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SubjectRe: [PATCH 04/24] VFS: Add LSM hooks for filesystem context [ver #7]
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On 04/24/2018 11:22 AM, David Howells wrote:
> Stephen Smalley <sds@tycho.nsa.gov> wrote:
>
>> Neither fsopen() nor fscontext_fs_write() appear to perform any kind of
>> up-front permission checking (DAC or MAC), although some security hooks may
>> be ultimately called to allocate structures, parse security options, etc.
>> Is there a reason not apply a may_mount() or similar check up front?
>
> may_mount() is called by fsmount() at the moment. It may make sense to move
> this earlier to fsopen(). Note that there's also going to be something that
> looks like:
>
> fd = fspick("/mnt");
> fsmount(fd, "/a", MNT_NOEXEC); // ie. bind mount
>
> or:
>
> fd = fspick("/mnt");
> write(fd, "o intr");
> write(fd, "x reconfigure"); // ie. something like remount
> close(fd);
>
> I guess we'd want to call may_mount() in fspick() too. But there's also the
> possibility of using this to create a query interfact too:
>
> fd = fspick("/mnt");
> write(fd, "q intr");
> read(fd, value_buffer);

My concern was that fsopen()/fscontext_fs_write() may expose attack surface (e.g. mount option parsing code) that might not be normally accessible to unprivileged userspace (i.e. gated by may_mount() and security_sb_mount()) prior to your changes.


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