On 25/09/2022 20:09, Günther Noack wrote:
On Fri, Sep 23, 2022 at 10:53:23PM +0200, Mickaël Salaün wrote:
On 23/09/2022 13:21, Günther Noack wrote:
On Mon, Sep 12, 2022 at 09:41:32PM +0200, Mickaël Salaün wrote:
On 08/09/2022 21:58, Günther Noack wrote:
Introduce the LANDLOCK_ACCESS_FS_TRUNCATE flag for file truncation.
[...]
+/**
+ * get_path_access_rights - Returns the subset of rights in access_request
+ * which are permitted for the given path.
+ *
+ * @domain: The domain that defines the current restrictions.
+ * @path: The path to get access rights for.
+ * @access_request: The rights we are interested in.
+ *
+ * Returns: The access mask of the rights that are permitted on the given path,
+ * which are also a subset of access_request (to save some calculation time).
+ */
+static inline access_mask_t
+get_path_access_rights(const struct landlock_ruleset *const domain,
+ const struct path *const path,
+ access_mask_t access_request)
+{
+ layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
+ unsigned long access_bit;
+ unsigned long access_req;
+
+ init_layer_masks(domain, access_request, &layer_masks);
+ if (!check_access_path_dual(domain, path, access_request, &layer_masks,
+ NULL, 0, NULL, NULL)) {
+ /*
+ * Return immediately for successful accesses and for cases
+ * where everything is permitted because the path belongs to an
+ * internal filesystem.
+ */
+ return access_request;
+ }
+
+ access_req = access_request;
+ for_each_set_bit(access_bit, &access_req, ARRAY_SIZE(layer_masks)) {
+ if (layer_masks[access_bit]) {
+ /* If any layer vetoed the access right, remove it. */
+ access_request &= ~BIT_ULL(access_bit);
+ }
+ }
This seems to be redundant with the value returned by init_layer_masks(),
which should be passed to check_access_path_dual() to avoid useless path
walk.
True, I'll use the result of init_layer_masks() to feed it back to
check_access_path_dual() to avoid a bit of computation.
Like this:
effective_access_request =
init_layer_masks(domain, access_request, &layer_masks);
if (!check_access_path_dual(domain, path, effective_access_request,
&layer_masks, NULL, 0, NULL, NULL)) {
// ...
}
correct
Overall, the approach here is:
* Initialize the layer_masks, so that it has a bit set for every
access right in access_request and layer where that access right is
handled.
* check_access_path_dual() with only the first few parameters -- this
will clear all the bits in layer masks which are actually permitted
according to the individual rules.
As a special case, this *may* return 0 immediately, in which case we
can (a) save a bit of calculation in the loop below and (b) we might
be in the case where access is permitted because it's a file from a
special file system (even though not all bits are cleared). If
check_access_path_dual() returns 0, we return the full requested
access_request that we received as input. >
* In the loop below, if there are any bits left in layer_masks, those
are rights which are not permitted for the given path. We remove
these from access_request and return the modified access_request.
This function is pretty similar to check_access_path(). Can't you change it
to use an access_mask_t pointer and get almost the same thing?
I'm shying away from this approach. Many of the existing different use
cases are already realized by "doing if checks deep down". I think it
would make the code more understandable if we managed to model these
differences between use cases already at the layer of function calls.
(This is particularly true for check_access_path_dual(), where in
order to find out how the "single" case works, you need to disentangle
to a large extent how the much more complicated dual case works.)
I agree that check_access_path_dual() is complex, but I couldn't find a
better way.
It seems out of the scope of this patch set, but I sometimes find it
OK to just duplicate the code and have a set of tests to demonstrate
that the two variants do the same thing.
check_access_path_dual() is mostly complex because of performance
reasons, as far as I can tell, and it might be possible to check its
results against a parallel implementation of it which runs slower,
uses more memory, but is more obviously correct. (I have used one
myself to check against when developing the truncate patch set.)
If you want to unify these two functions, what do you think of the
approach of just using get_path_access_rights() instead of
check_access_path()?
Basically, it would turn
return check_access_path(dom, path, access_request);
into
if (get_path_access_rights(dom, path, access_request) == access_request)
return 0;
return -EACCES;
This is slightly more verbose in the places where it's called, but it
would be more orthogonal, and it would also clarify that -EACCES is
the only possible error in the "single" path walk case.
Let me know what you think.
What about adding an additional argument `access_mask_t *const
access_allowed` to check_access_path_dual() which returns the set of
accesses (i.e. access_masked_parent1 & access_masked_parent2) that could
then be stored to landlock_file(file)->allowed_access? If this argument is
NULL it should just be ignored. What is left from get_path_access_rights()
could then be merged into hook_file_open().
IMHO, check_access_path_dual() does not seem like the right place to
add this. This functionality is not needed in any of the "dual path"
cases so far, and I'm not sure what it would mean. The necessary
information can also be easily derived from the resulting layer_masks,
which is already exposed in the check_access_path_dual() interface,
and I also believe that this approach is at least equally fast as
updating it on the fly when changing the layer_masks.
I could be convinced to add a `access_mask_t *const access_allowed`
argument to check_access_path() if you prefer that, but then again, in
that case the returned boolean can be reconstructed from the new
access_allowed variable, and we could as well make check_access_path()
return the access_allowed result instead of the boolean and let
callers check equality with what they expected...? (I admittedly don't
have a good setup to test the performance right now, but it looks like
a negligible difference to me?)
Good idea, let's try to make check_access_path_dual() returns the
allowed accesses (according to the request) and rename it to
get_access_path_dual(). unmask_layers() could be changed to return the
still-denied accesses instead of a boolean, and we could use this values
(for potential both parents) to return allowed_parent1 & allowed_parent2
(with access_mask_t types). This would also simplify is_eaccess() and
its calls could be moved to current_check_refer_path(). This would merge
get_path_access_rights() into check_access_path_dual() and make the
errno codes more explicit per hook or defined in check_access_path().
Here are the options we have discussed, in the order that I would
prefer them:
* to keep it as a separate function as it already is,
slightly duplicating check_access_path(). (I think it's cleaner,
because the code path for the rest of the hooks other than
security_file_open() stays simpler.)
* to make check_access_path() return the access_allowed access mask
and make callers check that it covers the access_request that they
asked for (see example from my previous mail on this thread). (This
is equivalent to discarding the existing check_access_path() and
using the get_path_access() function instead.)
* to add a `access_mask_t *const access_allowed` argument to
check_access_path(), which is calculated if it's non-NULL based on
the layer_masks result. It would be used from the security_file_open
hook.
* to add a `access_mask_t *const access_allowed` argument to
check_access_path_dual(). This doesn't make much sense, IMHO,
because an on-the-fly calculation of this result does not look like
a performance benefit to me, and calculating it based on the two
resulting layer_masks is already possible now. It's also not clear
to me what it would mean to calculate an access_allowed on two paths
at once, and what that would be used for.
Let me know which option you prefer. In the end, I don't feel that
strongly about it and I'm happy to do this either way.
