1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
|
.. SPDX-License-Identifier: GPL-2.0
======================
Memory Protection Keys
======================
Memory Protection Keys provide a mechanism for enforcing page-based
protections, but without requiring modification of the page tables when an
application changes protection domains.
Pkeys Userspace (PKU) is a feature which can be found on:
* Intel server CPUs, Skylake and later
* Intel client CPUs, Tiger Lake (11th Gen Core) and later
* Future AMD CPUs
* arm64 CPUs implementing the Permission Overlay Extension (FEAT_S1POE)
x86_64
======
Pkeys work by dedicating 4 previously Reserved bits in each page table entry to
a "protection key", giving 16 possible keys.
Protections for each key are defined with a per-CPU user-accessible register
(PKRU). Each of these is a 32-bit register storing two bits (Access Disable
and Write Disable) for each of 16 keys.
Being a CPU register, PKRU is inherently thread-local, potentially giving each
thread a different set of protections from every other thread.
There are two instructions (RDPKRU/WRPKRU) for reading and writing to the
register. The feature is only available in 64-bit mode, even though there is
theoretically space in the PAE PTEs. These permissions are enforced on data
access only and have no effect on instruction fetches.
arm64
=====
Pkeys use 3 bits in each page table entry, to encode a "protection key index",
giving 8 possible keys.
Protections for each key are defined with a per-CPU user-writable system
register (POR_EL0). This is a 64-bit register encoding read, write and execute
overlay permissions for each protection key index.
Being a CPU register, POR_EL0 is inherently thread-local, potentially giving
each thread a different set of protections from every other thread.
Unlike x86_64, the protection key permissions also apply to instruction
fetches.
Syscalls
========
There are 3 system calls which directly interact with pkeys::
int pkey_alloc(unsigned long flags, unsigned long init_access_rights)
int pkey_free(int pkey);
int pkey_mprotect(unsigned long start, size_t len,
unsigned long prot, int pkey);
Before a pkey can be used, it must first be allocated with pkey_alloc(). An
application writes to the architecture specific CPU register directly in order
to change access permissions to memory covered with a key. In this example
this is wrapped by a C function called pkey_set().
::
int real_prot = PROT_READ|PROT_WRITE;
pkey = pkey_alloc(0, PKEY_DISABLE_WRITE);
ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey);
... application runs here
Now, if the application needs to update the data at 'ptr', it can
gain access, do the update, then remove its write access::
pkey_set(pkey, 0); // clear PKEY_DISABLE_WRITE
*ptr = foo; // assign something
pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_WRITE again
Now when it frees the memory, it will also free the pkey since it
is no longer in use::
munmap(ptr, PAGE_SIZE);
pkey_free(pkey);
.. note:: pkey_set() is a wrapper around writing to the CPU register.
Example implementations can be found in
tools/testing/selftests/mm/pkey-{arm64,powerpc,x86}.h
Behavior
========
The kernel attempts to make protection keys consistent with the
behavior of a plain mprotect(). For instance if you do this::
mprotect(ptr, size, PROT_NONE);
something(ptr);
you can expect the same effects with protection keys when doing this::
pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ);
pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey);
something(ptr);
That should be true whether something() is a direct access to 'ptr'
like::
*ptr = foo;
or when the kernel does the access on the application's behalf like
with a read()::
read(fd, ptr, 1);
The kernel will send a SIGSEGV in both cases, but si_code will be set
to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when
the plain mprotect() permissions are violated.
Note that kernel accesses from a kthread (such as io_uring) will use a default
value for the protection key register and so will not be consistent with
userspace's value of the register or mprotect().
|
