Documentation: x86: convert protection-keys.txt to reST
This converts the plain text documentation to reStructuredText format and add it to Sphinx TOC tree. No essential content change. Signed-off-by: Changbin Du <changbin.du@gmail.com> Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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@ -18,3 +18,4 @@ x86-specific Documentation
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tlb
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mtrr
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pat
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protection-keys
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@ -1,3 +1,9 @@
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.. SPDX-License-Identifier: GPL-2.0
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======================
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Memory Protection Keys
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======================
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Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature
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which is found on Intel's Skylake "Scalable Processor" Server CPUs.
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It will be avalable in future non-server parts.
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@ -23,9 +29,10 @@ even though there is theoretically space in the PAE PTEs. These
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permissions are enforced on data access only and have no effect on
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instruction fetches.
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=========================== Syscalls ===========================
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Syscalls
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========
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There are 3 system calls which directly interact with pkeys:
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There are 3 system calls which directly interact with pkeys::
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int pkey_alloc(unsigned long flags, unsigned long init_access_rights)
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int pkey_free(int pkey);
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@ -37,6 +44,7 @@ pkey_alloc(). An application calls the WRPKRU instruction
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directly in order to change access permissions to memory covered
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with a key. In this example WRPKRU is wrapped by a C function
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called pkey_set().
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::
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int real_prot = PROT_READ|PROT_WRITE;
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pkey = pkey_alloc(0, PKEY_DISABLE_WRITE);
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@ -45,43 +53,44 @@ called pkey_set().
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... application runs here
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Now, if the application needs to update the data at 'ptr', it can
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gain access, do the update, then remove its write access:
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gain access, do the update, then remove its write access::
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pkey_set(pkey, 0); // clear PKEY_DISABLE_WRITE
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*ptr = foo; // assign something
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pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_WRITE again
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Now when it frees the memory, it will also free the pkey since it
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is no longer in use:
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is no longer in use::
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munmap(ptr, PAGE_SIZE);
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pkey_free(pkey);
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(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
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.. note:: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
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An example implementation can be found in
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tools/testing/selftests/x86/protection_keys.c)
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tools/testing/selftests/x86/protection_keys.c.
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=========================== Behavior ===========================
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Behavior
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========
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The kernel attempts to make protection keys consistent with the
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behavior of a plain mprotect(). For instance if you do this:
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behavior of a plain mprotect(). For instance if you do this::
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mprotect(ptr, size, PROT_NONE);
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something(ptr);
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you can expect the same effects with protection keys when doing this:
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you can expect the same effects with protection keys when doing this::
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pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ);
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pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey);
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something(ptr);
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That should be true whether something() is a direct access to 'ptr'
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like:
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like::
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*ptr = foo;
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or when the kernel does the access on the application's behalf like
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with a read():
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with a read()::
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read(fd, ptr, 1);
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