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Date: Tue, 22 Sep 2015 12:06:37 -0400
From: bfields@...ldses.org (J. Bruce Fields)
To: Andreas Gruenbacher <agruenba@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-nfs@...r.kernel.org, linux-api@...r.kernel.org,
linux-cifs@...r.kernel.org, linux-security-module@...r.kernel.org,
Andreas Gruenbacher <agruen@...nel.org>
Subject: Re: [RFC v7 25/41] richacl: Isolate the owner and group classes
On Sat, Sep 05, 2015 at 12:27:20PM +0200, Andreas Gruenbacher wrote:
> When applying the file masks to an acl, we need to ensure that no
> process gets more permissions than allowed by its file mask.
>
> This may require inserting an owner@ deny ace to ensure this if the
> owner mask contains fewer permissions than the group or other mask. For
> example, when applying mode 0466 to the following acl:
>
> everyone@:rw::allow
>
> A deny ace needs to be inserted so that the owner won't get elevated
> write access:
>
> owner@:w::deny
> everyone@:rw::allow
>
> Likewise, we may need to insert group class deny aces if the group mask
> contains fewer permissions than the other mask. For example, when
> applying mode 0646 to the following acl:
>
> owner@:rw::allow
> everyone@:rw::allow
>
> A deny ace needs to be inserted so that the owning group won't get
> elevated write access:
>
> owner@:rw::allow
> group@:w::deny
> everyone@:rw::allow
>
> Signed-off-by: Andreas Gruenbacher <agruen@...nel.org>
> ---
> fs/richacl_compat.c | 236 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 236 insertions(+)
>
> diff --git a/fs/richacl_compat.c b/fs/richacl_compat.c
> index 30bdc95..412844c 100644
> --- a/fs/richacl_compat.c
> +++ b/fs/richacl_compat.c
> @@ -494,3 +494,239 @@ richacl_set_other_permissions(struct richacl_alloc *alloc)
> richace_change_mask(alloc, &ace, other_mask);
> return 0;
> }
> +
> +/**
> + * richacl_max_allowed - maximum permissions that anybody is allowed
> + */
> +static unsigned int
> +richacl_max_allowed(struct richacl *acl)
> +{
> + struct richace *ace;
> + unsigned int allowed = 0;
> +
> + richacl_for_each_entry_reverse(ace, acl) {
> + if (richace_is_inherit_only(ace))
> + continue;
> + if (richace_is_allow(ace))
> + allowed |= ace->e_mask;
> + else if (richace_is_deny(ace)) {
> + if (richace_is_everyone(ace))
> + allowed &= ~ace->e_mask;
> + }
> + }
> + return allowed;
> +}
> +
> +/**
> + * richacl_isolate_owner_class - limit the owner class to the owner file mask
> + * @alloc: acl and number of allocated entries
> + *
> + * POSIX requires that after a chmod, the owner class is granted no more
> + * permissions than the owner file permission bits. For richacls, this
> + * means that the owner class must not be granted any permissions that the
> + * owner mask does not include.
> + *
> + * When we apply file masks to an acl which grant more permissions to the group
> + * or other class than to the owner class, we may end up in a situation where
> + * the owner is granted additional permissions from other aces. For example,
> + * given this acl:
> + *
> + * everyone:rwx::allow
> + *
> + * when file masks corresponding to mode 0466 are applied, after
> + * richacl_propagate_everyone() and __richacl_apply_masks(), we end up with:
> + *
> + * owner@:r::allow
> + * everyone@:rw::allow
Are you sure? I didn't think richacl_apply_masks actually creates an
owner@ entry in this case. Which is OK, just delete the owner@ ace from
here and the following example and it still makes sense, I think.
(But: thanks in general for the examples in these comments, they're
extremely helpful.)
> + *
> + * This acl still grants the owner rw access through the everyone@ allow ace.
> + * To fix this, we must deny the owner w access:
> + *
> + * owner@:w::deny
> + * owner@:r::allow
> + * everyone@:rw::allow
> + */
> +static int
> +richacl_isolate_owner_class(struct richacl_alloc *alloc)
> +{
> + struct richace *ace;
> + unsigned int allowed = 0;
> +
> + allowed = richacl_max_allowed(alloc->acl);
> + if (allowed & ~alloc->acl->a_owner_mask) {
> + /*
> + * Figure out if we can update an existig OWNER@ DENY entry.
> + */
> + richacl_for_each_entry(ace, alloc->acl) {
> + if (richace_is_inherit_only(ace))
> + continue;
> + if (richace_is_deny(ace)) {
> + if (richace_is_owner(ace))
> + break;
> + } else if (richace_is_allow(ace)) {
> + ace = alloc->acl->a_entries +
> + alloc->acl->a_count;
> + break;
> + }
> + }
> + if (ace != alloc->acl->a_entries + alloc->acl->a_count) {
> + if (richace_change_mask(alloc, &ace, ace->e_mask |
> + (allowed & ~alloc->acl->a_owner_mask)))
> + return -1;
> + } else {
> + /* Insert an owner@ deny entry at the front. */
> + ace = alloc->acl->a_entries;
> + if (richacl_insert_entry(alloc, &ace))
> + return -1;
> + ace->e_type = RICHACE_ACCESS_DENIED_ACE_TYPE;
> + ace->e_flags = RICHACE_SPECIAL_WHO;
> + ace->e_mask = allowed & ~alloc->acl->a_owner_mask;
> + ace->e_id.special = RICHACE_OWNER_SPECIAL_ID;
> + }
> + }
> + return 0;
Makes sense, though personally I'd find it simpler to follow without the
a_entries + a_count condition, maybe something like this (untested):
richacl_for_each_entry(ace, alloc->acl) {
if (richace_is_inherit_only(ace))
continue;
if (richace_is_allow(ace))
break;
if (richace_is_owner(ace))
goto found;
}
/* Insert an owner@ deny entry at the front. */
ace = alloc->acl->a_entries;
if (richacl_insert_entry(alloc, &ace))
return -1;
ace->e_type = RICHACE_ACCESS_DENIED_ACE_TYPE;
ace->e_flags = RICHACE_SPECIAL_WHO;
ace->e_mask = allowed & ~alloc->acl->a_owner_mask;
ace->e_id.special = RICHACE_OWNER_SPECIAL_ID;
return 0;
found:
if (richace_change_mask(alloc, &ace, ace->e_mask |
(allowed & ~alloc->acl->a_owner_mask)))
return -1;
return 0;
--b.
> +}
> +
> +/**
> + * __richacl_isolate_who - isolate entry from everyone@ allow entry
> + * @alloc: acl and number of allocated entries
> + * @who: identifier to isolate
> + * @deny: permissions this identifier should not be allowed
> + *
> + * See richacl_isolate_group_class().
> + */
> +static int
> +__richacl_isolate_who(struct richacl_alloc *alloc, struct richace *who,
> + unsigned int deny)
> +{
> + struct richacl *acl = alloc->acl;
> + struct richace *ace;
> + int n;
> + /*
> + * Compute the permissions already denied to @who.
> + */
> + richacl_for_each_entry(ace, acl) {
> + if (richace_is_inherit_only(ace))
> + continue;
> + if (richace_is_same_identifier(ace, who) &&
> + richace_is_deny(ace))
> + deny &= ~ace->e_mask;
> + }
> + if (!deny)
> + return 0;
> +
> + /*
> + * Figure out if we can update an existig deny entry. Start from the
> + * entry before the trailing everyone@ allow entry. We will not hit
> + * everyone@ entries in the loop.
> + */
> + for (n = acl->a_count - 2; n != -1; n--) {
> + ace = acl->a_entries + n;
> + if (richace_is_inherit_only(ace))
> + continue;
> + if (richace_is_deny(ace)) {
> + if (richace_is_same_identifier(ace, who))
> + break;
> + } else if (richace_is_allow(ace) &&
> + (ace->e_mask & deny)) {
> + n = -1;
> + break;
> + }
> + }
> + if (n != -1) {
> + if (richace_change_mask(alloc, &ace, ace->e_mask | deny))
> + return -1;
> + } else {
> + /*
> + * Insert a new entry before the trailing everyone@ deny entry.
> + */
> + struct richace who_copy;
> +
> + richace_copy(&who_copy, who);
> + ace = acl->a_entries + acl->a_count - 1;
> + if (richacl_insert_entry(alloc, &ace))
> + return -1;
> + richace_copy(ace, &who_copy);
> + ace->e_type = RICHACE_ACCESS_DENIED_ACE_TYPE;
> + ace->e_flags &= ~RICHACE_INHERITANCE_FLAGS;
> + ace->e_mask = deny;
> + }
> + return 0;
> +}
> +
> +/**
> + * richacl_isolate_group_class - limit the group class to the group file mask
> + * @alloc: acl and number of allocated entries
> + *
> + * POSIX requires that after a chmod, the group class is granted no more
> + * permissions than the group file permission bits. For richacls, this
> + * means that the group class must not be granted any permissions that the
> + * group mask does not include.
> + *
> + * When we apply file masks to an acl which grant more permissions to the other
> + * class than to the group class, we may end up in a situation where processes
> + * in the group class are granted additional permission from other aces. For
> + * example, given this acl:
> + *
> + * joe:rwx::allow
> + * everyone:rwx::allow
> + *
> + * when file masks corresponding to mode 0646 are applied, after
> + * richacl_propagate_everyone() and __richacl_apply_masks(), we end up with:
> + *
> + * joe:r::allow
> + * owner@:rw::allow
> + * group@:r::allow
> + * everyone@:rw::allow
> + *
> + * This acl still grants joe and group@ rw access through the everyone@ allow
> + * ace. To fix this, we must deny w access to group class aces before the
> + * everyone@ allow ace at the end of the acl:
> + *
> + * joe:r::allow
> + * owner@:rw::allow
> + * group@:r::allow
> + * joe:w::deny
> + * group@:w::deny
> + * everyone@:rw::allow
> + */
> +static int
> +richacl_isolate_group_class(struct richacl_alloc *alloc)
> +{
> + struct richace who = {
> + .e_flags = RICHACE_SPECIAL_WHO,
> + .e_id.special = RICHACE_GROUP_SPECIAL_ID,
> + };
> + struct richace *ace;
> + unsigned int deny;
> +
> + if (!alloc->acl->a_count)
> + return 0;
> + ace = alloc->acl->a_entries + alloc->acl->a_count - 1;
> + if (richace_is_inherit_only(ace) || !richace_is_everyone(ace))
> + return 0;
> + deny = ace->e_mask & ~alloc->acl->a_group_mask;
> +
> + if (deny) {
> + unsigned int n;
> +
> + if (__richacl_isolate_who(alloc, &who, deny))
> + return -1;
> + /*
> + * Start from the entry before the trailing everyone@ allow
> + * entry. We will not hit everyone@ entries in the loop.
> + */
> + for (n = alloc->acl->a_count - 2; n != -1; n--) {
> + ace = alloc->acl->a_entries + n;
> +
> + if (richace_is_inherit_only(ace) ||
> + richace_is_owner(ace) ||
> + richace_is_group(ace) ||
> + richace_is_everyone(ace))
> + continue;
> + if (__richacl_isolate_who(alloc, ace, deny))
> + return -1;
> + }
> + }
> + return 0;
> +}
> --
> 2.4.3
>
> --
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