stackprotector: actually use get_random_canary()

The RNG always mixes in the Linux version extremely early in boot. It
also always includes a cycle counter, not only during early boot, but
each and every time it is invoked prior to being fully initialized.
Together, this means that the use of additional xors inside of the
various stackprotector.h files is superfluous and over-complicated.
Instead, we can get exactly the same thing, but better, by just calling
`get_random_canary()`.

Acked-by: Guo Ren <guoren@kernel.org> # for csky
Acked-by: Catalin Marinas <catalin.marinas@arm.com> # for arm64
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

powerpc/64: add stack protector support

On PPC64, as register r13 points to the paca_struct at all time,
this patch adds a copy of the canary there, which is copied at
task_switch.
That new canary is then used by using the following GCC options:
-mstack-protector-guard=tls
-mstack-protector-guard-reg=r13
-mstack-protector-guard-offset=offsetof(struct paca_struct, canary))

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

powerpc/32: add stack protector support

This functionality was tentatively added in the past
(commit 6533b7c16ee5 ("powerpc: Initial stack protector
(-fstack-protector) support")) but had to be reverted
(commit f2574030b0e3 ("powerpc: Revert the initial stack
protector support") because of GCC implementing it differently
whether it had been built with libc support or not.

Now, GCC offers the possibility to manually set the
stack-protector mode (global or tls) regardless of libc support.

This time, the patch selects HAVE_STACKPROTECTOR only if
-mstack-protector-guard=tls is supported by GCC.

On PPC32, as register r2 points to current task_struct at
all time, the stack_canary located inside task_struct can be
used directly by using the following GCC options:
-mstack-protector-guard=tls
-mstack-protector-guard-reg=r2
-mstack-protector-guard-offset=offsetof(struct task_struct, stack_canary))

The protector is disabled for prom_init and bootx_init as
it is too early to handle it properly.

$ echo CORRUPT_STACK > /sys/kernel/debug/provoke-crash/DIRECT
[ 134.943666] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: lkdtm_CORRUPT_STACK+0x64/0x64
[ 134.943666]
[ 134.955414] CPU: 0 PID: 283 Comm: sh Not tainted 4.18.0-s3k-dev-12143-ga3272be41209 #835
[ 134.963380] Call Trace:
[ 134.965860] [c6615d60] [c001f76c] panic+0x118/0x260 (unreliable)
[ 134.971775] [c6615dc0] [c001f654] panic+0x0/0x260
[ 134.976435] [c6615dd0] [c032c368] lkdtm_CORRUPT_STACK_STRONG+0x0/0x64
[ 134.982769] [c6615e00] [ffffffff] 0xffffffff

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

powerpc: Revert the initial stack protector support

Unfortunately the stack protector support we merged recently only works
on some toolchains. If the toolchain is built without glibc support
everything works fine, but if glibc is built then it leads to a panic
at boot.

The solution is not rc5 material, so revert the support for now. This
reverts commits:

6533b7c16ee5 ("powerpc: Initial stack protector (-fstack-protector) support")
902e06eb86cd ("powerpc/32: Change the stack protector canary value per task")

Fixes: 6533b7c16ee5 ("powerpc: Initial stack protector (-fstack-protector) support")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

powerpc: Initial stack protector (-fstack-protector) support

Partialy copied from commit c743f38013aef ("ARM: initial stack protector
(-fstack-protector) support")

This is the very basic stuff without the changing canary upon
task switch yet. Just the Kconfig option and a constant canary
value initialized at boot time.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>