[RFC] mm: prevent set a value less than 0 to min_free_kbytes

[Han Pingtian]

If echo -1 > /proc/vm/sys/min_free_kbytes, the system will hang.
Changing proc_dointvec() to proc_dointvec_minmax() in the
min_free_kbytes_sysctl_handler() can prevent this to happen.

Signed-off-by: Han Pingtian <ha...@linux.vnet.ibm.com>
---
mm/page_alloc.c | 7 ++++++-
1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 77937e0..a9dcfd8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5692,7 +5692,12 @@ module_init(init_per_zone_wmark_min)
int min_free_kbytes_sysctl_handler(ctl_table *table, int write,
void __user *buffer, size_t *length, loff_t *ppos)
{
- proc_dointvec(table, write, buffer, length, ppos);
+ int rc;
+
+ rc = proc_dointvec_minmax(table, write, buffer, length, ppos);
+ if (rc)
+ return rc;
+
if (write) {
user_min_free_kbytes = min_free_kbytes;
setup_per_zone_wmarks();
--
1.7.7.6

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[Michal Hocko]

On Wed 08-01-14 16:42:42, Han Pingtian wrote:
> If echo -1 > /proc/vm/sys/min_free_kbytes, the system will hang.
> Changing proc_dointvec() to proc_dointvec_minmax() in the
> min_free_kbytes_sysctl_handler() can prevent this to happen.

You can still do echo $BIG_VALUE > /proc/vm/sys/min_free_kbytes and make
your machine unusable but I agree that proc_dointvec_minmax is more
suitable here as we already have:
.proc_handler = min_free_kbytes_sysctl_handler,
.extra1 = &zero,

It used to work properly but then 6fce56ec91b5 (sysctl: Remove
references to ctl_name and strategy from the generic sysctl table) has
removed sysctl_intvec strategy and so extra1 is ignored.

> Signed-off-by: Han Pingtian <ha...@linux.vnet.ibm.com>

That being said I do not think this will fix any real world problem but
just for sake of correctness

After changelog is updated feel free to add my
Acked-by: Michal Hocko <mho...@suse.cz>

> ---
> mm/page_alloc.c | 7 ++++++-
> 1 files changed, 6 insertions(+), 1 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 77937e0..a9dcfd8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5692,7 +5692,12 @@ module_init(init_per_zone_wmark_min)
> int min_free_kbytes_sysctl_handler(ctl_table *table, int write,
> void __user *buffer, size_t *length, loff_t *ppos)
> {
> - proc_dointvec(table, write, buffer, length, ppos);
> + int rc;
> +
> + rc = proc_dointvec_minmax(table, write, buffer, length, ppos);
> + if (rc)
> + return rc;
> +
> if (write) {
> user_min_free_kbytes = min_free_kbytes;
> setup_per_zone_wmarks();
> --
> 1.7.7.6
>

--

Michal Hocko
SUSE Labs

[David Rientjes]

On Wed, 8 Jan 2014, Han Pingtian wrote:

> If echo -1 > /proc/vm/sys/min_free_kbytes, the system will hang.
> Changing proc_dointvec() to proc_dointvec_minmax() in the
> min_free_kbytes_sysctl_handler() can prevent this to happen.
>
> Signed-off-by: Han Pingtian <ha...@linux.vnet.ibm.com>

Acked-by: David Rientjes <rien...@google.com>

Nice catch!