powerpc/livepatch: Add live patching support on ppc64le

Add the kconfig logic & assembly support for handling live patched
functions. This depends on DYNAMIC_FTRACE_WITH_REGS, which in turn
depends on the new -mprofile-kernel ftrace ABI, which is only supported
currently on ppc64le.

Live patching is handled by a special ftrace handler. This means it runs
from ftrace_caller(). The live patch handler modifies the NIP so as to
redirect the return from ftrace_caller() to the new patched function.

However there is one particularly tricky case we need to handle.

If a function A calls another function B, and it is known at link time
that they share the same TOC, then A will not save or restore its TOC,
and will call the local entry point of B.

When we live patch B, we replace it with a new function C, which may
not have the same TOC as A. At live patch time it's too late to modify A
to do the TOC save/restore, so the live patching code must interpose
itself between A and C, and do the TOC save/restore that A omitted.

An additionaly complication is that the livepatch code can not create a
stack frame in order to save the TOC. That is because if C takes > 8
arguments, or is varargs, A will have written the arguments for C in
A's stack frame.

To solve this, we introduce a "livepatch stack" which grows upward from
the base of the regular stack, and is used to store the TOC & LR when
calling a live patched function.

When the patched function returns, we retrieve the real LR & TOC from
the livepatch stack, restore them, and pop the livepatch "stack frame".

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Torsten Duwe <duwe@suse.de>
Reviewed-by: Balbir Singh <bsingharora@gmail.com>

3 files changed, 104 insertions, 0 deletions

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 91da283cd658..926c0eafbee3 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -159,6 +159,7 @@ config PPC
         select ARCH_HAS_DEVMEM_IS_ALLOWED
         select HAVE_ARCH_SECCOMP_FILTER
         select ARCH_HAS_UBSAN_SANITIZE_ALL
+        select HAVE_LIVEPATCH if HAVE_DYNAMIC_FTRACE_WITH_REGS
 
 config GENERIC_CSUM
         def_bool CPU_LITTLE_ENDIAN
@@ -1110,3 +1111,5 @@ config PPC_LIB_RHEAP
         bool
 
 source "arch/powerpc/kvm/Kconfig"
+
+source "kernel/livepatch/Kconfig"
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 07cebc3514f3..723efac2d917 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -86,6 +86,10 @@ int main(void)
         DEFINE(KSP_LIMIT, offsetof(struct thread_struct, ksp_limit));
 #endif /* CONFIG_PPC64 */
 
+#ifdef CONFIG_LIVEPATCH
+        DEFINE(TI_livepatch_sp, offsetof(struct thread_info, livepatch_sp));
+#endif
+
         DEFINE(KSP, offsetof(struct thread_struct, ksp));
         DEFINE(PT_REGS, offsetof(struct thread_struct, regs));
 #ifdef CONFIG_BOOKE
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index ec7f8aada697..47dbede3bddd 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -20,6 +20,7 @@
 
 #include <linux/errno.h>
 #include <linux/err.h>
+#include <linux/magic.h>
 #include <asm/unistd.h>
 #include <asm/processor.h>
 #include <asm/page.h>
@@ -1224,6 +1225,9 @@ _GLOBAL(ftrace_caller)
         addi    r3,r3,function_trace_op@toc@l
         ld      r5,0(r3)
 
+#ifdef CONFIG_LIVEPATCH
+        mr      r14,r7          /* remember old NIP */
+#endif
         /* Calculate ip from nip-4 into r3 for call below */
         subi    r3, r7, MCOUNT_INSN_SIZE
 
@@ -1248,6 +1252,9 @@ ftrace_call:
         /* Load ctr with the possibly modified NIP */
         ld      r3, _NIP(r1)
         mtctr   r3
+#ifdef CONFIG_LIVEPATCH
+        cmpd    r14,r3          /* has NIP been altered? */
+#endif
 
         /* Restore gprs */
         REST_8GPRS(0,r1)
@@ -1265,6 +1272,11 @@ ftrace_call:
         ld      r0, LRSAVE(r1)
         mtlr    r0
 
+#ifdef CONFIG_LIVEPATCH
+        /* Based on the cmpd above, if the NIP was altered handle livepatch */
+        bne-    livepatch_handler
+#endif
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
         stdu    r1, -112(r1)
 .globl ftrace_graph_call
@@ -1281,6 +1293,91 @@ _GLOBAL(ftrace_graph_stub)
 
 _GLOBAL(ftrace_stub)
         blr
+
+#ifdef CONFIG_LIVEPATCH
+        /*
+         * This function runs in the mcount context, between two functions. As
+         * such it can only clobber registers which are volatile and used in
+         * function linkage.
+         *
+         * We get here when a function A, calls another function B, but B has
+         * been live patched with a new function C.
+         *
+         * On entry:
+         *  - we have no stack frame and can not allocate one
+         *  - LR points back to the original caller (in A)
+         *  - CTR holds the new NIP in C
+         *  - r0 & r12 are free
+         *
+         * r0 can't be used as the base register for a DS-form load or store, so
+         * we temporarily shuffle r1 (stack pointer) into r0 and then put it back.
+         */
+livepatch_handler:
+        CURRENT_THREAD_INFO(r12, r1)
+
+        /* Save stack pointer into r0 */
+        mr      r0, r1
+
+        /* Allocate 3 x 8 bytes */
+        ld      r1, TI_livepatch_sp(r12)
+        addi    r1, r1, 24
+        std     r1, TI_livepatch_sp(r12)
+
+        /* Save toc & real LR on livepatch stack */
+        std     r2,  -24(r1)
+        mflr    r12
+        std     r12, -16(r1)
+
+        /* Store stack end marker */
+        lis     r12, STACK_END_MAGIC@h
+        ori     r12, r12, STACK_END_MAGIC@l
+        std     r12, -8(r1)
+
+        /* Restore real stack pointer */
+        mr      r1, r0
+
+        /* Put ctr in r12 for global entry and branch there */
+        mfctr   r12
+        bctrl
+
+        /*
+         * Now we are returning from the patched function to the original
+         * caller A. We are free to use r0 and r12, and we can use r2 until we
+         * restore it.
+         */
+
+        CURRENT_THREAD_INFO(r12, r1)
+
+        /* Save stack pointer into r0 */
+        mr      r0, r1
+
+        ld      r1, TI_livepatch_sp(r12)
+
+        /* Check stack marker hasn't been trashed */
+        lis     r2,  STACK_END_MAGIC@h
+        ori     r2,  r2, STACK_END_MAGIC@l
+        ld      r12, -8(r1)
+1:      tdne    r12, r2
+        EMIT_BUG_ENTRY 1b, __FILE__, __LINE__ - 1, 0
+
+        /* Restore LR & toc from livepatch stack */
+        ld      r12, -16(r1)
+        mtlr    r12
+        ld      r2,  -24(r1)
+
+        /* Pop livepatch stack frame */
+        CURRENT_THREAD_INFO(r12, r0)
+        subi    r1, r1, 24
+        std     r1, TI_livepatch_sp(r12)
+
+        /* Restore real stack pointer */
+        mr      r1, r0
+
+        /* Return to original caller of live patched function */
+        blr
+#endif
+
+
 #else
 _GLOBAL_TOC(_mcount)
         /* Taken from output of objdump from lib64/glibc */