100 lines
3.4 KiB
ReStructuredText
100 lines
3.4 KiB
ReStructuredText
.. 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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For anyone wishing to test or use this feature, it is available in
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Amazon's EC2 C5 instances and is known to work there using an Ubuntu
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17.04 image.
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Memory Protection Keys provides a mechanism for enforcing page-based
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protections, but without requiring modification of the page tables
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when an application changes protection domains. It works by
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dedicating 4 previously ignored bits in each page table entry to a
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"protection key", giving 16 possible keys.
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There is also a new user-accessible register (PKRU) with two separate
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bits (Access Disable and Write Disable) for each key. Being a CPU
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register, PKRU is inherently thread-local, potentially giving each
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thread a different set of protections from every other thread.
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There are two new instructions (RDPKRU/WRPKRU) for reading and writing
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to the new register. The feature is only available in 64-bit mode,
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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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========
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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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int pkey_mprotect(unsigned long start, size_t len,
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unsigned long prot, int pkey);
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Before a pkey can be used, it must first be allocated with
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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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ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
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ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey);
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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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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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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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An example implementation can be found in
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tools/testing/selftests/x86/protection_keys.c.
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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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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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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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*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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read(fd, ptr, 1);
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The kernel will send a SIGSEGV in both cases, but si_code will be set
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to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when
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the plain mprotect() permissions are violated.
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