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Merge tag 'fs.idmapped.v5.16-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux
Pull setattr idmapping fix from Christian Brauner:
"This contains a simple fix for setattr. When determining the validity
of the attributes the ia_{g,u}id fields contain the value that will be
written to inode->i_{g,u}id. When the {g,u}id attribute of the file
isn't altered and the caller's fs{g,u}id matches the current {g,u}id
attribute the attribute change is allowed.
The value in ia_{g,u}id does already account for idmapped mounts and
will have taken the relevant idmapping into account. So in order to
verify that the {g,u}id attribute isn't changed we simple need to
compare the ia_{g,u}id value against the inode's i_{g,u}id value.
This only has any meaning for idmapped mounts as idmapping helpers are
idempotent without them. And for idmapped mounts this really only has
a meaning when circular idmappings are used, i.e. mappings where e.g.
id 1000 is mapped to id 1001 and id 1001 is mapped to id 1000. Such
ciruclar mappings can e.g. be useful when sharing the same home
directory between multiple users at the same time.
Before this patch we could end up denying legitimate attribute changes
and allowing invalid attribute changes when circular mappings are
used. To even get into this situation the caller must've been
privileged both to create that mapping and to create that idmapped
mount.
This hasn't been seen in the wild anywhere but came up when expanding
the fstest suite during work on a series of hardening patches. All
idmapped fstests pass without any regressions and we're adding new
tests to verify the behavior of circular mappings.
The new tests can be found at [1]"
Link: https://lore.kernel.org/linux-fsdevel/20211109145713.1868404-2-brauner@kernel.org [1]
* tag 'fs.idmapped.v5.16-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
fs: handle circular mappings correctly
7cf7eed103