Re: [PATCH v4 12/20] Revert "powerpc/syscall: Save r3 in regs->orig_r3"

Save caller's original r3 state to the kernel stackframe before entering
system_call_exception. This allows for user registers to be cleared by
the time system_call_exception is entered, reducing the influence of
user registers on speculation within the kernel.

Prior to this commit, orig_r3 was saved at the beginning of
system_call_exception. Instead, save orig_r3 while the user value is
still live in r3.

Also replicate this early save in 32-bit. A similar save was removed in
commit 6f76a01173cc ("powerpc/syscall: implement system call entry/exit logic in C for PPC32")
when 32-bit adopted system_call_exception. Revert its removal of orig_r3
saves.

Signed-off-by: Rohan McLure <redacted>
---
V2 -> V3: New commit.
---
 arch/powerpc/kernel/entry_32.S     | 1 +
 arch/powerpc/kernel/interrupt_64.S | 2 ++
 arch/powerpc/kernel/syscall.c      | 1 -
 3 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 1d599df6f169..44dfce9a60c5 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S

@@ -101,6 +101,7 @@ __kuep_unlock:
 
 	.globl	transfer_to_syscall
 transfer_to_syscall:
+	stw	r3, ORIG_GPR3(r1)
 	stw	r11, GPR1(r1)
 	stw	r11, 0(r1)
 	mflr	r12

diff --git a/arch/powerpc/kernel/interrupt_64.S b/arch/powerpc/kernel/interrupt_64.S
index ce25b28cf418..71d2d9497283 100644
--- a/arch/powerpc/kernel/interrupt_64.S
+++ b/arch/powerpc/kernel/interrupt_64.S

@@ -91,6 +91,7 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_\name)
 	li	r11,\trapnr
 	std	r11,_TRAP(r1)
 	std	r12,_CCR(r1)
+	std	r3,ORIG_GPR3(r1)
 	addi	r10,r1,STACK_FRAME_OVERHEAD
 	ld	r11,exception_marker@toc(r2)
 	std	r11,-16(r10)		/* "regshere" marker */

@@ -275,6 +276,7 @@ END_BTB_FLUSH_SECTION
 	std	r10,_LINK(r1)
 	std	r11,_TRAP(r1)
 	std	r12,_CCR(r1)
+	std	r3,ORIG_GPR3(r1)
 	addi	r10,r1,STACK_FRAME_OVERHEAD
 	ld	r11,exception_marker@toc(r2)
 	std	r11,-16(r10)		/* "regshere" marker */

diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index 81ace9e8b72b..64102a64fd84 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c

@@ -25,7 +25,6 @@ notrace long system_call_exception(long r3, long r4, long r5,
 	kuap_lock();
 
 	add_random_kstack_offset();
-	regs->orig_gpr3 = r3;
 
 	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
 		BUG_ON(irq_soft_mask_return() != IRQS_ALL_DISABLED);

-- 
2.34.1