mseal: add documentation

Add documentation for mseal().

Link: https://lkml.kernel.org/r/20240415163527.626541-5-jeffxu@chromium.org
Signed-off-by: Jeff Xu <jeffxu@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jeff Xu <jeffxu@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Stephen Röttger <sroettger@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Amer Al Shanawany <amer.shanawany@gmail.com>
Cc: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/userspace-api/index.rst

@@ -20,6 +20,7 @@ System calls
    futex2
    ebpf/index
    ioctl/index
+   mseal
 
 Security-related interfaces
 ===========================

Documentation/userspace-api/mseal.rst

@@ -0,0 +1,199 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Introduction of mseal
+=====================
+
+:Author: Jeff Xu <jeffxu@chromium.org>
+
+Modern CPUs support memory permissions such as RW and NX bits. The memory
+permission feature improves security stance on memory corruption bugs, i.e.
+the attacker can’t just write to arbitrary memory and point the code to it,
+the memory has to be marked with X bit, or else an exception will happen.
+
+Memory sealing additionally protects the mapping itself against
+modifications. This is useful to mitigate memory corruption issues where a
+corrupted pointer is passed to a memory management system. For example,
+such an attacker primitive can break control-flow integrity guarantees
+since read-only memory that is supposed to be trusted can become writable
+or .text pages can get remapped. Memory sealing can automatically be
+applied by the runtime loader to seal .text and .rodata pages and
+applications can additionally seal security critical data at runtime.
+
+A similar feature already exists in the XNU kernel with the
+VM_FLAGS_PERMANENT flag [1] and on OpenBSD with the mimmutable syscall [2].
+
+User API
+========
+mseal()
+-----------
+The mseal() syscall has the following signature:
+
+``int mseal(void addr, size_t len, unsigned long flags)``
+
+**addr/len**: virtual memory address range.
+
+The address range set by ``addr``/``len`` must meet:
+   - The start address must be in an allocated VMA.
+   - The start address must be page aligned.
+   - The end address (``addr`` + ``len``) must be in an allocated VMA.
+   - no gap (unallocated memory) between start and end address.
+
+The ``len`` will be paged aligned implicitly by the kernel.
+
+**flags**: reserved for future use.
+
+**return values**:
+
+- ``0``: Success.
+
+- ``-EINVAL``:
+    - Invalid input ``flags``.
+    - The start address (``addr``) is not page aligned.
+    - Address range (``addr`` + ``len``) overflow.
+
+- ``-ENOMEM``:
+    - The start address (``addr``) is not allocated.
+    - The end address (``addr`` + ``len``) is not allocated.
+    - A gap (unallocated memory) between start and end address.
+
+- ``-EPERM``:
+    - sealing is supported only on 64-bit CPUs, 32-bit is not supported.
+
+- For above error cases, users can expect the given memory range is
+  unmodified, i.e. no partial update.
+
+- There might be other internal errors/cases not listed here, e.g.
+  error during merging/splitting VMAs, or the process reaching the max
+  number of supported VMAs. In those cases, partial updates to the given
+  memory range could happen. However, those cases should be rare.
+
+**Blocked operations after sealing**:
+    Unmapping, moving to another location, and shrinking the size,
+    via munmap() and mremap(), can leave an empty space, therefore
+    can be replaced with a VMA with a new set of attributes.
+
+    Moving or expanding a different VMA into the current location,
+    via mremap().
+
+    Modifying a VMA via mmap(MAP_FIXED).
+
+    Size expansion, via mremap(), does not appear to pose any
+    specific risks to sealed VMAs. It is included anyway because
+    the use case is unclear. In any case, users can rely on
+    merging to expand a sealed VMA.
+
+    mprotect() and pkey_mprotect().
+
+    Some destructive madvice() behaviors (e.g. MADV_DONTNEED)
+    for anonymous memory, when users don't have write permission to the
+    memory. Those behaviors can alter region contents by discarding pages,
+    effectively a memset(0) for anonymous memory.
+
+    Kernel will return -EPERM for blocked operations.
+
+    For blocked operations, one can expect the given address is unmodified,
+    i.e. no partial update. Note, this is different from existing mm
+    system call behaviors, where partial updates are made till an error is
+    found and returned to userspace. To give an example:
+
+    Assume following code sequence:
+
+    - ptr = mmap(null, 8192, PROT_NONE);
+    - munmap(ptr + 4096, 4096);
+    - ret1 = mprotect(ptr, 8192, PROT_READ);
+    - mseal(ptr, 4096);
+    - ret2 = mprotect(ptr, 8192, PROT_NONE);
+
+    ret1 will be -ENOMEM, the page from ptr is updated to PROT_READ.
+
+    ret2 will be -EPERM, the page remains to be PROT_READ.
+
+**Note**:
+
+- mseal() only works on 64-bit CPUs, not 32-bit CPU.
+
+- users can call mseal() multiple times, mseal() on an already sealed memory
+  is a no-action (not error).
+
+- munseal() is not supported.
+
+Use cases:
+==========
+- glibc:
+  The dynamic linker, during loading ELF executables, can apply sealing to
+  non-writable memory segments.
+
+- Chrome browser: protect some security sensitive data-structures.
+
+Notes on which memory to seal:
+==============================
+
+It might be important to note that sealing changes the lifetime of a mapping,
+i.e. the sealed mapping won’t be unmapped till the process terminates or the
+exec system call is invoked. Applications can apply sealing to any virtual
+memory region from userspace, but it is crucial to thoroughly analyze the
+mapping's lifetime prior to apply the sealing.
+
+For example:
+
+- aio/shm
+
+  aio/shm can call mmap()/munmap() on behalf of userspace, e.g. ksys_shmdt() in
+  shm.c. The lifetime of those mapping are not tied to the lifetime of the
+  process. If those memories are sealed from userspace, then munmap() will fail,
+  causing leaks in VMA address space during the lifetime of the process.
+
+- Brk (heap)
+
+  Currently, userspace applications can seal parts of the heap by calling
+  malloc() and mseal().
+  let's assume following calls from user space:
+
+  - ptr = malloc(size);
+  - mprotect(ptr, size, RO);
+  - mseal(ptr, size);
+  - free(ptr);
+
+  Technically, before mseal() is added, the user can change the protection of
+  the heap by calling mprotect(RO). As long as the user changes the protection
+  back to RW before free(), the memory range can be reused.
+
+  Adding mseal() into the picture, however, the heap is then sealed partially,
+  the user can still free it, but the memory remains to be RO. If the address
+  is re-used by the heap manager for another malloc, the process might crash
+  soon after. Therefore, it is important not to apply sealing to any memory
+  that might get recycled.
+
+  Furthermore, even if the application never calls the free() for the ptr,
+  the heap manager may invoke the brk system call to shrink the size of the
+  heap. In the kernel, the brk-shrink will call munmap(). Consequently,
+  depending on the location of the ptr, the outcome of brk-shrink is
+  nondeterministic.
+
+
+Additional notes:
+=================
+As Jann Horn pointed out in [3], there are still a few ways to write
+to RO memory, which is, in a way, by design. Those cases are not covered
+by mseal(). If applications want to block such cases, sandbox tools (such as
+seccomp, LSM, etc) might be considered.
+
+Those cases are:
+
+- Write to read-only memory through /proc/self/mem interface.
+- Write to read-only memory through ptrace (such as PTRACE_POKETEXT).
+- userfaultfd.
+
+The idea that inspired this patch comes from Stephen Röttger’s work in V8
+CFI [4]. Chrome browser in ChromeOS will be the first user of this API.
+
+Reference:
+==========
+[1] https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/osfmk/mach/vm_statistics.h#L274
+
+[2] https://man.openbsd.org/mimmutable.2
+
+[3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426FkcgnfUGLvA@mail.gmail.com
+
+[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc