[RFC,v5,4/4] mseal:add documentation

Commit Message

Jeff Xu Jan. 9, 2024, 3:45 p.m. UTC

From: Jeff Xu <[e-mail]>

Add documentation for mseal().

Signed-off-by: Jeff Xu <[e-mail]>
---
 Documentation/userspace-api/mseal.rst | 181 ++++++++++++++++++++++++++
 1 file changed, 181 insertions(+)
 create mode 100644 Documentation/userspace-api/mseal.rst

Comments

Randy Dunlap Jan. 11, 2024, 3:16 a.m. UTC | #1

On 1/9/24 07:45, [e-mail] wrote:
> From: Jeff Xu <[e-mail]>
> 
> Add documentation for mseal().
> 
> Signed-off-by: Jeff Xu <[e-mail]>
> ---
>  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 <[e-mail]>
> +
> +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:

                       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.
> +
> +- ``-EACCES``:
> +    - ``MAP_SEALABLE`` is not set during mmap().
> +
> +- ``-EPERM``:
> +    - sealing is supported only on 64 bit CPUs, 32-bit is not supported.

                                      64-bit

> +
> +- 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 shall be rare.

s/shall/should/
unless you are predicting the future.

> +
> +**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.
> +
> +**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.
> +
> +mmap()
> +----------
> +``void *mmap(void* addr, size_t length, int prot, int flags, int fd,
> +off_t offset);``
> +
> +We add two changes in ``prot`` and ``flags`` of  mmap() related to
> +memory sealing.
> +
> +**prot**
> +
> +The ``PROT_SEAL`` bit in ``prot`` field of mmap().
> +
> +When present, it marks the memory is sealed since creation.
> +
> +This is useful as optimization because it avoids having to make two
> +system calls: one for mmap() and one for mseal().
> +
> +It's worth noting that even though the sealing is set via the
> +``prot`` field in mmap(), it can't be set in the ``prot``
> +field in later mprotect(). This is unlike the ``PROT_READ``,
> +``PROT_WRITE``, ``PROT_EXEC`` bits, e.g. if ``PROT_WRITE`` is not set in
> +mprotect(), it means that the region is not writable.
> +
> +Setting ``PROT_SEAL`` implies setting ``MAP_SEALABLE`` below.
> +
> +**flags**
> +
> +The ``MAP_SEALABLE`` bit in the ``flags`` field of mmap().
> +
> +When present, it marks the map as sealable. A map created
> +without ``MAP_SEALABLE`` will not support sealing; In other words,

                                             sealing. In

> +mseal() will fail for such a map.
> +
> +
> +Applications that don't care about sealing will expect their
> +behavior unchanged. For those that need sealing support, opt-in

                                                            opt in

> +by adding ``MAP_SEALABLE`` in mmap().
> +
> +Note: for a map created without ``MAP_SEALABLE`` or a map created
> +with ``MAP_SEALABLE`` but not sealed yet, mmap(MAP_FIXED) can
> +change the sealable or sealing bit.
> +
> +Use Case:
> +=========
> +- glibc:
> +  The dynamic linker, during loading ELF executables, can apply sealing to
> +  non-writable memory segments.
> +
> +- Chrome browser: protect some security sensitive data-structures.
> +
> +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/[e-mail]
> +
> +[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc

Jeff Xu Jan. 11, 2024, 5:19 a.m. UTC | #2

On Wed, Jan 10, 2024 at 7:16 PM Randy Dunlap <[e-mail]> wrote:
>
>
>
> On 1/9/24 07:45, [e-mail] wrote:
> > From: Jeff Xu <[e-mail]>
> >
> > Add documentation for mseal().
> >
> > Signed-off-by: Jeff Xu <[e-mail]>
> > ---
> >  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 <[e-mail]>
> > +
> > +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:
>
>                        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.
> > +
> > +- ``-EACCES``:
> > +    - ``MAP_SEALABLE`` is not set during mmap().
> > +
> > +- ``-EPERM``:
> > +    - sealing is supported only on 64 bit CPUs, 32-bit is not supported.
>
>                                       64-bit
>
> > +
> > +- 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 shall be rare.
>
> s/shall/should/
> unless you are predicting the future.
>
> > +
> > +**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.
> > +
> > +**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.
> > +
> > +mmap()
> > +----------
> > +``void *mmap(void* addr, size_t length, int prot, int flags, int fd,
> > +off_t offset);``
> > +
> > +We add two changes in ``prot`` and ``flags`` of  mmap() related to
> > +memory sealing.
> > +
> > +**prot**
> > +
> > +The ``PROT_SEAL`` bit in ``prot`` field of mmap().
> > +
> > +When present, it marks the memory is sealed since creation.
> > +
> > +This is useful as optimization because it avoids having to make two
> > +system calls: one for mmap() and one for mseal().
> > +
> > +It's worth noting that even though the sealing is set via the
> > +``prot`` field in mmap(), it can't be set in the ``prot``
> > +field in later mprotect(). This is unlike the ``PROT_READ``,
> > +``PROT_WRITE``, ``PROT_EXEC`` bits, e.g. if ``PROT_WRITE`` is not set in
> > +mprotect(), it means that the region is not writable.
> > +
> > +Setting ``PROT_SEAL`` implies setting ``MAP_SEALABLE`` below.
> > +
> > +**flags**
> > +
> > +The ``MAP_SEALABLE`` bit in the ``flags`` field of mmap().
> > +
> > +When present, it marks the map as sealable. A map created
> > +without ``MAP_SEALABLE`` will not support sealing; In other words,
>
>                                              sealing. In
>
> > +mseal() will fail for such a map.
> > +
> > +
> > +Applications that don't care about sealing will expect their
> > +behavior unchanged. For those that need sealing support, opt-in
>
>                                                             opt in
>
> > +by adding ``MAP_SEALABLE`` in mmap().
> > +
> > +Note: for a map created without ``MAP_SEALABLE`` or a map created
> > +with ``MAP_SEALABLE`` but not sealed yet, mmap(MAP_FIXED) can
> > +change the sealable or sealing bit.
> > +
> > +Use Case:
> > +=========
> > +- glibc:
> > +  The dynamic linker, during loading ELF executables, can apply sealing to
> > +  non-writable memory segments.
> > +
> > +- Chrome browser: protect some security sensitive data-structures.
> > +
> > +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/[e-mail]
> > +
> > +[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc
>
Thanks. Will update in the next version.
-Jeff

> --
> #Randy

Patch

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 <[e-mail]>
+
+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.
+
+**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.
+
+- ``-EACCES``:
+    - ``MAP_SEALABLE`` is not set during mmap().
+
+- ``-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 shall 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.
+
+**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.
+
+mmap()
+----------
+``void *mmap(void* addr, size_t length, int prot, int flags, int fd,
+off_t offset);``
+
+We add two changes in ``prot`` and ``flags`` of  mmap() related to
+memory sealing.
+
+**prot**
+
+The ``PROT_SEAL`` bit in ``prot`` field of mmap().
+
+When present, it marks the memory is sealed since creation.
+
+This is useful as optimization because it avoids having to make two
+system calls: one for mmap() and one for mseal().
+
+It's worth noting that even though the sealing is set via the
+``prot`` field in mmap(), it can't be set in the ``prot``
+field in later mprotect(). This is unlike the ``PROT_READ``,
+``PROT_WRITE``, ``PROT_EXEC`` bits, e.g. if ``PROT_WRITE`` is not set in
+mprotect(), it means that the region is not writable.
+
+Setting ``PROT_SEAL`` implies setting ``MAP_SEALABLE`` below.
+
+**flags**
+
+The ``MAP_SEALABLE`` bit in the ``flags`` field of mmap().
+
+When present, it marks the map as sealable. A map created
+without ``MAP_SEALABLE`` will not support sealing; In other words,
+mseal() will fail for such a map.
+
+
+Applications that don't care about sealing will expect their
+behavior unchanged. For those that need sealing support, opt-in
+by adding ``MAP_SEALABLE`` in mmap().
+
+Note: for a map created without ``MAP_SEALABLE`` or a map created
+with ``MAP_SEALABLE`` but not sealed yet, mmap(MAP_FIXED) can
+change the sealable or sealing bit.
+
+Use Case:
+=========
+- glibc:
+  The dynamic linker, during loading ELF executables, can apply sealing to
+  non-writable memory segments.
+
+- Chrome browser: protect some security sensitive data-structures.
+
+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/[e-mail]
+
+[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc