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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id k14sm8919617pgn.94.2020.05.30.09.07.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 30 May 2020 09:07:17 -0700 (PDT) Date: Sat, 30 May 2020 09:07:16 -0700 From: Kees Cook To: Al Viro Cc: Sargun Dhillon , christian.brauner@ubuntu.com, containers@lists.linux-foundation.org, cyphar@cyphar.com, jannh@google.com, jeffv@google.com, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, palmer@google.com, rsesek@google.com, tycho@tycho.ws, Matt Denton Subject: Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier Message-ID: <202005300834.6419E818A7@keescook> References: <20200528110858.3265-1-sargun@sargun.me> <20200528110858.3265-3-sargun@sargun.me> <202005282345.573B917@keescook> <20200530011054.GA14852@ircssh-2.c.rugged-nimbus-611.internal> <202005291926.E9004B4@keescook> <20200530140837.GM23230@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200530140837.GM23230@ZenIV.linux.org.uk> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 30, 2020 at 03:08:37PM +0100, Al Viro wrote: > On Fri, May 29, 2020 at 07:43:10PM -0700, Kees Cook wrote: > > > Can anyone clarify the expected failure mode from SCM_RIGHTS? Can we > > move the put_user() after instead? I think cleanup would just be: > > replace_fd(fd, NULL, 0) > > Bollocks. > > Repeat after me: descriptor tables can be shared. There is no > "cleanup" after you've put something there. Right -- this is what I was trying to ask about, and why I didn't like the idea of just leaving the fd in the table on failure. But yeah, there is a race if the process is about to fork or something. So the choice here is how to handle the put_user() failure: - add the put_user() address to the new helper, as I suggest in [1]. (exactly duplicates current behavior) - just leave the fd in place (not current behavior: dumps a fd into the process without "agreed" notification). - do a double put_user (once before and once after), also in [1]. (sort of a best-effort combo of the above two. and SCM_RIGHTS is hardly fast-pth). -Kees [1] https://lore.kernel.org/linux-api/202005282345.573B917@keescook/ -- Kees Cook