2023-11-18 11:34:23

by Kamil Duljas

Subject: [PATCH] genetlink: Prevent memory leak when krealloc fail

genl_allocate_reserve_groups() allocs new memory in while loop
but if krealloc fail, the memory allocated by kzalloc is not freed.
It seems allocated memory is unnecessary when the function
returns -ENOMEM

Signed-off-by: Kamil Duljas <[email protected]>
---
net/netlink/genetlink.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
index 92ef5ed2e7b0..82273d6eaea3 100644
--- a/net/netlink/genetlink.c
+++ b/net/netlink/genetlink.c
@@ -437,8 +437,10 @@ static int genl_allocate_reserve_groups(int n_groups, int *first_id)
} else {
new_groups = krealloc(mc_groups, nlen,
GFP_KERNEL);
- if (!new_groups)
+ if (!new_groups) {
+ kfree(mc_groups);
return -ENOMEM;
+ }
mc_groups = new_groups;
for (i = 0; i < BITS_TO_LONGS(n_groups); i++)
mc_groups[mc_groups_longs + i] = 0;
--
2.42.0.windows.2

2023-11-18 12:03:03

by Florian Westphal

Subject: Re: [PATCH] genetlink: Prevent memory leak when krealloc fail

Kamil Duljas <[email protected]> wrote:
> genl_allocate_reserve_groups() allocs new memory in while loop
> but if krealloc fail, the memory allocated by kzalloc is not freed.
> It seems allocated memory is unnecessary when the function
> returns -ENOMEM

Why should it be free'd? mc_groups is not a local variable.

> new_groups = krealloc(mc_groups, nlen,
> GFP_KERNEL);
> - if (!new_groups)
> + if (!new_groups) {
> + kfree(mc_groups);
> return -ENOMEM;
> + }

How did you test this? AFAICS this results in use-after-free for every
access to mc_groups after this error path is taken.

Existing code looks correct, we can't grow mc_groups and return an
error.

2023-11-18 13:28:24

by Kamil Duljas

Subject: Re: [PATCH] genetlink: Prevent memory leak when krealloc fail

Yes, you're right. I did not think about it. So if we have a static
pointer that may be resued, should not restore the pointer as at the
beginning?
static unsigned long *mc_groups = &mc_group_start;

At this moment we don't know how much memory is allocated. What do you
think about this?

> new_groups = krealloc(mc_groups, nlen,
> GFP_KERNEL);
> - if (!new_groups)
> + if (!new_groups) {
> + kfree(mc_groups);
> + mc_groups = &mc_group_start;
> return -ENOMEM;
> + }


sob., 18 lis 2023 o 13:02 Florian Westphal <[email protected]> napisał(a):
>
> Kamil Duljas <[email protected]> wrote:
> > genl_allocate_reserve_groups() allocs new memory in while loop
> > but if krealloc fail, the memory allocated by kzalloc is not freed.
> > It seems allocated memory is unnecessary when the function
> > returns -ENOMEM
>
> Why should it be free'd? mc_groups is not a local variable.
>
> > new_groups = krealloc(mc_groups, nlen,
> > GFP_KERNEL);
> > - if (!new_groups)
> > + if (!new_groups) {
> > + kfree(mc_groups);
> > return -ENOMEM;
> > + }
>
> How did you test this? AFAICS this results in use-after-free for every
> access to mc_groups after this error path is taken.
>
> Existing code looks correct, we can't grow mc_groups and return an
> error.



--
Pozdrawiam,
Kamil Duljas

2023-11-18 13:43:11

by Florian Westphal

Subject: Re: [PATCH] genetlink: Prevent memory leak when krealloc fail

Kamil Duljas <[email protected]> wrote:
> Yes, you're right. I did not think about it. So if we have a static
> pointer that may be resued, should not restore the pointer as at the
> beginning?
> static unsigned long *mc_groups = &mc_group_start;
>
> At this moment we don't know how much memory is allocated. What do you
> think about this?

We do: mc_groups_longs.

> > new_groups = krealloc(mc_groups, nlen,
> > GFP_KERNEL);
> > - if (!new_groups)
> > + if (!new_groups) {
> > + kfree(mc_groups);
> > + mc_groups = &mc_group_start;
> > return -ENOMEM;
> > + }

Seems wrong to shrink when we can't grow. Whats the point?