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h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=STAvvTVzBYeeYtMAR4rcP0Cgi9iM9Rx+w3MaVMYJjlI1iQG2xRjQWByb5+m1YRGu9 W7DZ+g/eSDTYcq4y3JfQ60Ej5nCw+KN20a5eILdppvOw/UNJaiqXvbnIT2j/YqK8Ah y7X1KdxHtJniVxNxiff7jTO0AI57+Bw1xgJo0oAA= Date: Thu, 23 May 2024 12:45:21 -0700 From: Andrew Morton To: Jeff Xu Cc: =?UTF-8?Q?Barnab=C3=A1s_P=C5=91cze?= , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, dmitry.torokhov@gmail.com, dverkamp@chromium.org, hughd@google.com, jorgelo@chromium.org, skhan@linuxfoundation.org, keescook@chromium.org Subject: Re: [PATCH v1] memfd: `MFD_NOEXEC_SEAL` should not imply `MFD_ALLOW_SEALING` Message-Id: <20240523124521.99a798d645b0939d331d70c1@linux-foundation.org> In-Reply-To: References: <20240513191544.94754-1-pobrn@protonmail.com> <20240522162324.0aeba086228eddd8aff4f628@linux-foundation.org> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 56E15140020 X-Stat-Signature: 9ks4w6r9d8p6ym31btueytwijny8hnrh X-HE-Tag: 1716493523-854206 X-HE-Meta: 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 7H4t18mX K4CX6NiLWSJ2gJG6GMeZa812RO/NHurVayqDlYumYcrgf4yEqEO5Oasb5l9UzQuaoXnJsbARXy/hwFR0a2Kc05Ro8w0K19eqsIRhuZHBHL1PeXPTGuzPPUU82T8RmjWZPnN8nnrIUbU/xJw8LmRjuG9WzdMDZTXw5EIa50qLR+5OgjMg/VO4Syg/brlguUhkSBlji75AGV8TfJdExkRhc7uv2BQnzDGHQe9f0ewz9w/sNFSVUM2hTxeYbOyZxEntn4oVZF6wCDVpV8M3fHgj8JK8sIF7wD5KvkQ/COxH1ulvc4obtOzwbk0mvW3gakhQYT5nnoiCNcqmKbGVEQbMTFepi8wCKfGrFz9z1fBG1LSzAZTTZV1BPp8pylw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 22 May 2024 19:32:35 -0700 Jeff Xu wrote: > > > > It's a change to a userspace API, yes? Please let's have a detailed > > description of why this is OK. Why it won't affect any existing users. > > > Unfortunately, this is a breaking change that might break a > application if they do below: > memfd_create("", MFD_NOEXEC_SEAL) > fcntl(fd, F_ADD_SEALS, F_SEAL_WRITE); <-- this will fail in new > semantics, due to mfd not being sealable. > > However, I still think the new semantics is a better, the reason is > due to the sysctl: memfd_noexec_scope > Currently, when the sysctl is set to MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL > kernel adds MFD_NOEXEC_SEAL to memfd_create, and the memfd becomes sealable. > E.g. > When the sysctl is set to MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL > The app calls memfd_create("",0) > application will get sealable memfd, which might be a surprise to application. > > If the app doesn't want this behavior, they will need one of two below > in current implementation. > 1> > set the sysctl: memfd_noexec_scope to 0. > So the kernel doesn't overwrite the mdmfd_create > > 2> > modify their code to get non-sealable NOEXEC memfd. > memfd_create("", MEMFD_NOEXEC_SCOPE_NOEXEC) > fcntl(fd, F_ADD_SEALS, F_SEAL_SEAL) > > The new semantics works better with the sysctl. > > Since memfd noexec is new, maybe there is no application using the > MFD_NOEXEC_SEAL to create > sealable memfd. They mostly likely use > memfd(MFD_NOEXEC_SEAL|MFD_ALLOW_SEALING) instead. > I think it might benefit in the long term with the new semantics. Yes, it's new so I expect any damage will be small. Please prepare a v2 which fully explains/justifies the thinking for this non-backward-compatible change and which include the cc:stable.