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commit 7d5dcd2358df55353eed94a0e84b77bb3597634e
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Author: J. Bruce Fields <bfields@redhat.com>
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Date: Fri Mar 27 13:11:28 2020 -0400
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exports man page: warn about subdirectory exports
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Subdirectory exports have a number of problems which have been poorly
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documented.
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Signed-off-by: J. Bruce Fields <bfields@redhat.com>
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Signed-off-by: Steve Dickson <steved@redhat.com>
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diff --git a/utils/exportfs/exports.man b/utils/exportfs/exports.man
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index e3a16f6b..1d171849 100644
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--- a/utils/exportfs/exports.man
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+++ b/utils/exportfs/exports.man
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@@ -494,6 +494,33 @@ export entry for
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.B /home/joe
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in the example section below, which maps all requests to uid 150 (which
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is supposedly that of user joe).
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+
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+.SS Subdirectory Exports
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+
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+Normally you should only export only the root of a filesystem. The NFS
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+server will also allow you to export a subdirectory of a filesystem,
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+however, this has drawbacks:
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+
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+First, it may be possible for a malicious user to access files on the
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+filesystem outside of the exported subdirectory, by guessing filehandles
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+for those other files. The only way to prevent this is by using the
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+.IR no_subtree_check
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+option, which can cause other problems.
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+
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+Second, export options may not be enforced in the way that you would
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+expect. For example, the
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+.IR security_label
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+option will not work on subdirectory exports, and if nested subdirectory
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+exports change the
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+.IR security_label
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+or
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+.IR sec=
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+options, NFSv4 clients will normally see only the options on the parent
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+export. Also, where security options differ, a malicious client may use
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+filehandle-guessing attacks to access the files from one subdirectory
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+using the options from another.
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+
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+
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.SS Extra Export Tables
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After reading
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.I /etc/exports
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