[PATCH 5.15 07/26] Revert "proc/wchan: use printk format instead of lookup_symbol_name()"

[Greg Kroah-Hartman]

From: Kees Cook <keescook@xxxxxxxxxxxx>

commit 54354c6a9f7fd5572d2b9ec108117c4f376d4d23 upstream.

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@xxxxxxxxx>
Reported-by: Vito Caputo <vcaputo@xxxxxxxxxxx>
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20211008111626.090829198@xxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 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 */