mm: do file ownership checks with the proper mount idmap

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.

Link: https://lore.kernel.org/20260625153853.913949-1-pfalcato@suse.de
Fixes: 9caccd41541a ("fs: introduce MOUNT_ATTR_IDMAP")
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christian Brauner (Amutable) <brauner@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>