On Thu, Jan 4, 2024 at 3:47 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote: > > > > On 1/4/24 10:51, jeffxu@xxxxxxxxxxxx wrote: > > From: Jeff Xu <jeffxu@xxxxxxxxxxxx> > > > > Add documentation for mseal(). > > > > Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxxxx> > > --- > > Documentation/userspace-api/mseal.rst | 181 ++++++++++++++++++++++++++ > > 1 file changed, 181 insertions(+) > > create mode 100644 Documentation/userspace-api/mseal.rst > > > > diff --git a/Documentation/userspace-api/mseal.rst b/Documentation/userspace-api/mseal.rst > > new file mode 100644 > > index 000000000000..1700ce5af218 > > --- /dev/null > > +++ b/Documentation/userspace-api/mseal.rst > > @@ -0,0 +1,181 @@ > > +.. SPDX-License-Identifier: GPL-2.0 > > + > > +===================== > > +Introduction of mseal > > +===================== > > + > > +:Author: Jeff Xu <jeffxu@xxxxxxxxxxxx> > > + > > +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 > > +======== > > +Two system calls are involved in virtual memory sealing, mseal() and mmap(). > > + > > +mseal() > > +----------- > > +The mseal() syscall has 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. > > Does that mean that the <len> will be extended to be page aligned > if it's not already page aligned? > Yes. the code (do_mseal) calls PAGE_ALIGNED(len). mprotect() also has this. Two test cases cover this part. test_seal_mprotect_unalign_len test_seal_mprotect_unalign_len_variant_2 -Jeff > -- > #Randy
