On Fri, Sep 25, 2020 at 11:00:30AM +0200, David Hildenbrand wrote: > On 25.09.20 09:41, Peter Zijlstra wrote: > > On Thu, Sep 24, 2020 at 04:29:03PM +0300, Mike Rapoport wrote: > >> From: Mike Rapoport <rppt@xxxxxxxxxxxxx> > >> > >> Removing a PAGE_SIZE page from the direct map every time such page is > >> allocated for a secret memory mapping will cause severe fragmentation of > >> the direct map. This fragmentation can be reduced by using PMD-size pages > >> as a pool for small pages for secret memory mappings. > >> > >> Add a gen_pool per secretmem inode and lazily populate this pool with > >> PMD-size pages. > > > > What's the actual efficacy of this? Since the pmd is per inode, all I > > need is a lot of inodes and we're in business to destroy the directmap, > > no? > > > > Afaict there's no privs needed to use this, all a process needs is to > > stay below the mlock limit, so a 'fork-bomb' that maps a single secret > > page will utterly destroy the direct map. > > > > I really don't like this, at all. > > As I expressed earlier, I would prefer allowing allocation of secretmem > only from a previously defined CMA area. This would physically locally > limit the pain. Given that this thing doesn't have a migrate hook, that seems like an eminently reasonable contraint. Because not only will it mess up the directmap, it will also destroy the ability of the page-allocator / compaction to re-form high order blocks by sprinkling holes throughout. Also, this is all very close to XPFO, yet I don't see that mentioned anywhere. Further still, it has this HAVE_SECRETMEM_UNCACHED nonsense which is completely unused. I'm not at all sure exposing UNCACHED to random userspace is a sane idea.
