From: Mike Rapoport <rppt@xxxxxxxxxxxxx> Subject: mmap: make mlock_future_check() global Patch series "mm: introduce memfd_secret system call to create "secret" memory areas", v20. This is an implementation of "secret" mappings backed by a file descriptor. 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 be present only in the page table of the owning mm. 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. It's designed to provide the following protections: * Enhanced protection (in conjunction with all the other in-kernel attack prevention systems) against ROP attacks. Seceretmem makes "simple" ROP insufficient to perform exfiltration, which increases the required complexity of the attack. Along with other protections like the kernel stack size limit and address space layout randomization which make finding gadgets is really hard, absence of any in-kernel primitive for accessing secret memory means the one gadget ROP attack can't work. Since the only way to access secret memory is to reconstruct the missing mapping entry, the attacker has to recover the physical page and insert a PTE pointing to it in the kernel and then retrieve the contents. That takes at least three gadgets which is a level of difficulty beyond most standard attacks. * Prevent cross-process secret userspace memory exposures. Once the secret memory is allocated, the user can't accidentally pass it into the kernel to be transmitted somewhere. The secreremem pages cannot be accessed via the direct map and they are disallowed in GUP. * Harden against exploited kernel flaws. In order to access secretmem, a kernel-side attack would need to either walk the page tables and create new ones, or spawn a new privileged uiserspace process to perform secrets exfiltration using ptrace. In the future the secret mappings may be used as a mean to protect guest memory in a virtual machine host. For demonstration of secret memory usage we've created a userspace library https://git.kernel.org/pub/scm/linux/kernel/git/jejb/secret-memory-preloader.git that does two things: the first is act as a preloader for openssl to redirect all the OPENSSL_malloc calls to secret memory meaning any secret keys get automatically protected this way and the other thing it does is expose the API to the user who needs it. We anticipate that a lot of the use cases would be like the openssl one: many toolkits that deal with secret keys already have special handling for the memory to try to give them greater protection, so this would simply be pluggable into the toolkits without any need for user application modification. Hiding secret memory mappings behind an anonymous file allows usage of the page cache for tracking pages allocated for the "secret" mappings as well as using address_space_operations for e.g. page migration callbacks. The anonymous file may be also used implicitly, like hugetlb files, to implement mmap(MAP_SECRET) and use the secret memory areas with "native" mm ABIs in the future. Removing of the pages from the direct map may cause its fragmentation on architectures that use large pages to map the physical memory which affects the system performance. However, the original Kconfig text for CONFIG_DIRECT_GBPAGES said that gigabyte pages in the direct map "... can improve the kernel's performance a tiny bit ..." (commit 00d1c5e05736 ("x86: add gbpages switches")) and the recent report [1] showed that "... although 1G mappings are a good default choice, there is no compelling evidence that it must be the only choice". Hence, it is sufficient to have secretmem disabled by default with the ability of a system administrator to enable it at boot time. In addition, there is also a long term goal to improve management of the direct map. [1] https://lore.kernel.org/linux-mm/213b4567-46ce-f116-9cdf-bbd0c884eb3c@xxxxxxxxxxxxxxx/ This patch (of 7): It will be used by the upcoming secret memory implementation. Link: https://lkml.kernel.org/r/20210518072034.31572-1-rppt@xxxxxxxxxx Link: https://lkml.kernel.org/r/20210518072034.31572-2-rppt@xxxxxxxxxx Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> Acked-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> Cc: Christopher Lameter <cl@xxxxxxxxx> Cc: Dan Williams <dan.j.williams@xxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: David Hildenbrand <david@xxxxxxxxxx> Cc: Elena Reshetova <elena.reshetova@xxxxxxxxx> Cc: Hagen Paul Pfeifer <hagen@xxxxxxxx> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: James Bottomley <jejb@xxxxxxxxxxxxx> Cc: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> Cc: Mark Rutland <mark.rutland@xxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx> Cc: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx> Cc: Paul Walmsley <paul.walmsley@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx> Cc: Roman Gushchin <guro@xxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Shuah Khan <shuah@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Tycho Andersen <tycho@xxxxxxxx> Cc: Will Deacon <will@xxxxxxxxxx> Cc: kernel test robot <lkp@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/internal.h | 3 +++ mm/mmap.c | 5 ++--- 2 files changed, 5 insertions(+), 3 deletions(-) --- a/mm/internal.h~mmap-make-mlock_future_check-global +++ a/mm/internal.h @@ -360,6 +360,9 @@ static inline void munlock_vma_pages_all extern void mlock_vma_page(struct page *page); extern unsigned int munlock_vma_page(struct page *page); +extern int mlock_future_check(struct mm_struct *mm, unsigned long flags, + unsigned long len); + /* * Clear the page's PageMlocked(). This can be useful in a situation where * we want to unconditionally remove a page from the pagecache -- e.g., --- a/mm/mmap.c~mmap-make-mlock_future_check-global +++ a/mm/mmap.c @@ -1352,9 +1352,8 @@ static inline unsigned long round_hint_t return hint; } -static inline int mlock_future_check(struct mm_struct *mm, - unsigned long flags, - unsigned long len) +int mlock_future_check(struct mm_struct *mm, unsigned long flags, + unsigned long len) { unsigned long locked, lock_limit; _
