Re: [PATCH v4 0/6] mm: introduce memfd_secret system call to create "secret" memory areas

On Wed, Aug 19, 2020 at 12:47:54PM +0200, David Hildenbrand wrote: > On 18.08.20 16:15, Mike Rapoport wrote: >> From: Mike Rapoport <rppt(a)linux.ibm.com&gt; >> >> Hi, >> >> This is an implementation of "secret" mappings backed by a file descriptor. >> >> v4 changes: >> * rebase on v5.9-rc1 >> * Do not redefine PMD_PAGE_ORDER in fs/dax.c, thanks Kirill >> * Make secret mappings exclusive by default and only require flags to >> memfd_secret() system call for uncached mappings, thanks again Kirill :) >> >> v3 changes: >> * Squash kernel-parameters.txt update into the commit that added the >> command line option. >> * Make uncached mode explicitly selectable by architectures. For now enable >> it only on x86. >> >> v2 changes: >> * Follow Michael's suggestion and name the new system call 'memfd_secret' >> * Add kernel-parameters documentation about the boot option >> * Fix i386-tinyconfig regression reported by the kbuild bot. >> CONFIG_SECRETMEM now depends on !EMBEDDED to disable it on small systems >> from one side and still make it available unconditionally on >> architectures that support SET_DIRECT_MAP. >> >> >> The file descriptor backing secret memory mappings is created using a >> dedicated memfd_secret system call The desired protection mode for the >> memory is configured using flags parameter of the system call. The mmap() >> of the file descriptor created with memfd_secret() will create a "secret" >> memory mapping. The pages in that mapping will be marked as not present in >> the direct map and will have desired protection bits set in the user page >> table. For instance, current implementation allows uncached mappings. >> >> Although normally Linux userspace mappings are protected from other users, >> such secret mappings are useful for environments where a hostile tenant is >> trying to trick the kernel into giving them access to other tenants >> mappings. >> >> Additionally, the secret mappings may be used as a mean to protect guest >> memory in a virtual machine host. >> > > Just a general question. I assume such pages (where the direct mapping > was changed) cannot get migrated - I can spot a simple alloc_page(). So > essentially a process can just allocate a whole bunch of memory that is > unmovable, correct? Is there any limit? Is it properly accounted towards > the process (memctl) ? The memory as accounted in the same way like with mlock(), so normal user won't be able to allocate more than RLIMIT_MEMLOCK.