From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 18 Jan 2019 22:32:49 +0900 From: Masami Hiramatsu Subject: Re: [PATCH 17/17] module: Prevent module removal racing with text_poke() Message-Id: <20190118223249.94436b58fbf5f9592d92dfca@kernel.org> In-Reply-To: <69EA2C81-826F-46BA-8D80-241C39B0B70B@gmail.com> References: <20190117003259.23141-1-rick.p.edgecombe@intel.com> <20190117003259.23141-18-rick.p.edgecombe@intel.com> <20190117165422.d33d1af83db8716e24960a3c@kernel.org> <69EA2C81-826F-46BA-8D80-241C39B0B70B@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-2022-JP Content-Transfer-Encoding: 7bit To: Nadav Amit Cc: "H. Peter Anvin" , Masami Hiramatsu , Rick Edgecombe , Andy Lutomirski , Ingo Molnar , LKML , X86 ML , Thomas Gleixner , Borislav Petkov , Dave Hansen , Peter Zijlstra , Damian Tometzki , linux-integrity , LSM List , Andrew Morton , Kernel Hardening , Linux-MM , Will Deacon , ard.biesheuvel@linaro.org, kristen@linux.intel.com, deneen.t.dock@intel.com List-ID: On Thu, 17 Jan 2019 17:15:27 -0800 Nadav Amit wrote: > > On Jan 17, 2019, at 3:58 PM, H. Peter Anvin wrote: > > > > On 1/16/19 11:54 PM, Masami Hiramatsu wrote: > >> On Wed, 16 Jan 2019 16:32:59 -0800 > >> Rick Edgecombe wrote: > >> > >>> From: Nadav Amit > >>> > >>> It seems dangerous to allow code modifications to take place > >>> concurrently with module unloading. So take the text_mutex while the > >>> memory of the module is freed. > >> > >> At that point, since the module itself is removed from module list, > >> it seems no actual harm. Or would you have any concern? > > > > The issue isn't the module list, but rather when it is safe to free the > > contents, so we don't clobber anything. We absolutely need to enforce > > that we can't text_poke() something that might have already been freed. > > > > That being said, we *also* really would prefer to enforce that we can't > > text_poke() memory that doesn't actually contain code; as far as I can > > tell we don't currently do that check. > > Yes, that what the mutex was supposed to achieve. It’s not supposed just > to check whether it is a code page, but also that it is the same code > page that you wanted to patch. > > > This, again, is a good use for a separate mm context. We can enforce > > that that context will only ever contain valid page mappings for actual > > code pages. > > This will not tell you that you have the *right* code-page. The module > notifiers help to do so, since they synchronize the text poking with > the module removal. > > > (Note: in my proposed algorithm, with a separate mm, replace INVLPG with > > switching CR3 if we have to do a rollback or roll forward in the > > breakpoint handler.) > > I really need to read your patches more carefully to see what you mean. > > Anyhow, so what do you prefer? I’m ok with either one: > 1. Keep this patch > 2. Remove this patch and change into a comment on text_poke() > 3. Just drop the patch I would prefer 2. so at least we should add a comment to text_poke(). Thank you, -- Masami Hiramatsu