[PATCH] proc: Disable /proc/$pid/wchan

[Kees Cook]

The /proc/$pid/wchan file has been broken by default on x86_64 for 4
years now[1]. As this remains a potential leak of either kernel
addresses (when symbolization fails) or limited observation of kernel
function progress, just remove the contents for good.

Unconditionally set the contents to "0" and also mark the wchan
field in /proc/$pid/stat with 0.

This leaves kernel/sched/fair.c as the only user of get_wchan(). But
again, since this was broken for 4 years, was this profiling logic
actually doing anything useful?

[1] https://lore.kernel.org/lkml/20210922001537.4ktg3r2ky3b3r6yp@treble/

Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Vito Caputo <vcaputo@xxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/x86/kernel/process.c |  2 +-
 fs/proc/array.c           | 16 +++++-----------
 fs/proc/base.c            | 16 +---------------
 3 files changed, 7 insertions(+), 27 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 1d9463e3096b..84a4f9f3f0c2 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -937,7 +937,7 @@ unsigned long arch_randomize_brk(struct mm_struct *mm)
 }
 
 /*
- * Called from fs/proc with a reference on @p to find the function
+ * Called from scheduler with a reference on @p to find the function
  * which called into schedule(). This needs to be done carefully
  * because the task might wake up and we might look at a stack
  * changing under us.
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 49be8c8ef555..8a4ecfd901b8 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -452,7 +452,7 @@ int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
 static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 			struct pid *pid, struct task_struct *task, int whole)
 {
-	unsigned long vsize, eip, esp, wchan = 0;
+	unsigned long vsize, eip, esp;
 	int priority, nice;
 	int tty_pgrp = -1, tty_nr = 0;
 	sigset_t sigign, sigcatch;
@@ -540,8 +540,6 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 		unlock_task_sighand(task, &flags);
 	}
 
-	if (permitted && (!whole || num_threads < 2))
-		wchan = get_wchan(task);
 	if (!whole) {
 		min_flt = task->min_flt;
 		maj_flt = task->maj_flt;
@@ -600,16 +598,12 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	seq_put_decimal_ull(m, " ", sigcatch.sig[0] & 0x7fffffffUL);
 
 	/*
-	 * We used to output the absolute kernel address, but that's an
-	 * information leak - so instead we show a 0/1 flag here, to signal
-	 * to user-space whether there's a wchan field in /proc/PID/wchan.
-	 *
+	 * We used to output the absolute kernel address, and then just
+	 * a symbol. But both are information leaks, so just report 0
+	 * to indicate there is no wchan field in /proc/$PID/wchan.
 	 * This works with older implementations of procps as well.
 	 */
-	if (wchan)
-		seq_puts(m, " 1");
-	else
-		seq_puts(m, " 0");
+	seq_puts(m, " 0");
 
 	seq_put_decimal_ull(m, " ", 0);
 	seq_put_decimal_ull(m, " ", 0);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 533d5836eb9a..52484cd77f99 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -378,24 +378,10 @@ static const struct file_operations proc_pid_cmdline_ops = {
 };
 
 #ifdef CONFIG_KALLSYMS
-/*
- * Provides a wchan file via kallsyms in a proper one-value-per-file format.
- * Returns the resolved symbol.  If that fails, simply return the address.
- */
 static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
 			  struct pid *pid, struct task_struct *task)
 {
-	unsigned long wchan;
-
-	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');
+	seq_putc(m, '0');
 
 	return 0;
 }
-- 
2.30.2