[PATCH 2/2] sysctl: handle overflow for file-max

[Christian Brauner <christian@brauner.io>]

Currently, when writing

echo 18446744073709551616 > /proc/sys/fs/file-max

/proc/sys/fs/file-max will overflow and be set to 0. That quickly
crashes the system. Let's detect the overflow and set to ULONG_MAX in
this case effectively capping the value.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Christian Brauner <christian@brauner.io>
---
 kernel/sysctl.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index a9409375380c..a3e4321b8ffa 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -127,6 +127,7 @@ static int __maybe_unused one = 1;
 static int __maybe_unused two = 2;
 static int __maybe_unused four = 4;
 static unsigned long one_ul = 1;
+static unsigned long ulong_max = ULONG_MAX;
 static int one_hundred = 100;
 static int one_thousand = 1000;
 #ifdef CONFIG_PRINTK
@@ -1696,6 +1697,7 @@ static struct ctl_table fs_table[] = {
 		.maxlen		= sizeof(files_stat.max_files),
 		.mode		= 0644,
 		.proc_handler	= proc_doulongvec_minmax,
+		.extra2		= &ulong_max,
 	},
 	{
 		.procname	= "nr_open",
@@ -2789,17 +2791,20 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
 		unsigned long val;
 
 		if (write) {
-			bool neg;
+			bool neg, overflow;
 
 			left -= proc_skip_spaces(&p);
 
 			err = proc_get_long(&p, &left, &val, &neg,
 					     proc_wspace_sep,
-					     sizeof(proc_wspace_sep), NULL);
+					     sizeof(proc_wspace_sep), NULL,
+					     &overflow);
 			if (err)
 				break;
 			if (neg)
 				continue;
+			if (overflow && max)
+				val = *max;
 			val = convmul * val / convdiv;
 			if ((min && val < *min) || (max && val > *max))
 				continue;
-- 
2.17.1