netdev - Re: [PATCH v2] net: skbuff: fix uninitialized memory use in pskb_expand

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Message-ID: <CANn89i+txkMjnxtaXtFJucNCLehhOdCw=5KBGpxCMj2v2erLNQ@mail.gmail.com>
Date: Mon, 26 Jan 2026 14:26:35 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Soham Metha <sohammetha01@...il.com>
Cc: linux-kernel-mentees@...ts.linuxfoundation.org, shuah@...nel.org, 
	skhan@...uxfoundation.org, linux-kernel@...r.kernel.org, 
	syzbot+619b9ef527f510a57cfc@...kaller.appspotmail.com, 
	syzkaller-bugs@...glegroups.com, andrii@...nel.org, ast@...nel.org, 
	bpf@...r.kernel.org, daniel@...earbox.net, eddyz87@...il.com, 
	haoluo@...gle.com, john.fastabend@...il.com, jolsa@...nel.org, 
	kpsingh@...nel.org, martin.lau@...ux.dev, sdf@...ichev.me, song@...nel.org, 
	yonghong.song@...ux.dev, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Jason Xing <kernelxing@...cent.com>, Kuniyuki Iwashima <kuniyu@...gle.com>, Michal Luczaj <mhal@...x.co>, 
	Mina Almasry <almasrymina@...gle.com>, Eric Biggers <ebiggers@...gle.com>, 
	Alexander Lobakin <aleksander.lobakin@...el.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v2] net: skbuff: fix uninitialized memory use in pskb_expand_head()

On Mon, Jan 26, 2026 at 2:22 PM Soham Metha <sohammetha01@...il.com> wrote:
>
> pskb_expand_head() allocates a new skb data buffer using
> kmalloc_reserve(), which does not initialize memory. skb helpers may
> later copy or move padding bytes from the buffer.
>
> Initialize the newly allocated skb buffer to avoid propagating
> uninitialized memory.
>
> Reported-by: syzbot+619b9ef527f510a57cfc@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=619b9ef527f510a57cfc
> Tested-by: syzbot+619b9ef527f510a57cfc@...kaller.appspotmail.com
> Signed-off-by: Soham Metha <sohammetha01@...il.com>
> ---
>
> v2:
> - No code changes
> - Resent to netdev list
> - Added Closes tag
> - Added Tested-by tag
>
>  net/core/skbuff.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index a56133902c0d..b0f0d3a0310b 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -2282,6 +2282,9 @@ int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail,
>         data = kmalloc_reserve(&size, gfp_mask, NUMA_NO_NODE, NULL);
>         if (!data)
>                 goto nodata;
> +
> +       memset(data, 0, size);
> +
>


Certainly not.

You might wonder why we have GFP_ZERO ?

Answer : we do not generally want to pay the price of zeroing memory
_unless_ absolutely needed.

Fix the caller instead, ie root-cause the issue, thank you