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@xxxxxxxxxxxxx> > > > > 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. > -- > Thanks, > > David / dhildenb > -- Sincerely yours, Mike.
