x86,rethook: Fix arch_rethook_trampoline() to generate a complete pt_regs

Currently arch_rethook_trampoline() generates an almost complete
pt_regs on-stack, everything except regs->ss that is, that currently
points to the fake return address, which is not a valid segment
descriptor.

Since interpretation of regs->[sb]p should be done in the context of
regs->ss, and we have code actually doing that (see
arch/x86/lib/insn-eval.c for instance), complete the job by also
pushing ss.

This ensures that anybody who does do look at regs->ss doesn't
mysteriously malfunction, avoiding much future pain.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Link: https://lore.kernel.org/bpf/164826164851.2455864.17272661073069737350.stgit@devnote2

Author: Peter Zijlstra

arch/x86/kernel/rethook.c

@@ -29,29 +29,31 @@ asm(
 	/* Push a fake return address to tell the unwinder it's a rethook. */
 	"	pushq $arch_rethook_trampoline\n"
 	UNWIND_HINT_FUNC
-	/* Save the 'sp - 8', this will be fixed later. */
+	"       pushq $" __stringify(__KERNEL_DS) "\n"
+	/* Save the 'sp - 16', this will be fixed later. */
 	"	pushq %rsp\n"
 	"	pushfq\n"
 	SAVE_REGS_STRING
 	"	movq %rsp, %rdi\n"
 	"	call arch_rethook_trampoline_callback\n"
 	RESTORE_REGS_STRING
-	/* In the callback function, 'regs->flags' is copied to 'regs->sp'. */
-	"	addq $8, %rsp\n"
+	/* In the callback function, 'regs->flags' is copied to 'regs->ss'. */
+	"	addq $16, %rsp\n"
 	"	popfq\n"
 #else
 	/* Push a fake return address to tell the unwinder it's a rethook. */
 	"	pushl $arch_rethook_trampoline\n"
 	UNWIND_HINT_FUNC
-	/* Save the 'sp - 4', this will be fixed later. */
+	"	pushl %ss\n"
+	/* Save the 'sp - 8', this will be fixed later. */
 	"	pushl %esp\n"
 	"	pushfl\n"
 	SAVE_REGS_STRING
 	"	movl %esp, %eax\n"
 	"	call arch_rethook_trampoline_callback\n"
 	RESTORE_REGS_STRING
-	/* In the callback function, 'regs->flags' is copied to 'regs->sp'. */
-	"	addl $4, %esp\n"
+	/* In the callback function, 'regs->flags' is copied to 'regs->ss'. */
+	"	addl $8, %esp\n"
 	"	popfl\n"
 #endif
 	ASM_RET
@@ -73,8 +75,8 @@ __used __visible void arch_rethook_trampoline_callback(struct pt_regs *regs)
 #endif
 	regs->ip = (unsigned long)&arch_rethook_trampoline;
 	regs->orig_ax = ~0UL;
-	regs->sp += sizeof(long);
-	frame_pointer = &regs->sp + 1;
+	regs->sp += 2*sizeof(long);
+	frame_pointer = (long *)(regs + 1);
 
 	/*
 	 * The return address at 'frame_pointer' is recovered by the
@@ -84,10 +86,10 @@ __used __visible void arch_rethook_trampoline_callback(struct pt_regs *regs)
 	rethook_trampoline_handler(regs, (unsigned long)frame_pointer);
 
 	/*
-	 * Copy FLAGS to 'pt_regs::sp' so that arch_rethook_trapmoline()
+	 * Copy FLAGS to 'pt_regs::ss' so that arch_rethook_trapmoline()
 	 * can do RET right after POPF.
 	 */
-	regs->sp = regs->flags;
+	*(unsigned long *)&regs->ss = regs->flags;
 }
 NOKPROBE_SYMBOL(arch_rethook_trampoline_callback);
 
@@ -105,7 +107,7 @@ STACK_FRAME_NON_STANDARD_FP(arch_rethook_trampoline);
 void arch_rethook_fixup_return(struct pt_regs *regs,
 			       unsigned long correct_ret_addr)
 {
-	unsigned long *frame_pointer = &regs->sp + 1;
+	unsigned long *frame_pointer = (void *)(regs + 1);
 
 	/* Replace fake return address with real one. */
 	*frame_pointer = correct_ret_addr;