Re: [PATCH v2 1/1] xdp: avoid calling kfree twice

Re: [PATCH v2 1/1] xdp: avoid calling kfree twice

[Daniel Borkmann]

On 12/9/20 6:03 AM, Zhu Yanjun wrote:
> In the function xdp_umem_pin_pages, if npgs != umem->npgs and
> npgs >= 0, the function xdp_umem_unpin_pages is called. In this
> function, kfree is called to handle umem->pgs, and then in the
> function xdp_umem_pin_pages, kfree is called again to handle
> umem->pgs. Eventually, umem->pgs is freed twice.
> 
> Acked-by: Björn Töpel <bjorn.topel@intel.com>
> Signed-off-by: Zhu Yanjun <yanjun.zhu@intel.com>

Please also fix up the commit log according to Bjorn's prior feedback [0].
If it's just a cleanup, it should state so, the commit message right now
makes it sound like an actual double free bug.

   [0] https://lore.kernel.org/netdev/0fef898d-cf5e-ef1b-6c35-c98669e9e0ed@intel.com/

Re: [PATCH v2 1/1] xdp: avoid calling kfree twice

[Zhu Yanjun]

On Wed, Dec 9, 2020 at 1:12 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 12/9/20 6:03 AM, Zhu Yanjun wrote:
> > In the function xdp_umem_pin_pages, if npgs != umem->npgs and
> > npgs >= 0, the function xdp_umem_unpin_pages is called. In this
> > function, kfree is called to handle umem->pgs, and then in the
> > function xdp_umem_pin_pages, kfree is called again to handle
> > umem->pgs. Eventually, umem->pgs is freed twice.
> >
> > Acked-by: Björn Töpel <bjorn.topel@intel.com>
> > Signed-off-by: Zhu Yanjun <yanjun.zhu@intel.com>
>
> Please also fix up the commit log according to Bjorn's prior feedback [0].
> If it's just a cleanup, it should state so, the commit message right now
> makes it sound like an actual double free bug.

The umem->pgs is actually freed twice. Since umem->pgs is set to NULL
after the first kfree,
the second kfree would not trigger call trace.
IMO, the commit log is very clear about this.

Zhu Yanjun
>
>    [0] https://lore.kernel.org/netdev/0fef898d-cf5e-ef1b-6c35-c98669e9e0ed@intel.com/

[PATCH v2 1/1] xdp: avoid calling kfree twice

[Zhu Yanjun]

In the function xdp_umem_pin_pages, if npgs != umem->npgs and
npgs >= 0, the function xdp_umem_unpin_pages is called. In this
function, kfree is called to handle umem->pgs, and then in the
function xdp_umem_pin_pages, kfree is called again to handle
umem->pgs. Eventually, umem->pgs is freed twice.

Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@intel.com>
---
 net/xdp/xdp_umem.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
index 56a28a686988..01b31c56cead 100644
--- a/net/xdp/xdp_umem.c
+++ b/net/xdp/xdp_umem.c
@@ -97,7 +97,6 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
 {
 	unsigned int gup_flags = FOLL_WRITE;
 	long npgs;
-	int err;
 
 	umem->pgs = kcalloc(umem->npgs, sizeof(*umem->pgs),
 			    GFP_KERNEL | __GFP_NOWARN);
@@ -112,20 +111,14 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem, unsigned long address)
 	if (npgs != umem->npgs) {
 		if (npgs >= 0) {
 			umem->npgs = npgs;
-			err = -ENOMEM;
-			goto out_pin;
+			xdp_umem_unpin_pages(umem);
+			return -ENOMEM;
 		}
-		err = npgs;
-		goto out_pgs;
+		kfree(umem->pgs);
+		umem->pgs = NULL;
+		return (int)npgs;
 	}
 	return 0;
-
-out_pin:
-	xdp_umem_unpin_pages(umem);
-out_pgs:
-	kfree(umem->pgs);
-	umem->pgs = NULL;
-	return err;
 }
 
 static int xdp_umem_account_pages(struct xdp_umem *umem)
-- 
2.18.4

Re: [PATCH v2 1/1] xdp: avoid calling kfree twice

[Toke Høiland-Jørgensen]

Zhu Yanjun <zyjzyj2000@gmail.com> writes:

> On Wed, Dec 9, 2020 at 1:12 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>>
>> On 12/9/20 6:03 AM, Zhu Yanjun wrote:
>> > In the function xdp_umem_pin_pages, if npgs != umem->npgs and
>> > npgs >= 0, the function xdp_umem_unpin_pages is called. In this
>> > function, kfree is called to handle umem->pgs, and then in the
>> > function xdp_umem_pin_pages, kfree is called again to handle
>> > umem->pgs. Eventually, umem->pgs is freed twice.
>> >
>> > Acked-by: Björn Töpel <bjorn.topel@intel.com>
>> > Signed-off-by: Zhu Yanjun <yanjun.zhu@intel.com>
>>
>> Please also fix up the commit log according to Bjorn's prior feedback [0].
>> If it's just a cleanup, it should state so, the commit message right now
>> makes it sound like an actual double free bug.
>
> The umem->pgs is actually freed twice. Since umem->pgs is set to NULL
> after the first kfree,
> the second kfree would not trigger call trace.
> IMO, the commit log is very clear about this.

Yes, it is very clear; and also wrong. As someone already pointed out,
passing a NULL pointer to kfree() doesn't actually lead to a double
free:

https://elixir.bootlin.com/linux/latest/source/mm/slub.c#L4106

-Toke

Re: [PATCH v2 1/1] xdp: avoid calling kfree twice

[Zhu Yanjun]

On Wed, Dec 9, 2020 at 6:44 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Zhu Yanjun <zyjzyj2000@gmail.com> writes:
>
> > On Wed, Dec 9, 2020 at 1:12 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >>
> >> On 12/9/20 6:03 AM, Zhu Yanjun wrote:
> >> > In the function xdp_umem_pin_pages, if npgs != umem->npgs and
> >> > npgs >= 0, the function xdp_umem_unpin_pages is called. In this
> >> > function, kfree is called to handle umem->pgs, and then in the
> >> > function xdp_umem_pin_pages, kfree is called again to handle
> >> > umem->pgs. Eventually, umem->pgs is freed twice.
> >> >
> >> > Acked-by: Björn Töpel <bjorn.topel@intel.com>
> >> > Signed-off-by: Zhu Yanjun <yanjun.zhu@intel.com>
> >>
> >> Please also fix up the commit log according to Bjorn's prior feedback [0].
> >> If it's just a cleanup, it should state so, the commit message right now
> >> makes it sound like an actual double free bug.
> >
> > The umem->pgs is actually freed twice. Since umem->pgs is set to NULL
> > after the first kfree,
> > the second kfree would not trigger call trace.
> > IMO, the commit log is very clear about this.
>
> Yes, it is very clear; and also wrong. As someone already pointed out,
> passing a NULL pointer to kfree() doesn't actually lead to a double
> free:

In your commit, does "double free" mean the call trace bug? If so, I
will correct it in my commit log.
In my commit log, I just mean that kfree is called twice. And the
second kfree is meaningless. And since NULL
is passed to it, this will not trigger "double free" CallTrace.

I will send the latest version soon.

Zhu Yanjun
>
> https://elixir.bootlin.com/linux/latest/source/mm/slub.c#L4106
>
> -Toke
>