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Wong" , John Groves , Joanne Koong , Bernd Schubert , John Groves , Dan Williams , Bernd Schubert , Alison Schofield , John Groves , Jonathan Corbet , Shuah Khan , Vishal Verma , Dave Jiang , Jan Kara , Alexander Viro , Christian Brauner , Randy Dunlap , Jeff Layton , Amir Goldstein , Jonathan Cameron , Stefan Hajnoczi , Josef Bacik , Bagas Sanjaya , Chen Linxuan , James Morse , Fuad Tabba , Sean Christopherson , Shivank Garg , Ackerley Tng , Aravind Ramesh , Ajay Joshi , "venkataravis@micron.com" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "nvdimm@lists.linux.dev" , "linux-cxl@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , djbw@kernel.org Subject: Re: [PATCH V10 00/10] famfs: port into fuse Message-ID: References: <20260414185740.GA604658@frogsfrogsfrogs> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Apr 15, 2026 at 04:10:00PM +0100, Matthew Wilcox wrote: > On Wed, Apr 15, 2026 at 04:04:50PM +0200, Miklos Szeredi wrote: > > On Wed, 15 Apr 2026 at 15:35, Gregory Price wrote: > > > > > This was my first reaction when I realized the BPF program would be > > > controlling iomap return value in the fault path. Big ol' (!) popped > > > up over my head. > > > > I'm wondering which part of this triggers the big (!). > > > > BPF program being run in the fault path? > > > > Or that the return value from the BPF function is used as iomap? > > > > Or something else? > > If a BPF program controls what memory address a fault now allows access > to, who validates that this is a memory address within the purview of > the BPF program, and not, say, the address of the kernel page tables? > > (I have done no looking to determine if this is already considered) >From an initial look at the existing bpf ops structures, I do not see any other struct with a similar (obvious) pattern - so it's not clear to me such a concern has been exposed elsewhere or directly addressed. There is a verifier step for the BPF program that in theory would validate the range matches the DAX ranges, but i think that only validates the types are right and only on load - I think the BPF program itself would be the address validater, which is a strong no. BPF folks please correct me if i'm off base here. My initial take is that it's a real concern a "bug" in a BPF program could let userland map arbitrary memory into userland page tables, and such an extension would not be a quick fix to the FAMFS problem. ~Gregory