctx.TrustedUser) if err != nil { return &ProtectorOption{ProtectorInfo{}, nil, err} } @@ -155,7 +171,7 @@ func (ctx *Context) ProtectorOptions() ([]*ProtectorOption, error) { if err := ctx.checkContext(); err != nil { return nil, err } - descriptors, err := ctx.Mount.ListProtectors() + descriptors, err := ctx.Mount.ListProtectors(ctx.TrustedUser) if err != nil { return nil, err } -- cgit v1.2.3 From 85a747493ff368a72f511619ecd391016ecb933c Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Wed, 23 Feb 2022 12:35:04 -0800 Subject: Extend ownership validation to entire directory structure A previous commit extended file ownership validation to policy and protector files (by default -- there's an opt-out in /etc/fscrypt.conf). However, that didn't apply to the parent directories: MOUNTPOINT MOUNTPOINT/.fscrypt MOUNTPOINT/.fscrypt/policies MOUNTPOINT/.fscrypt/protectors The problem is that if the parent directories aren't trusted (owned by another non-root user), then untrusted changes to their contents can be made at any time, including the introduction of symlinks and so on. While it's debatable how much of a problem this really is, given the other validations that are done, it seems to be appropriate to validate the parent directories too. Therefore, this commit applies the same ownership validations to the above four directories as are done on the metadata files themselves. In addition, it is validated that none of these directories are symlinks except for ".fscrypt" where this is explicitly supported. --- actions/context.go | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'actions/context.go') diff --git a/actions/context.go b/actions/context.go index 1ee0d60..ac3f6d3 100644 --- a/actions/context.go +++ b/actions/context.go @@ -138,7 +138,7 @@ func (ctx *Context) checkContext() error { if err := ctx.Config.CheckValidity(); err != nil { return &ErrBadConfig{ctx.Config, err} } - return ctx.Mount.CheckSetup() + return ctx.Mount.CheckSetup(ctx.TrustedUser) } func (ctx *Context) getKeyringOptions() *keyring.Options { -- cgit v1.2.3