In the Linux kernel, the following vulnerability has been resolved:

mm: do file ownership checks with the proper mount idmap

Ever since idmapped mounts were introduced, inode ownership checks (for
side-channel protection) in mincore() and madvise(MADV_PAGEOUT) were done
against the nop_mnt_idmap, which completely ignores the file's mount's
idmap.  This results in odd edgecases like:

1) mount/bind-mount with an idmap userA:userB:1
2) userB runs an owner_or_capable() check on file that is owned by userA
on-disk/in-memory, but owned by userB after idmap translation
3) owner_or_capable() mysteriously fails as the correct idmap wasn't supplied

In the case of mincore/madvise MADV_PAGEOUT, this is usually benign,
because file_permission(file, MAY_WRITE) will probably succeed, as it uses
the proper idmap internally, but it does not need to be the case on e.g a
0444 file where even the owner itself doesn't have permissions to write to
it.

Since this is clearly not trivial to get right, introduce a
file_owner_or_capable() that can carry the correct semantics, and switch
the various users in mm to it.

The issue was found by manual code inspection & an off-list discussion
with Jan Kara.

https://git.kernel.org/stable/c/04ba248d02d9eaa3d9077b00a6134caa75fa3e90

https://git.kernel.org/stable/c/5c942ad7df75925ee166e7f0fb36892d8dde376b

https://git.kernel.org/stable/c/744b23aa430d52f5c8e4dbff7d71496d6643bed2

https://git.kernel.org/stable/c/8344bdf0629457e532797b42d9d2bbf2a2900bbf

https://git.kernel.org/stable/c/e187bc02f8fa4226d62814592cf064ee4557c470

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