[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PATCH v3 5/8] xen/hypfs: add support for id-based dynamic directories
On 17.12.20 13:14, Jan Beulich wrote:
On 17.12.2020 12:32, Jürgen Groß wrote:
On 17.12.20 12:28, Jan Beulich wrote:
On 09.12.2020 17:09, Juergen Gross wrote:
+static const struct hypfs_entry *hypfs_dyndir_enter(
+ const struct hypfs_entry *entry)
+{
+ const struct hypfs_dyndir_id *data;
+
+ data = hypfs_get_dyndata();
+
+ /* Use template with original enter function. */
+ return data->template->e.funcs->enter(&data->template->e);
+}
At the example of this (applies to other uses as well): I realize
hypfs_get_dyndata() asserts that the pointer is non-NULL, but
according to the bottom of ./CODING_STYLE this may not be enough
when considering the implications of a NULL deref in the context
of a PV guest. Even this living behind a sysctl doesn't really
help, both because via XSM not fully privileged domains can be
granted access, and because speculation may still occur all the
way into here. (I'll send a patch to address the latter aspect in
a few minutes.) While likely we have numerous existing examples
with similar problems, I guess in new code we'd better be as
defensive as possible.
What do you suggest? BUG_ON()?
Well, BUG_ON() would be a step in the right direction, converting
privilege escalation to DoS. The question is if we can't do better
here, gracefully failing in such a case (the usual pair of
ASSERT_UNREACHABLE() plus return/break/goto approach doesn't fit
here, at least not directly).
You are aware that this is nothing a user can influence, so it would
be a clear coding error in the hypervisor?
A user (or guest) can't arrange for there to be a NULL pointer,
but if there is one that can be run into here, this would still
require an XSA afaict.
I still don't see how this could happen without a major coding bug,
which IMO wouldn't go unnoticed during a really brief test (this is
the reason for ASSERT() in hypfs_get_dyndata() after all).
Its not as if the control flow would allow many different ways to reach
any of the hypfs_get_dyndata() calls.
I can add security checks at the appropriate places, but I think this
would be just dead code. OTOH if you are feeling strong here lets go
with it.
Juergen
Attachment:
OpenPGP_0xB0DE9DD628BF132F.asc
Description: application/pgp-keys
Attachment:
OpenPGP_signature
Description: OpenPGP digital signature
|
