From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1D2F83F8E11; Tue, 24 Mar 2026 12:31:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774355481; cv=none; b=J2Qx/U+NOSkcuCPtkhXiARPTqEEWeQ/mzAtDCm17Sd9VziFZFIa0TKOdQ0c/kU+66GrIYc08l/GS5jGCf++AC/WivOjD4x7vizSggOjQDJt2vt5mj9hloy2GixA227AojbqvQtH3J/IoegWxPmugLLtGKorSTXAGshIbL8KJN3o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774355481; c=relaxed/simple; bh=Z63/zjcHqfFV5Qn4B9BpNQIoOK6HDNDi+XQVR+t8PjI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=NmN9pSq343ikm34qVHfhll+P6d0jm49ldf5TaBZrHhYfNOxTMMemsKnJbmkK0R6xXswmm7QU/OrbBIhfk/rEShqHDV6V9Af9vK5qUCt5Lrgn/a52fDi2rzXr39j7ze9Zd3en+fQcEExERXFxMhtV4UfhTW5RXZTnSFpTlYDVfBs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=X9VEy6yd; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="X9VEy6yd" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 58838C19424; Tue, 24 Mar 2026 12:31:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1774355480; bh=Z63/zjcHqfFV5Qn4B9BpNQIoOK6HDNDi+XQVR+t8PjI=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=X9VEy6ydihgcVFTq1LZcft7aQ6mXSCotcDWGosw+P4bUq5yI8iqsYI6iPz0tudSAM FIpzDNOzrKGbKBVQ8ab5EBi+y6VhNbqfiOJsacsLc+bwELmEDj/okZ/q4eE2ipysCn VUGo1CX4hHqwsDcq7nysyCcFOULs+QMLVEJIt9IlKWBOSuJyC+jnE1c3VtQtDPriYs DdTe8D6z1HWwxWhZb08E0EbSB74l5Ksup82R/KRmmrqVLiqwUbmg2xeH7LUWY7RnPi DVlyesXSMrTrN7ie9J7FP+OQ/ZdwXLO7NOeEz+DNnO2EPn1Yo/r+JGIW0AfvIqaZVn r1ZueWgELVYOw== Message-ID: <1f9db20a9701e3d79805c75ebee5a5129a72ead0.camel@kernel.org> Subject: Re: [RFC PATCH 0/1] vfs: transitive upgrade restrictions for fds From: Jeff Layton To: Jori Koolstra , Alexander Viro , Christian Brauner , Jan Kara , Chuck Lever , Alexander Aring , Arnd Bergmann , gregkh@linuxfoundation.org Cc: Andrew Morton , "Liam R . Howlett" , Mike Rapoport , David Hildenbrand , Lorenzo Stoakes , zhang jiao , Kees Cook , Penglei Jiang , Ethan Tidmore , Oleg Nesterov , Suren Baghdasaryan , Vlastimil Babka , wangzijie , NeilBrown , Amir Goldstein , Mateusz Guzik , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org Date: Tue, 24 Mar 2026 08:31:16 -0400 In-Reply-To: <20260323220029.765874-1-jkoolstra@xs4all.nl> References: <20260323220029.765874-1-jkoolstra@xs4all.nl> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV 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Extend struct open_how to allow > setting transitive restrictions on using file descriptors to open other > files. A use case for this feature is to block services or containers > from re-opening/upgrading an O_PATH file descriptor through e.g. > /proc//fd/ or OPENAT2_EMPTY_PATH (if upstreamed) as O_WRONLY. >=20 > The implementation idea is this: magic paths like /proc//fd/ > (currently the only one of its sort AFAIK) go through nd_jump_link() to > hard set current->nameidata. To include information about the fd > yielding the magic link, we add a new struct jump_how as a parameter. > This struct may include restictions or other metadata attached to the > magic link jump other than the struct path to jump to. So far it has > only one unsigned int field: allowed_upgrades. This is a flag int that > (for now) may be either READ_UPGRADABLE, WRITE_UPGRADABLE, or > DENY_UPGRADES. >=20 > The idea is that you can restrict what kind of open flags may be used > to open files in any way using this fd as a starting point > (transitively). The check is enforced in may_open_upgrade(), which is > just the old may_open() with an extra test. To keep this state attached > to the fds, we add a field f_allowed_upgrades to struct file. Then > in do_open(), after success, we compute: >=20 > file->f_allowed_upgrades =3D > op->allowed_upgrades & nd->allowed_upgrades; >=20 > where op is the struct open_flags that is build from open_how in > build_open_flags(), and nd->allowed_upgrades is set during path > traversal either in path_init() or nd_jump_link(). >=20 > The implementation and the idea are a bit rough; it is the first bit of > less trivial work I have done on the kernel, hence the RFC status. I did > create some self tests already which this patch passes, and nothing > seems to break on a fresh vng kernel. But obviously there may be MANY > things I am overlooking. >=20 > The original idea for this features comes form the UAPI group kernel > feature idea list [1]. >=20 > [1] https://github.com/uapi-group/kernel-features?tab=3Dreadme-ov-file#up= grade-masks-in-openat2 >=20 > Jori Koolstra (1): > vfs: transitive upgrade restrictions for fds >=20 > fs/file_table.c | 2 ++ > fs/internal.h | 1 + > fs/namei.c | 38 ++++++++++++++++++++++++++++---- > fs/open.c | 9 ++++++++ > fs/proc/base.c | 24 ++++++++++++++------ > fs/proc/fd.c | 6 ++++- > fs/proc/internal.h | 4 +++- > include/linux/fcntl.h | 6 ++++- > include/linux/fs.h | 1 + > include/linux/namei.h | 15 ++++++++++++- > include/uapi/asm-generic/fcntl.h | 4 ++++ > include/uapi/linux/openat2.h | 1 + > 12 files changed, 96 insertions(+), 15 deletions(-) It's an interesting idea, but I could see it being difficult to track the result of this across a large chain of open fd's. If you are going to do this, then at the very least you should add a mechanism (fcntl() command?) to query the current f_allowed_upgrade mask, so that this can be debugged in some fashion. --=20 Jeff Layton