[PATCH v4 2/9] x86/kthread,dumpstack: Set task_pt_regs->cs.RPL=3 for kernel threads

[Andy Lutomirski <luto@kernel.org>]

For kernel threads, task_pt_regs is currently all zeros, a valid user state
(if kernel_execve() has been called), or some combination thereof during
execution of kernel_execve().  If a stack trace is printed, the unwinder
might get confused and treat task_pt_regs as a kernel state.  Indeed,
forcing a stack dump results in an attempt to display _kernel_ code bytes
from a bogus address at the very top of kernel memory.

Fix the confusion by setting cs=3 so that user_mode(task_pt_regs(...)) ==
true for kernel threads.

Also improve the error when failing to fetch code bytes to show which type
of fetch failed.  This makes it much easier to understand what's happening.

Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/kernel/dumpstack.c |  4 ++--
 arch/x86/kernel/process.c   | 13 +++++++++++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 55cf3c8325c6..9b7b69bb12e5 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -128,8 +128,8 @@ void show_opcodes(struct pt_regs *regs, const char *loglvl)
 		/* No access to the user space stack of other tasks. Ignore. */
 		break;
 	default:
-		printk("%sCode: Unable to access opcode bytes at RIP 0x%lx.\n",
-		       loglvl, prologue);
+		printk("%sCode: Unable to access %s opcode bytes at RIP 0x%lx.\n",
+		       loglvl, user_mode(regs) ? "user" : "kernel", prologue);
 		break;
 	}
 }
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 145a7ac0c19a..f6f16df04cb9 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -163,6 +163,19 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
 	/* Kernel thread ? */
 	if (unlikely(p->flags & PF_KTHREAD)) {
 		memset(childregs, 0, sizeof(struct pt_regs));
+
+		/*
+		 * Even though there is no real user state here, these
+		 * are were user regs belong, and kernel_execve() will
+		 * overwrite them with user regs.  Put an obviously
+		 * invalid value that nonetheless causes user_mode(regs)
+		 * to return true in CS.
+		 *
+		 * This also prevents the unwinder from thinking that there
+		 * is invalid kernel state at the top of the stack.
+		 */
+		childregs->cs = 3;
+
 		kthread_frame_init(frame, sp, arg);
 		return 0;
 	}
-- 
2.30.2