To avoid a useless nop on top of every uaccess enable/disable and
make life easier for objtool, replace static branches by ASM feature
fixups that will nop KUAP enabling instructions out in the unlikely
case KUAP is disabled at boottime.
Leave it as is on book3s/64 for now, it will be handled later when
objtool is activated on PPC64.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/671948788024fd890ec4ed175bc332dab8664ea5.1689091022.git.christophe.leroy@csgroup.eu
Objtool reports following warnings:
arch/powerpc/kernel/signal_32.o: warning: objtool:
__prevent_user_access.constprop.0+0x4 (.text+0x4):
redundant UACCESS disable
arch/powerpc/kernel/signal_32.o: warning: objtool: user_access_begin+0x2c
(.text+0x4c): return with UACCESS enabled
arch/powerpc/kernel/signal_32.o: warning: objtool: handle_rt_signal32+0x188
(.text+0x360): call to __prevent_user_access.constprop.0() with UACCESS enabled
arch/powerpc/kernel/signal_32.o: warning: objtool: handle_signal32+0x150
(.text+0x4d4): call to __prevent_user_access.constprop.0() with UACCESS enabled
This is due to some KUAP enabling/disabling functions being outline
allthough they are marked inline. Use __always_inline instead.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/ca5e50ddbec3867db5146ebddbc9a1dc0e443bc8.1689091022.git.christophe.leroy@csgroup.eu
On book3s/32 KUAP is performed at segment level. At the moment,
when enabling userspace access, only current segment is modified.
Then if a write is performed on another user segment, a fault is
taken and all other user segments get enabled for userspace
access. This then require special attention when disabling
userspace access.
Having a userspace write access crossing a segment boundary is
unlikely. Having a userspace write access crossing a segment boundary
back and forth is even more unlikely. So, instead of enabling
userspace access on all segments when a write fault occurs, just
change which segment has userspace access enabled in order to
eliminate the case when more than one segment has userspace access
enabled. That simplifies userspace access deactivation.
There is however a corner case which is even more unlikely but has
to be handled anyway: an unaligned access which is crossing a
segment boundary. That would definitely require at least having
userspace access enabled on the two segments. To avoid complicating
the likely case for a so unlikely happening, handle such situation
like an alignment exception and emulate the store.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/8de8580513c1a6e880bad1ba9a69d3efad3d4fa5.1689091022.git.christophe.leroy@csgroup.eu
All but book3s/64 use a static branch key for disabling kuap.
book3s/64 uses an mmu feature.
Refactor all targets to use MMU_FTR_KUAP like book3s/64.
For PPC32 that implies updating mmu features fixups once KUAP
has been initialised.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/6b3d7c977bad73378ea368bc6818e9c94ea95ab0.1689091022.git.christophe.leroy@csgroup.eu
kuep_is_disabled() was introduced by commit 91bb30822a ("powerpc/32s:
Refactor update of user segment registers") but then all users but one
were removed by commit 526d4a4c77 ("powerpc/32s: Do kuep_lock() and
kuep_unlock() in assembly").
Fold kuep_is_disabled() into init_new_context() which is its only user.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/b2247147c0a8c830ac82966451647850df4a64da.1689091022.git.christophe.leroy@csgroup.eu
Add kuap_lock() and call it when entering interrupts from user.
It is called kuap_lock() as it is similar to kuap_save_and_lock()
without the save.
However book3s/32 already have a kuap_lock(). Rename it
kuap_lock_addr().
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4437e2deb9f6f549f7089d45e9c6f96a7e77905a.1634627931.git.christophe.leroy@csgroup.eu
Make the following functions generic to all platforms.
- bad_kuap_fault()
- kuap_assert_locked()
- kuap_save_and_lock() (PPC32 only)
- kuap_kernel_restore()
- kuap_get_and_assert_locked()
And for all platforms except book3s/64
- allow_user_access()
- prevent_user_access()
- prevent_user_access_return()
- restore_user_access()
Prepend __ in front of the name of platform specific ones.
For now the generic just calls the platform specific, but
next patch will move redundant parts of specific functions
into the generic one.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eaef143a8dae7288cd34565ffa7b49c16aee1ec3.1634627931.git.christophe.leroy@csgroup.eu
When interrupt and syscall entries where converted to C, KUEP locking
and unlocking was also converted. It improved performance by unrolling
the loop, and allowed easily implementing boot time deactivation of
KUEP.
However, null_syscall selftest shows that KUEP is still heavy
(361 cycles with KUEP, 212 cycles without).
A way to improve more is to group 'mtsr's together, instead of
repeating 'addi' + 'mtsr' several times.
In order to do that, more registers need to be available. In C, GCC
will always be able to provide the requested number of registers, but
at the cost of saving some data on the stack, which is counter
performant here.
So let's do it in assembly, when we have full control of which
register can be used. It also has the advantage of locking earlier
and unlocking later and it helps GCC generating less tricky code.
The only drawback is to make boot time deactivation less straight
forward and require 'hand' instruction patching.
Group 'mtsr's by 4.
With this change, null_syscall selftest reports 336 cycles. Without
the change it was 361 cycles, that's a 7% reduction.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/115cb279e9b9948dfd93a065e047081c59e3a2a6.1634627931.git.christophe.leroy@csgroup.eu
At interrupt exit, kuap_kernel_restore() calls kuap_unlock() with the
value contained in regs->kuap. However, when regs->kuap contains
0xffffffff it means that KUAP was not unlocked so calling kuap_unlock()
is unrelevant and results in jeopardising the contents of kernel space
segment registers.
So check that regs->kuap doesn't contain KUAP_NONE before calling
kuap_unlock(). In the meantime it also means that if KUAP has not
been correcly locked back at interrupt exit, it must be locked
before continuing. This is done by checking the content of
current->thread.kuap which was returned by kuap_get_and_assert_locked()
Fixes: 16132529ce ("powerpc/32s: Rework Kernel Userspace Access Protection")
Reported-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0d0c4d0f050a637052287c09ba521bad960a2790.1631715131.git.christophe.leroy@csgroup.eu
Commit b5efec00b6 ("powerpc/32s: Move KUEP locking/unlocking in C")
removed the 'isync' instruction after adding/removing NX bit in user
segments. The reasoning behind this change was that when setting the
NX bit we don't mind it taking effect with delay as the kernel never
executes text from userspace, and when clearing the NX bit this is
to return to userspace and then the 'rfi' should synchronise the
context.
However, it looks like on book3s/32 having a hash page table, at least
on the G3 processor, we get an unexpected fault from userspace, then
this is followed by something wrong in the verification of MSR_PR
at end of another interrupt.
This is fixed by adding back the removed isync() following update
of NX bit in user segment registers. Only do it for cores with an
hash table, as 603 cores don't exhibit that problem and the two isync
increase ./null_syscall selftest by 6 cycles on an MPC 832x.
First problem: unexpected WARN_ON() for mysterious PROTFAULT
WARNING: CPU: 0 PID: 1660 at arch/powerpc/mm/fault.c:354 do_page_fault+0x6c/0x5b0
Modules linked in:
CPU: 0 PID: 1660 Comm: Xorg Not tainted 5.13.0-pmac-00028-gb3c15b60339a #40
NIP: c001b5c8 LR: c001b6f8 CTR: 00000000
REGS: e2d09e40 TRAP: 0700 Not tainted (5.13.0-pmac-00028-gb3c15b60339a)
MSR: 00021032 <ME,IR,DR,RI> CR: 42d04f30 XER: 20000000
GPR00: c000424c e2d09f00 c301b680 e2d09f40 0000001e 42000000 00cba028 00000000
GPR08: 08000000 48000010 c301b680 e2d09f30 22d09f30 00c1fff0 00cba000 a7b7ba4c
GPR16: 00000031 00000000 00000000 00000000 00000000 00000000 a7b7b0d0 00c5c010
GPR24: a7b7b64c a7b7d2f0 00000004 00000000 c1efa6c0 00cba02c 00000300 e2d09f40
NIP [c001b5c8] do_page_fault+0x6c/0x5b0
LR [c001b6f8] do_page_fault+0x19c/0x5b0
Call Trace:
[e2d09f00] [e2d09f04] 0xe2d09f04 (unreliable)
[e2d09f30] [c000424c] DataAccess_virt+0xd4/0xe4
--- interrupt: 300 at 0xa7a261dc
NIP: a7a261dc LR: a7a253bc CTR: 00000000
REGS: e2d09f40 TRAP: 0300 Not tainted (5.13.0-pmac-00028-gb3c15b60339a)
MSR: 0000d032 <EE,PR,ME,IR,DR,RI> CR: 228428e2 XER: 20000000
DAR: 00cba02c DSISR: 42000000
GPR00: a7a27448 afa6b0e0 a74c35c0 a7b7b614 0000001e a7b7b614 00cba028 00000000
GPR08: 00020fd9 00000031 00cb9ff8 a7a273b0 220028e2 00c1fff0 00cba000 a7b7ba4c
GPR16: 00000031 00000000 00000000 00000000 00000000 00000000 a7b7b0d0 00c5c010
GPR24: a7b7b64c a7b7d2f0 00000004 00000002 0000001e a7b7b614 a7b7aff4 00000030
NIP [a7a261dc] 0xa7a261dc
LR [a7a253bc] 0xa7a253bc
--- interrupt: 300
Instruction dump:
7c4a1378 810300a0 75278410 83820298 83a300a4 553b018c 551e0036 4082038c
2e1b0000 40920228 75280800 41820220 <0fe00000> 3b600000 41920214 81420594
Second problem: MSR PR is seen unset allthough the interrupt frame shows it set
kernel BUG at arch/powerpc/kernel/interrupt.c:458!
Oops: Exception in kernel mode, sig: 5 [#1]
BE PAGE_SIZE=4K MMU=Hash SMP NR_CPUS=2 PowerMac
Modules linked in:
CPU: 0 PID: 1660 Comm: Xorg Tainted: G W 5.13.0-pmac-00028-gb3c15b60339a #40
NIP: c0011434 LR: c001629c CTR: 00000000
REGS: e2d09e70 TRAP: 0700 Tainted: G W (5.13.0-pmac-00028-gb3c15b60339a)
MSR: 00029032 <EE,ME,IR,DR,RI> CR: 42d09f30 XER: 00000000
GPR00: 00000000 e2d09f30 c301b680 e2d09f40 83440000 c44d0e68 e2d09e8c 00000000
GPR08: 00000002 00dc228a 00004000 e2d09f30 22d09f30 00c1fff0 afa6ceb4 00c26144
GPR16: 00c25fb8 00c26140 afa6ceb8 90000000 00c944d8 0000001c 00000000 00200000
GPR24: 00000000 000001fb afa6d1b4 00000001 00000000 a539a2a0 a530fd80 00000089
NIP [c0011434] interrupt_exit_kernel_prepare+0x10/0x70
LR [c001629c] interrupt_return+0x9c/0x144
Call Trace:
[e2d09f30] [c000424c] DataAccess_virt+0xd4/0xe4 (unreliable)
--- interrupt: 300 at 0xa09be008
NIP: a09be008 LR: a09bdfe8 CTR: a09bdfc0
REGS: e2d09f40 TRAP: 0300 Tainted: G W (5.13.0-pmac-00028-gb3c15b60339a)
MSR: 0000d032 <EE,PR,ME,IR,DR,RI> CR: 420028e2 XER: 20000000
DAR: a539a308 DSISR: 0a000000
GPR00: a7b90d50 afa6b2d0 a74c35c0 a0a8b690 a0a8b698 a5365d70 a4fa82a8 00000004
GPR08: 00000000 a09bdfc0 00000000 a5360000 a09bde7c 00c1fff0 afa6ceb4 00c26144
GPR16: 00c25fb8 00c26140 afa6ceb8 90000000 00c944d8 0000001c 00000000 00200000
GPR24: 00000000 000001fb afa6d1b4 00000001 00000000 a539a2a0 a530fd80 00000089
NIP [a09be008] 0xa09be008
LR [a09bdfe8] 0xa09bdfe8
--- interrupt: 300
Instruction dump:
80010024 83e1001c 7c0803a6 4bffff80 3bc00800 4bffffd0 486b42fd 4bffffcc
81430084 71480002 41820038 554a0462 <0f0a0000> 80620060 74630001 40820034
Fixes: b5efec00b6 ("powerpc/32s: Move KUEP locking/unlocking in C")
Cc: stable@vger.kernel.org # v5.13+
Reported-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4856f5574906e2aec0522be17bf3848a22b2cd0b.1629269345.git.christophe.leroy@csgroup.eu
PPC64 uses MMU features to enable/disable KUAP at boot time.
But feature fixups are applied way too early on PPC32.
Now that all KUAP related actions are in C following the
conversion of KUAP initial setup and context switch in C,
static branches can be used to enable/disable KUAP.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Export disable_kuap_key to fix build errors]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/cd79e8008455fba5395d099f9bb1305c039b931c.1622708530.git.christophe.leroy@csgroup.eu
PPC64 uses MMU features to enable/disable KUEP at boot time.
But feature fixups are applied way too early on PPC32.
Now that all KUEP related actions are in C following the
conversion of KUEP initial setup and context switch in C,
static branches can be used to enable/disable KUEP.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7745a2c3a08ec46302920a3f48d1cb9b5469dbbb.1622708530.git.christophe.leroy@csgroup.eu
switch_mmu_context() does things that can easily be done in C.
For updating user segments, we have update_user_segments().
As mentionned in commit b5efec00b6 ("powerpc/32s: Move KUEP
locking/unlocking in C"), update_user_segments() has the loop
unrolled which is a significant performance gain.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/05c0875ad8220c03452c3a334946e207c6ca04d6.1622708530.git.christophe.leroy@csgroup.eu
KUEP implements the update of user segment registers.
Move it into mmu-hash.h in order to use it from other places.
And inline kuep_lock() and kuep_unlock(). Inlining kuep_lock() is
important for system_call_exception(), otherwise system_call_exception()
has to save into stack the system call parameters that are used just
after, and doing that takes more instructions than kuep_lock() itself.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/24591ca480d14a62ef910e38a5273d551262c4a2.1622708530.git.christophe.leroy@csgroup.eu
In most cases, kuap_update_sr() will update a single segment
register.
We know that first update will always be done, if there is no
segment register to update at all, kuap_update_sr() is not
called.
Avoid recurring calculations and tests in that case.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/848f18d213b8341939add7302dc4ef80cc7a12e3.1620307636.git.christophe.leroy@csgroup.eu
In preparation of porting PPC32 to C syscall entry/exit,
create C version of kuap_save_and_lock() and kuap_user_restore() and
kuap_kernel_restore() and kuap_assert_locked() and
kuap_get_and_assert_locked() on book3s/32.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2be8fb729da4a0f9863b25e1b9d547174fcd5056.1615552867.git.christophe.leroy@csgroup.eu
Function names should tell what the function does, not how.
mfsrin() and mtsrin() are read/writing segment registers.
They are called that way because they are using mfsrin and mtsrin
instructions, but it doesn't matter for the caller.
In preparation of following patch, change their name to mfsr() and mtsr()
in order to make it obvious they manipulate segment registers without
messing up with how they do it.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f92d99f4349391b77766745900231aa880a0efb5.1612612022.git.christophe.leroy@csgroup.eu
This partially reverts commit eb232b1624 ("powerpc/book3s64/kuap: Improve
error reporting with KUAP") and update the fault handler to print
[ 55.022514] Kernel attempted to access user page (7e6725b70000) - exploit attempt? (uid: 0)
[ 55.022528] BUG: Unable to handle kernel data access on read at 0x7e6725b70000
[ 55.022533] Faulting instruction address: 0xc000000000e8b9bc
[ 55.022540] Oops: Kernel access of bad area, sig: 11 [#1]
....
when the kernel access userspace address without unlocking AMR.
bad_kuap_fault() is added as part of commit 5e5be3aed2 ("powerpc/mm: Detect
bad KUAP faults") to catch userspace access incorrectly blocked by AMR. Hence
retain the full stack dump there even with hash translation. Also, add a comment
explaining the difference between hash and radix.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201208031539.84878-1-aneesh.kumar@linux.ibm.com
With hash translation use DSISR_KEYFAULT to identify a wrong access.
With Radix we look at the AMR value and type of fault.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201127044424.40686-17-aneesh.kumar@linux.ibm.com
Merge our fixes branch from this cycle. It contains several important
fixes we need in next for testing purposes, and also some that will
conflict with upcoming changes.
Implement user_access_save() and user_access_restore()
On 8xx and radix:
- On save, get the value of the associated special register then
prevent user access.
- On restore, set back the saved value to the associated special
register.
On book3s/32:
- On save, get the value stored in current->thread.kuap and prevent
user access.
- On restore, regenerate address range from the stored value and
reopen read/write access for that range.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/54f2f74938006b33c55a416674807b42ef222068.1579866752.git.christophe.leroy@c-s.fr
In preparation of implementing user_access_begin and friends
on powerpc, the book3s/32 version of prevent_user_access() need
to be prepared for user_access_end().
user_access_end() doesn't provide the address and size which
were passed to user_access_begin(), required by prevent_user_access()
to know which segment to modify.
The list of segments which where unprotected by allow_user_access()
are available in current->kuap. But we don't want prevent_user_access()
to read this all the time, especially everytime it is 0 (for instance
because the access was not a write access).
Implement a special direction named KUAP_CURRENT. In this case only,
the addr and end are retrieved from current->kuap.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/55bcc1f25d8200892a31f67a0b024ff3b816c3cc.1579866752.git.christophe.leroy@c-s.fr
NULL addr is a user address. Don't waste time checking it. If
someone tries to access it, it will SIGFAULT the same way as for
address 1, so no need to make it special.
The special case is when not doing a write, in that case we want
to drop the entire function. This is now handled by 'dir' param
and not by the nulity of 'to' anymore.
Also make beginning of prevent_user_access() similar
to beginning of allow_user_access(), and tell the compiler
that writing in kernel space or with a 0 length is unlikely
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/85e971223dfe6ace734637db1841678939a76155.1579866752.git.christophe.leroy@c-s.fr
At the moment, bad_kuap_fault() reports a fault only if a bad access
to userspace occurred while access to userspace was not granted.
But if a fault occurs for a write outside the allowed userspace
segment(s) that have been unlocked, bad_kuap_fault() fails to
detect it and the kernel loops forever in do_page_fault().
Fix it by checking that the accessed address is within the allowed
range.
Fixes: a68c31fc01 ("powerpc/32s: Implement Kernel Userspace Access Protection")
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f48244e9485ada0a304ed33ccbb8da271180c80d.1579866752.git.christophe.leroy@c-s.fr
Make sure starting addr is aligned to segment boundary so that when
incrementing the segment, the starting address of the new segment is
below the end address. Otherwise the last segment might get missed.
Fixes: a68c31fc01 ("powerpc/32s: Implement Kernel Userspace Access Protection")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/067a1b09f15f421d40797c2d04c22d4049a1cee8.1571071875.git.christophe.leroy@c-s.fr
This patch implements Kernel Userspace Access Protection for
book3s/32.
Due to limitations of the processor page protection capabilities,
the protection is only against writing. read protection cannot be
achieved using page protection.
The previous patch modifies the page protection so that RW user
pages are RW for Key 0 and RO for Key 1, and it sets Key 0 for
both user and kernel.
This patch changes userspace segment registers are set to Ku 0
and Ks 1. When kernel needs to write to RW pages, the associated
segment register is then changed to Ks 0 in order to allow write
access to the kernel.
In order to avoid having the read all segment registers when
locking/unlocking the access, some data is kept in the thread_struct
and saved on stack on exceptions. The field identifies both the
first unlocked segment and the first segment following the last
unlocked one. When no segment is unlocked, it contains value 0.
As the hash_page() function is not able to easily determine if a
protfault is due to a bad kernel access to userspace, protfaults
need to be handled by handle_page_fault when KUAP is set.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Drop allow_read/write_to/from_user() as they're now in kup.h,
and adapt allow_user_access() to do nothing when to == NULL]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
To implement Kernel Userspace Execution Prevention, this patch
sets NX bit on all user segments on kernel entry and clears NX bit
on all user segments on kernel exit.
Note that powerpc 601 doesn't have the NX bit, so KUEP will not
work on it. A warning is displayed at startup.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
