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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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| bad_inode.c | ||
| binfmt_aout.c | ||
| binfmt_elf_fdpic.c | ||
| binfmt_elf.c | ||
| binfmt_em86.c | ||
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| compat_binfmt_elf.c | ||
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| eventfd.c | ||
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| exec.c | ||
| fcntl.c | ||
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| file_table.c | ||
| file.c | ||
| filesystems.c | ||
| fs_context.c | ||
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| Kconfig | ||
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| locks.c | ||
| Makefile | ||
| mbcache.c | ||
| mount.h | ||
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| namespace.c | ||
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| open.c | ||
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