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for-linus-2021-03-10
-----BEGIN PGP SIGNATURE----- iHUEABYKAB0WIQRAhzRXHqcMeLMyaSiRxhvAZXjcogUCYEiQUgAKCRCRxhvAZXjc omY/AQDEn45Gx6YxDOiWXxyreS3JYx8jVNyy85uDDFRwR0qK+QEAgsmdOxOJoFfe zzNA8dKmXx2t+upuK8htmqtQTrGV/wg= =9MKn -----END PGP SIGNATURE----- Merge tag 'for-linus-2021-03-10' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux Pull detached mounts fix from Christian Brauner: "Creating a series of detached mounts, attaching them to the filesystem, and unmounting them can be used to trigger an integer overflow in ns->mounts causing the kernel to block any new mounts in count_mounts() and returning ENOSPC because it falsely assumes that the maximum number of mounts in the mount namespace has been reached, i.e. it thinks it can't fit the new mounts into the mount namespace anymore. Without this fix heavy use of the new mount API with move_mount() will cause the host to become unuseable and thus blocks some xfstest patches I want to resend. Depending on the number of mounts in your system, this can be reproduced on any kernel that supportes open_tree() and move_mount(). A reproducer has been sent for inclusion with xfstests. It takes care to do this in another mount namespace, not in the host's mount namespace so there shouldn't be any risk in running it but if one did run it on the host it would require a reboot in order to be able to mount again. See https://lore.kernel.org/fstests/20210309121041.753359-1-christian.brauner@ubuntu.com The root cause of this is that detached mounts aren't handled correctly when source and target mount are identical and reside on a shared mount causing a broken mount tree where the detached source itself is propagated which propagation prevents for regular bind-mounts and new mounts. This ultimately leads to a miscalculation of the number of mounts in the mount namespace. Detached mounts created via 'open_tree(fd, path, OPEN_TREE_CLONE)' are essentially like an unattached bind-mount. They can then later on be attached to the filesystem via move_mount() which calls into attach_recursive_mount(). Part of attaching it to the filesystem is making sure that mounts get correctly propagated in case the destination mountpoint is MS_SHARED, i.e. is a shared mountpoint. This is done by calling into propagate_mnt() which walks the list of peers calling propagate_one() on each mount in this list making sure it receives the propagation event. The propagate_one() function thereby skips both new mounts and bind mounts to not propagate them "into themselves". Both are identified by checking whether the mount is already attached to any mount namespace in mnt->mnt_ns. The is what the IS_MNT_NEW() helper is responsible for. However, detached mounts have an anonymous mount namespace attached to them stashed in mnt->mnt_ns which means that IS_MNT_NEW() doesn't realize they need to be skipped causing the mount to propagate "into itself" breaking the mount table and causing a disconnect between the number of mounts recorded as being beneath or reachable from the target mountpoint and the number of mounts actually recorded/counted in ns->mounts ultimately causing an overflow which in turn prevents any new mounts via the ENOSPC issue. So teach propagation to handle detached mounts by making it aware of them. I've been tracking this issue down for the last couple of days and then verifying that the fix is correct by unmounting everything in my current mount table leaving only /proc and /sys mounted and running the reproducer above overnight verifying the number of mounts counted in ns->mounts. With this fix the counts are correct and the ENOSPC issue can't be reproduced. This change will only have an effect on mounts created with the new mount API since detached mounts cannot be created with the old mount API so regressions are extremely unlikely. Here's an illustration: #### mount(): ubuntu@f1-vm:~$ sudo mount --bind /mnt/ /mnt/ ubuntu@f1-vm:~$ findmnt | grep -i mnt ├─/mnt /dev/sda2[/mnt] ext4 rw,relatime #### open_tree(OPEN_TREE_CLONE) + move_mount() with bug: ubuntu@f1-vm:~$ sudo ./mount-new /mnt/ /mnt/ ubuntu@f1-vm:~$ findmnt | grep -i mnt ├─/mnt /dev/sda2[/mnt] ext4 rw,relatime │ └─/mnt /dev/sda2[/mnt] ext4 rw,relatime #### open_tree(OPEN_TREE_CLONE) + move_mount() with the fix: ubuntu@f1-vm:~$ sudo ./mount-new /mnt /mnt ubuntu@f1-vm:~$ findmnt | grep -i mnt └─/mnt /dev/sda2[/mnt] ext4 rw,relatime" * tag 'for-linus-2021-03-10' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux: mount: fix mounting of detached mounts onto targets that reside on shared mounts
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#define IS_MNT_SHARED(m) ((m)->mnt.mnt_flags & MNT_SHARED)
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#define IS_MNT_SLAVE(m) ((m)->mnt_master)
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#define IS_MNT_NEW(m) (!(m)->mnt_ns)
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#define IS_MNT_NEW(m) (!(m)->mnt_ns || is_anon_ns((m)->mnt_ns))
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#define CLEAR_MNT_SHARED(m) ((m)->mnt.mnt_flags &= ~MNT_SHARED)
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#define IS_MNT_UNBINDABLE(m) ((m)->mnt.mnt_flags & MNT_UNBINDABLE)
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#define IS_MNT_MARKED(m) ((m)->mnt.mnt_flags & MNT_MARKED)
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