On 6/20/2025 8:32 PM, 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 the info. I have posted the revised patch with EXPORT_SYMBOL_GPL_FOR_MODULES: https://lore.kernel.org/linux-mm/20250620070328.803704-3-shivankg@xxxxxxx Please review when you have a chance. Thanks, Shivank
