On Fri, Jun 20, 2025 at 08:02:18AM -0700, Sean Christopherson wrote: > On Thu, Jun 19, 2025, Mike Rapoport wrote: > > On Thu, Jun 19, 2025 at 02:06:17PM +0200, Christian Brauner wrote: > > > On Thu, Jun 19, 2025 at 02:01:22PM +0300, Mike Rapoport wrote: > > > > On Thu, Jun 19, 2025 at 12:38:25PM +0200, Christian Brauner wrote: > > > > > On Thu, Jun 19, 2025 at 11:13:49AM +0200, Vlastimil Babka wrote: > > > > > > On 6/19/25 09:31, Shivank Garg wrote: > > > > > > > Export anon_inode_make_secure_inode() to allow KVM guest_memfd to create > > > > > > > anonymous inodes with proper security context. This replaces the current > > > > > > > pattern of calling alloc_anon_inode() followed by > > > > > > > inode_init_security_anon() for creating security context manually. > > > > > > > > > > > > > > This change also fixes a security regression in secretmem where the > > > > > > > S_PRIVATE flag was not cleared after alloc_anon_inode(), causing > > > > > > > LSM/SELinux checks to be bypassed for secretmem file descriptors. > > > > > > > > > > > > > > As guest_memfd currently resides in the KVM module, we need to export this > > > > > > > > > > > > Could we use the new EXPORT_SYMBOL_GPL_FOR_MODULES() thingy to make this > > > > > > explicit for KVM? > > > > > > > > > > Oh? Enlighten me about that, if you have a second, please. > > > > > > > > From Documentation/core-api/symbol-namespaces.rst: > > > > > > > > The macro takes a comma separated list of module names, allowing only those > > > > modules to access this symbol. Simple tail-globs are supported. > > > > > > > > For example:: > > > > > > > > EXPORT_SYMBOL_GPL_FOR_MODULES(preempt_notifier_inc, "kvm,kvm-*") > > > > > > > > will limit usage of this symbol to modules whoes name matches the given > > > > patterns. > > > > > > Is that still mostly advisory and can still be easily circumenvented? > > Yes and no. For out-of-tree modules, it's mostly advisory. Though I can imagine > if someone tries to report a bug because their module is masquerading as e.g. kvm, > then they will be told to go away (in far less polite words :-D). > > For in-tree modules, the restriction is much more enforceable. Renaming a module > to circumvent a restricted export will raise major red flags, and getting "proper" > access to a symbol would require an ack from the relevant maintainers. E.g. for > many KVM-induced exports, it's not that other module writers are trying to misbehave, > there simply aren't any guardrails to deter them from using a "dangerous" export. > > The other big benefit I see is documentation, e.g. both for readers/developers to > understand the intent, and for auditing purposes (I would be shocked if there > aren't exports that were KVM-induced, but that are no longer necessary). > > And we can utilize the framework to do additional hardening. E.g. for exports > that exist solely for KVM, I plan on adding wrappers so that the symbols are > exproted if and only if KVM is enabled in the kernel .config[*]. Again, that's > far from perfect, e.g. AFAIK every distro enables KVM, but it should help keep > everyone honest. > > [*] https://lore.kernel.org/all/ZzJOoFFPjrzYzKir@xxxxxxxxxx > > > The commit message says > > > > will limit the use of said function to kvm.ko, any other module trying > > to use this symbol will refure to load (and get modpost build > > failures). > > To Christian's point, the restrictions are trivial to circumvent by out-of-tree > modules. E.g. to get access to the above, simply name your module kvm-lol.ko or > whatever. Thanks for all the details! I'm more than happy to switch a bunch of our exports so that we only allow them for specific modules. But for that we also need EXPOR_SYMBOL_FOR_MODULES() so we can switch our non-gpl versions.
