Now that explicit capability checking is carried out in the core kernel and the drivers that are already supported in PCuABI, add a section to the porting guide explaining in which situations explicit checking might be required and what should be done.
Also add a few notes to the table of code examples, to make it clear that using the new user pointer API is not necessarily enough in itself.
Signed-off-by: Kevin Brodsky kevin.brodsky@arm.com ---
A follow-up to Luca's explicit checking series.
Rendered doc:
https://git.morello-project.org/kbrodsky-arm/linux/-/blob/porting_guide_expl...
Documentation/cheri/pcuabi-porting.rst | 81 +++++++++++++++++++++----- 1 file changed, 67 insertions(+), 14 deletions(-)
diff --git a/Documentation/cheri/pcuabi-porting.rst b/Documentation/cheri/pcuabi-porting.rst index 2a38862e869a..fcda3b0f1d83 100644 --- a/Documentation/cheri/pcuabi-porting.rst +++ b/Documentation/cheri/pcuabi-porting.rst @@ -15,20 +15,25 @@ The appropriate API to represent and convert user pointers is described in the `user pointer documentation`_. A few examples of modifications required for PCuABI compliance:
-+--------------------------------+------------------------------------+ -| Invalid code | Potential replacement | -+================================+====================================+ -| ``(unsigned long)uptr`` | ``user_ptr_addr(uptr)`` | -+--------------------------------+------------------------------------+ -| ``(void __user *)u64`` | | ``uaddr_to_user_ptr(u64)`` | -| | | ``as_user_ptr(u64)`` | -+--------------------------------+------------------------------------+ -| ``get_user(uptr, &uarg->ptr)`` | ``get_user_ptr(uptr, &uarg->ptr)`` | -+--------------------------------+------------------------------------+ -| ``IS_ERR(ubuf)`` | ``USER_PTR_IS_ERR(ubuf)`` | -+--------------------------------+------------------------------------+ -| ... | ... | -+--------------------------------+------------------------------------+ ++--------------------------------+------------------------------------+------------------------------------------+ +| Invalid code | Potential replacement | Notes | ++================================+====================================+==========================================+ +| ``(unsigned long)uptr`` | ``user_ptr_addr(uptr)`` | Extracting the address of a user pointer | +| | | may indicate that explicit capability | +| | | checking is required, see the | +| | | `Explicit capability checking`_ section. | ++--------------------------------+------------------------------------+------------------------------------------+ +| ``(void __user *)u64`` | | ``uaddr_to_user_ptr(u64)`` | Creating a user pointer from an address | +| | | ``as_user_ptr(u64)`` | should be avoided. | +| | | uapi structs may need to be modified to | +| | | hold full user pointers. | ++--------------------------------+------------------------------------+------------------------------------------+ +| ``get_user(uptr, &uarg->ptr)`` | ``get_user_ptr(uptr, &uarg->ptr)`` | | ++--------------------------------+------------------------------------+------------------------------------------+ +| ``IS_ERR(ubuf)`` | ``USER_PTR_IS_ERR(ubuf)`` | | ++--------------------------------+------------------------------------+------------------------------------------+ +| ... | ... | | ++--------------------------------+------------------------------------+------------------------------------------+
``ioctl`` handlers' third argument ================================== @@ -363,6 +368,54 @@ of the `PCuABI documentation`_. For instance:: Fortunately, ``__user`` is mostly used in simple types, and such fixups are rarely needed in driver code.
+Explicit capability checking +============================ + +In the vast majority of cases, the memory referenced by user pointers is +accessed through the user mapping, using uaccess functions such as +``copy_from_user()``. As long as the original user pointer is wholly +propagated to the uaccess function, no particular attention is required. + +In certain situations, such accesses may instead occur via a kernel +mapping (of the same underlying pages). Often, this kernel mapping is +created by a function in the GUP family (``get_user_pages()``, +``pin_user_pages()``). These cases should be carefully considered and +typically require the user pointer to be explicitly checked (see below). + +The following code patterns may indicate that user memory is being +accessed via a kernel mapping: + +* A call to any function named ``get_user_pages*`` or + ``pin_user_pages*``. Explicit checking is generally required before + calling such a function. Note that this does not normally apply to + ``{get,pin}_user_pages_remote()``, because they are intended to access + another process's memory and such an operation does not need to (and + typically cannot) be authorised by a capability. + +* Casting a user pointer to an integer (``(unsigned long)uptr``). This + typically indicates that an address-based operation, such as GUP, is + going to be carried out. ``user_ptr_addr()`` should be used instead of + the cast, but the way the address is used should also be carefully + considered. + +* Calling ``access_ok()``. Standard uaccess functions call that function + themselves, so an explicit call indicates either that a low-level + uaccess function (e.g. ``__copy_from_user()``) is going to be used - + which is fine - or that the access is not going to be done via uaccess + at all - which requires explicit checking. Note that ``access_ok()`` + does not itself require a valid capability (i.e. it only considers the + address) and ``as_user_ptr()`` may occasionally be needed to pass it + a raw user address, but in general a full user pointer should be + provided by userspace and validated (either by uaccess or explicit + checking). + +Explicit checking should be done using one of the ``check_user_ptr_*()`` +functions, see the "Explicit checking" section of the `user pointer +documentation`_. The required permissions (R/W/RW) should be minimal: if +the kernel only reads memory via the pointer, then +``check_user_ptr_read()`` should be used, so that a pointer without +write permission will pass the check. + .. _user pointer documentation: Documentation/core-api/user_ptr.rst .. _PCuABI documentation: Documentation/cheri/pcuabi.rst .. _pure-capability kernel-user ABI: `PCuABI documentation`_
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