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Shutemov" Subject: Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel Message-ID: <20180628104610.czsnq4w3lfhxrn53@ltop.local> References: <20180626172900.ufclp2pfrhwkxjco@armageddon.cambridge.arm.com> <0cef1643-a523-98e7-95e2-9ec595137642@arm.com> <20180627171757.amucnh5znld45cpc@armageddon.cambridge.arm.com> <20180628061758.j6bytsaj5jk4aocg@ltop.local> <20180628102741.vk6vphfinlj3lvhv@armageddon.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180628102741.vk6vphfinlj3lvhv@armageddon.cambridge.arm.com> User-Agent: NeoMutt/20180622 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 28, 2018 at 11:27:42AM +0100, Catalin Marinas wrote: > On Thu, Jun 28, 2018 at 08:17:59AM +0200, Luc Van Oostenryck wrote: > > On Wed, Jun 27, 2018 at 06:17:58PM +0100, Catalin Marinas wrote: > > > sparse is indeed an option. The current implementation doesn't warn on > > > an explicit cast from (void __user *) to (unsigned long) since that's a > > > valid thing in the kernel. I couldn't figure out if there's any other > > > __attribute__ that could be used to warn of such conversion. > > > > sparse doesn't have such attribute but would an new option that would warn > > on such cast be a solution for your case? > > I can't tell for sure whether such sparse option would be the full > solution but detecting explicit __user pointer casts to long is a good > starting point. So far this patchset pretty much relies on detecting > a syscall failure and trying to figure out why, patching the kernel. It > doesn't really scale. OK, I'll add such an option this evening. > As a side note, we have cases in the user-kernel ABI where the user > address type is "unsigned long": mmap() and friends. My feedback on an > early version of this patchset was to always require untagged pointers > coming from user space on such syscalls, so no need for explicit > untagging. Mmmm yes. I tend to favor a sort of opposite approach. When we have an address that must not be dereferenced as-such (and sometimes when the address can be from both __user & __kernel space) I prefer to use a ulong which will force the use of the required operation before being able to do any sort of dereferencing and this won't need horrible casts with __force (it, of course, all depends on the full context). -- Luc