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Subject[PATCH v1 2/2] sysctl: handle overflow for file-max
Date
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@chromium.org>
Signed-off-by: Christian Brauner <christian@brauner.io>
---
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
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