linux-kernel - [PATCH 1/7] Revert "proc/wchan: use printk format instead of lookup_symbol

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Date:   Fri, 08 Oct 2021 13:15:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     keescook@...omium.org, jannh@...gle.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        vcaputo@...garu.com, mingo@...hat.com, juri.lelli@...hat.com,
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        stable@...r.kernel.org
Subject: [PATCH 1/7] Revert "proc/wchan: use printk format instead of lookup_symbol_name()"

From: Kees Cook <keescook@...omium.org>

This reverts commit 152c432b128cb043fc107e8f211195fe94b2159c.

When a kernel address couldn't be symbolized for /proc/$pid/wchan, it
would leak the raw value, a potential information exposure. This is a
regression compared to the safer pre-v5.12 behavior.

Reported-by: kernel test robot <oliver.sang@...el.com>
Reported-by: Vito Caputo <vcaputo@...garu.com>
Reported-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: stable@...r.kernel.org
---
 fs/proc/base.c |   21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -67,6 +67,7 @@
 #include <linux/mm.h>
 #include <linux/swap.h>
 #include <linux/rcupdate.h>
+#include <linux/kallsyms.h>
 #include <linux/stacktrace.h>
 #include <linux/resource.h>
 #include <linux/module.h>
@@ -386,17 +387,19 @@ static int proc_pid_wchan(struct seq_fil
 			  struct pid *pid, struct task_struct *task)
 {
 	unsigned long wchan;
+	char symname[KSYM_NAME_LEN];
 
-	if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
-		wchan = get_wchan(task);
-	else
-		wchan = 0;
-
-	if (wchan)
-		seq_printf(m, "%ps", (void *) wchan);
-	else
-		seq_putc(m, '0');
+	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
+		goto print0;
 
+	wchan = get_wchan(task);
+	if (wchan && !lookup_symbol_name(wchan, symname)) {
+		seq_puts(m, symname);
+		return 0;
+	}
+
+print0:
+	seq_putc(m, '0');
 	return 0;
 }
 #endif /* CONFIG_KALLSYMS */