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 053F633F8A8; Thu, 4 Dec 2025 14:11:27 +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=1764857488; cv=none; b=ao8U/SHUaEJMvH28qrc1TERp6jZ6rb78aPwQpY7m42mdj648B0ilCZ+K7Z3BYnhwBWbyo49Ursw5QW0yKB1+dwCU2c89uLsqQHltdLn/qi29oR5Cyo+hR/yxBbquBUP3O+9KXaeaSa4d4OdCIMzhTJhrt2PfhVTd4touo6l5nSg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764857488; c=relaxed/simple; bh=m3H/3ioNp9U4j7DEHrzqol59bKijaqgK49gOb72xL0M=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=ZuQeDGl/gA+LTa1ZsFU/DjuMWeKUPT5sR62hfNp9/sif9t0ykBqVXwzt4tkKd+5UUi/QwxuwjgY7kZ8GFIiWvTE/NXk6UQC09GieFcVg9deuh8+iyvevedcd6+tIDvp/13IxMMsmFrtBnrBDgRC4oAv1R5W3XIYLYQP7edzUvTo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=tm70lfgF; 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="tm70lfgF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1572EC116D0; Thu, 4 Dec 2025 14:11:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764857487; bh=m3H/3ioNp9U4j7DEHrzqol59bKijaqgK49gOb72xL0M=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=tm70lfgFyib2UED/ePDToCcQsDnLXjzfwjRX99vC2YxT5gzG/ZLZj4cPGybJXqrId iSWaNx9HfMsvNN32luCQh6Si5GuSr2OU0q4+KGNtyaZEPasaolyMWRRL6FHDS1er8d mApW7+4L6zWRv0pNVTqT1iM3IklHtMBPgznpvij8n0QoU15RjHFw45lYl8ZpFkfUaF wNU6kuH19ha45AhKDKp3UZ6jIYZOQFWD1Ck8rnUcHhpPfVNh6heVST1W2zHaYYutdT eiABNhMO2t0av1i0rXL6yq1cdRYmm9T5ninVzrbZMoTOBztvQe8wBcrTNwa3pxqmnw rHnr5TXZRG5jw== Message-ID: <37d39ff38dfef8bae73c5bc6a8593784bb2d3774.camel@kernel.org> Subject: Re: [PATCH v7 0/3] statmount: accept fd as a parameter From: Jeff Layton To: Bhavik Sachdev , Alexander Viro , Christian Brauner , Shuah Khan Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, criu@lists.linux.dev, Jan Kara , Aleksa Sarai , Miklos Szeredi , Pavel Tikhomirov , Andrei Vagin , Alexander Mikhalitsyn , John Hubbard , Amir Goldstein , "Martin K . Petersen" , Andrew Donnellan Date: Thu, 04 Dec 2025 09:11:24 -0500 In-Reply-To: <20251129091455.757724-1-b.sachdev1904@gmail.com> References: <20251129091455.757724-1-b.sachdev1904@gmail.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ cDEsvv5IMmeMe1/qLZ6NaVkNuL3WOXvxaVT9USW1+/SGipO2IpKJjeDZfehlB/kpfF24+RrK+seQf CBYyUE8QJpvTZyfUHNYldXlrjO6n5MdOempLqWpfOmcGkwnyNRBR46g/jf8KnPRwXs509yAqDB6sE 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AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.58.2 (3.58.2-1.fc43) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sat, 2025-11-29 at 14:41 +0530, Bhavik Sachdev wrote: > We would like to add support for checkpoint/restoring file descriptors > open on these "unmounted" mounts to CRIU (Checkpoint/Restore in > Userspace) [1]. >=20 > Currently, we have no way to get mount info for these "unmounted" mounts > since they do appear in /proc//mountinfo and statmount does not > work on them, since they do not belong to any mount namespace. >=20 > This patch helps us by providing a way to get mountinfo for these > "unmounted" mounts by using a fd on the mount. >=20 > Changes from v6 [2] to v7: > * Add kselftests for STATMOUNT_BY_FD flag. >=20 > * Instead of renaming mnt_id_req.mnt_ns_fd to mnt_id_req.fd introduce a > union so struct mnt_id_req looks like this: >=20 > struct mnt_id_req { > __u32 size; > union { > __u32 mnt_ns_fd; > __u32 mnt_fd; > }; > __u64 mnt_id; > __u64 param; > __u64 mnt_ns_id; > }; >=20 > * In case of STATMOUNT_BY_FD grab mnt_ns inside of do_statmount(), > since we get mnt_ns from mnt, which should happen under namespace lock. >=20 > * Remove the modifications made to grab_requested_mnt_ns, those were > never needed. >=20 > Changes from v5 [3] to v6: > * Instead of returning "[unmounted]" as the mount point for "unmounted" > mounts, we unset the STATMOUNT_MNT_POINT flag in statmount.mask. >=20 > * Instead of returning 0 as the mnt_ns_id for "unmounted" mounts, we > unset the STATMOUNT_MNT_NS_ID flag in statmount.mask. >=20 > * Added comment in `do_statmount` clarifying that the caller sets s->mnt > in case of STATMOUNT_BY_FD. >=20 > * In `do_statmount` move the mnt_ns_id and mnt_ns_empty() check just > before lookup_mnt_in_ns(). >=20 > * We took another look at the capability checks for getting information > for "unmounted" mounts using an fd and decided to remove them for the > following reasons: >=20 > - All fs related information is available via fstatfs() without any > capability check. >=20 > - Mount information is also available via /proc/pid/mountinfo (without > any capability check). >=20 > - Given that we have access to a fd on the mount which tells us that > we had access to the mount at some point (or someone that had access > gave us the fd). So, we should be able to access mount info. >=20 > Changes from v4 [4] to v5: > Check only for s->root.mnt to be NULL instead of checking for both > s->root.mnt and s->root.dentry (I did not find a case where only one of > them would be NULL). >=20 > * Only allow system root (CAP_SYS_ADMIN in init_user_ns) to call > statmount() on fd's on "unmounted" mounts. We (mostly Pavel) spent some > time thinking about how our previous approach (of checking the opener's > file credentials) caused problems. >=20 > Please take a look at the linked pictures they describe everything more > clearly. >=20 > Case 1: A fd is on a normal mount (Link to Picture: [5]) > Consider, a situation where we have two processes P1 and P2 and a file > F1. F1 is opened on mount ns M1 by P1. P1 is nested inside user > namespace U1 and U2. P2 is also in U1. P2 is also in a pid namespace and > mount namespace separate from M1. >=20 > P1 sends F1 to P2 (using a unix socket). But, P2 is unable to call > statmount() on F1 because since it is a separate pid and mount > namespace. This is good and expected. >=20 > Case 2: A fd is on a "unmounted" mount (Link to Picture: [6]) > Consider a similar situation as Case 1. But now F1 is on a mounted that > has been "unmounted". Now, since we used openers credentials to check > for permissions P2 ends up having the ability call statmount() and get > mount info for this "unmounted" mount. >=20 > Hence, It is better to restrict the ability to call statmount() on fds > on "unmounted" mounts to system root only (There could also be other > cases than the one described above). >=20 > Changes from v3 [7] to v4: > * Change the string returned when there is no mountpoint to be > "[unmounted]" instead of "[detached]". > * Remove the new DEFINE_FREE put_file and use the one already present in > include/linux/file.h (fput) [8]. > * Inside listmount consistently pass 0 in flags to copy_mnt_id_req and > prepare_klistmount()->grab_requested_mnt_ns() and remove flags from the > prepare_klistmount prototype. > * If STATMOUNT_BY_FD is set, check for mnt_ns_id =3D=3D 0 && mnt_id =3D= =3D 0. >=20 > Changes from v2 [9] to v3: > * Rename STATMOUNT_FD flag to STATMOUNT_BY_FD. > * Fixed UAF bug caused by the reference to fd_mount being bound by scope > of CLASS(fd_raw, f)(kreq.fd) by using fget_raw instead. > * Reused @spare parameter in mnt_id_req instead of adding new fields to > the struct. >=20 > Changes from v1 [10] to v2: > v1 of this patchset, took a different approach and introduced a new > umount_mnt_ns, to which "unmounted" mounts would be moved to (instead of > their namespace being NULL) thus allowing them to be still available via > statmount. >=20 > Introducing umount_mnt_ns complicated namespace locking and modified > performance sensitive code [11] and it was agreed upon that fd-based > statmount would be better. >=20 > This code is also available on github [12]. >=20 > [1]: https://github.com/checkpoint-restore/criu/pull/2754 > [2]: https://lore.kernel.org/all/20251118084836.2114503-1-b.sachdev1904@g= mail.com/ > [3]: https://lore.kernel.org/criu/20251109053921.1320977-2-b.sachdev1904@= gmail.com/T/#u > [4]: https://lore.kernel.org/all/20251029052037.506273-2-b.sachdev1904@gm= ail.com/ > [5]: https://github.com/bsach64/linux/blob/statmount-fd-v5/fd_on_normal_m= ount.png > [6]: https://github.com/bsach64/linux/blob/statmount-fd-v5/file_on_unmoun= ted_mount.png > [7]: https://lore.kernel.org/all/20251024181443.786363-1-b.sachdev1904@gm= ail.com/ > [8]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/t= ree/include/linux/file.h#n97 > [9]: https://lore.kernel.org/linux-fsdevel/20251011124753.1820802-1-b.sac= hdev1904@gmail.com/ > [10]: https://lore.kernel.org/linux-fsdevel/20251002125422.203598-1-b.sac= hdev1904@gmail.com/ > [11]: https://lore.kernel.org/linux-fsdevel/7e4d9eb5-6dde-4c59-8ee3-35823= 3f082d0@virtuozzo.com/ > [12]: https://github.com/bsach64/linux/tree/statmount-fd-v7 >=20 > Bhavik Sachdev (3): > statmount: permission check should return EPERM > statmount: accept fd as a parameter > selftests: statmount: tests for STATMOUNT_BY_FD >=20 > fs/namespace.c | 102 ++++--- > include/uapi/linux/mount.h | 10 +- > .../filesystems/statmount/statmount.h | 15 +- > .../filesystems/statmount/statmount_test.c | 261 +++++++++++++++++- > .../filesystems/statmount/statmount_test_ns.c | 101 ++++++- > 5 files changed, 430 insertions(+), 59 deletions(-) This looks useful. Reviewed-by: Jeff Layton