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[198.0.35.241]) by smtp.gmail.com with ESMTPSA id h10-20020a170902f54a00b0019c13c4b175sm72200plf.189.2023.03.02.11.54.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 02 Mar 2023 11:54:03 -0800 (PST) Message-ID: <6400fedb.170a0220.ece29.04b8@mx.google.com> X-Google-Original-Message-ID: <202303021147.@keescook> Date: Thu, 2 Mar 2023 11:54:03 -0800 From: Kees Cook To: Al Viro Cc: Linus Torvalds , Mateusz Guzik , Eric Biggers , Alexander Potapenko , Christian Brauner , serge@hallyn.com, paul@paul-moore.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible References: <20230125155557.37816-2-mjguzik@gmail.com> <20230302083025.khqdizrnjkzs2lt6@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 02, 2023 at 07:19:49PM +0000, Al Viro wrote: > On Thu, Mar 02, 2023 at 11:10:03AM -0800, Linus Torvalds wrote: > > On Thu, Mar 2, 2023 at 11:03 AM Linus Torvalds > > wrote: > > > > > > It might be best if we actually exposed it as a SLAB_SKIP_ZERO thing, > > > just to make it possible to say - exactly in situations like this - > > > that this particular slab cache has no advantage from pre-zeroing. > > > > Actually, maybe it's just as well to keep it per-allocation, and just > > special-case getname_flags() itself. > > > > We could replace the __getname() there with just a > > > > kmem_cache_alloc(names_cachep, GFP_KERNEL | __GFP_SKIP_ZERO); > > > > we're going to overwrite the beginning of the buffer with the path we > > copy from user space, and then we'd have to make people comfortable > > with the fact that even with zero initialization hardening on, the > > space after the filename wouldn't be initialized... > > ACK; same in getname_kernel() and sys_getcwd(), at the very least. FWIW, much earlier analysis suggested opting out these kmem caches: buffer_head names_cache mm_struct anon_vma skbuff_head_cache skbuff_fclone_cache Alexander's analysis more recently[2] of skbuff went a bit further, I think, and allowed opt-out for non-kmem cache page allocations too. -Kees [1] https://lore.kernel.org/all/20190514143537.10435-5-glider@google.com/ -- Kees Cook