[PATCH v1 2/2] sysctl: handle overflow for file-max

[Christian Brauner]

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.
This commit explicitly caps the value for file-max to ULONG_MAX.

Note, this isn't technically necessary since proc_get_long() will already
return ULONG_MAX. However, two reason why we still should do this:
1. it makes it explicit what the upper bound of file-max is instead of
   making readers of the code infer it from proc_get_long() themselves
2. other tunebles than file-max may want to set a lower max value than
   ULONG_MAX and we need to enable __do_proc_doulongvec_minmax() to handle
   such cases too

Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Christian Brauner <christian@xxxxxxxxxx>
---
v0->v1:
- if max value is < than ULONG_MAX use max as upper bound
- (Dominik) remove double "the" from commit message
---
 kernel/sysctl.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 97551eb42946..226d4eaf4b0e 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",
@@ -2795,6 +2797,8 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
 				break;
 			if (neg)
 				continue;
+			if (max && val > *max)
+				val = *max;
 			val = convmul * val / convdiv;
 			if ((min && val < *min) || (max && val > *max))
 				continue;
-- 
2.17.1