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Date: Wed, 23 Sep 2015 15:11:45 +0200
From: Andreas Gruenbacher <agruenba@...hat.com>
To: "J. Bruce Fields" <bfields@...ldses.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
linux-fsdevel <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
2015-09-22 18:06 GMT+02:00 J. Bruce Fields <bfields@...ldses.org>:
> 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.
Hmm, the example can be fixed by applying more 0406 here instead of 0466.
> (But: thanks in general for the examples in these comments, they're
> extremely helpful.)
Yes, I think without them, the code cannot be reviewed properly.
> I'd find it simpler to follow without the a_entries + a_count condition,
> maybe something like this (untested):
>
> [...]
Great, let me further simplify this to:
static int
richacl_isolate_owner_class(struct richacl_alloc *alloc)
{
struct richacl *acl = alloc->acl;
unsigned int deny = richacl_max_allowed(acl) & ~acl->a_owner_mask;
if (deny) {
struct richace *ace;
/*
* Figure out if we can update an existig OWNER@ DENY entry.
*/
richacl_for_each_entry(ace, acl) {
if (richace_is_inherit_only(ace))
continue;
if (richace_is_allow(ace))
break;
if (richace_is_owner(ace)) {
return richace_change_mask(alloc, &ace,
ace->e_mask | deny);
}
}
/* Insert an owner@ deny entry at the front. */
ace = 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 = deny;
ace->e_id.special = RICHACE_OWNER_SPECIAL_ID;
}
return 0;
}
Thanks,
Andreas
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