* [PATCH] net: ppp: Add bound checking for skb d on ppp_sync_txmung
@ 2025-04-07 15:26 Arnaud Lecomte
2025-04-08 15:33 ` Simon Horman
0 siblings, 1 reply; 3+ messages in thread
From: Arnaud Lecomte @ 2025-04-07 15:26 UTC (permalink / raw)
To: Andrew Lunn, David S. Miller, Eric Dumazet, Jakub Kicinski,
Paolo Abeni
Cc: linux-ppp, netdev, linux-kernel, contact, skhan,
syzbot+29fc8991b0ecb186cf40
Ensure we have enough data in linear buffer from skb before accessing
initial bytes. This prevents potential out-of-bounds accesses
when processing short packets.
When ppp_sync_txmung receives an incoming package with an empty
payload:
(remote) gef➤ p *(struct pppoe_hdr *) (skb->head + skb->network_header)
$18 = {
type = 0x1,
ver = 0x1,
code = 0x0,
sid = 0x2,
length = 0x0,
tag = 0xffff8880371cdb96
}
from the skb struct (trimmed)
tail = 0x16,
end = 0x140,
head = 0xffff88803346f400 "4",
data = 0xffff88803346f416 ":\377",
truesize = 0x380,
len = 0x0,
data_len = 0x0,
mac_len = 0xe,
hdr_len = 0x0,
it is not safe to access data[2].
Reported-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
Tested-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
Fixes: 9946eaf552b1 ("Merge tag 'hardening-v6.14-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux")
Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
---
drivers/net/ppp/ppp_synctty.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
index 644e99fc3623..520d895acc60 100644
--- a/drivers/net/ppp/ppp_synctty.c
+++ b/drivers/net/ppp/ppp_synctty.c
@@ -506,6 +506,11 @@ ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
unsigned char *data;
int islcp;
+ /* Ensure we can safely access protocol field and LCP code */
+ if (!skb || !pskb_may_pull(skb, 3)) {
+ kfree_skb(skb);
+ return NULL;
+ }
data = skb->data;
proto = get_unaligned_be16(data);
---
base-commit: 9946eaf552b194bb352c2945b54ff98c8193b3f1
change-id: 20250405-bound-checking-ppp_txmung-4807c854ed85
Best regards,
--
Arnaud Lecomte <contact@arnaud-lcm.com>
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH] net: ppp: Add bound checking for skb d on ppp_sync_txmung
2025-04-07 15:26 [PATCH] net: ppp: Add bound checking for skb d on ppp_sync_txmung Arnaud Lecomte
@ 2025-04-08 15:33 ` Simon Horman
2025-04-08 15:58 ` Arnaud Lecomte
0 siblings, 1 reply; 3+ messages in thread
From: Simon Horman @ 2025-04-08 15:33 UTC (permalink / raw)
To: Arnaud Lecomte
Cc: Andrew Lunn, David S. Miller, Eric Dumazet, Jakub Kicinski,
Paolo Abeni, linux-ppp, netdev, linux-kernel, skhan,
syzbot+29fc8991b0ecb186cf40
On Mon, Apr 07, 2025 at 05:26:21PM +0200, Arnaud Lecomte wrote:
> Ensure we have enough data in linear buffer from skb before accessing
> initial bytes. This prevents potential out-of-bounds accesses
> when processing short packets.
>
> When ppp_sync_txmung receives an incoming package with an empty
> payload:
> (remote) gef➤ p *(struct pppoe_hdr *) (skb->head + skb->network_header)
> $18 = {
> type = 0x1,
> ver = 0x1,
> code = 0x0,
> sid = 0x2,
> length = 0x0,
> tag = 0xffff8880371cdb96
> }
>
> from the skb struct (trimmed)
> tail = 0x16,
> end = 0x140,
> head = 0xffff88803346f400 "4",
> data = 0xffff88803346f416 ":\377",
> truesize = 0x380,
> len = 0x0,
> data_len = 0x0,
> mac_len = 0xe,
> hdr_len = 0x0,
>
> it is not safe to access data[2].
>
> Reported-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
> Tested-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
> Fixes: 9946eaf552b1 ("Merge tag 'hardening-v6.14-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux")
It doesn't seem right to use a Merge commit in a fixes tag.
Looking over the code, the access to data[2] seems to have existed since
the beginning of git history, in which case I think we can use this:
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
> ---
> drivers/net/ppp/ppp_synctty.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
> index 644e99fc3623..520d895acc60 100644
> --- a/drivers/net/ppp/ppp_synctty.c
> +++ b/drivers/net/ppp/ppp_synctty.c
> @@ -506,6 +506,11 @@ ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
> unsigned char *data;
> int islcp;
>
> + /* Ensure we can safely access protocol field and LCP code */
> + if (!skb || !pskb_may_pull(skb, 3)) {
I doubt that skb can be NULL.
> + kfree_skb(skb);
> + return NULL;
> + }
> data = skb->data;
> proto = get_unaligned_be16(data);
>
>
> ---
> base-commit: 9946eaf552b194bb352c2945b54ff98c8193b3f1
> change-id: 20250405-bound-checking-ppp_txmung-4807c854ed85
>
> Best regards,
> --
> Arnaud Lecomte <contact@arnaud-lcm.com>
>
>
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH] net: ppp: Add bound checking for skb d on ppp_sync_txmung
2025-04-08 15:33 ` Simon Horman
@ 2025-04-08 15:58 ` Arnaud Lecomte
0 siblings, 0 replies; 3+ messages in thread
From: Arnaud Lecomte @ 2025-04-08 15:58 UTC (permalink / raw)
To: Simon Horman
Cc: Andrew Lunn, David S. Miller, Eric Dumazet, Jakub Kicinski,
Paolo Abeni, linux-ppp, netdev, linux-kernel, skhan,
syzbot+29fc8991b0ecb186cf40
Thanks for the review.
You're right, using a merge commit in the `Fixes:` tag was not relevant.
I'll send a v2 shortly with the fix as well as the removal of the skb
check as it is already done before.
On 08/04/2025 17:33, Simon Horman wrote:
> On Mon, Apr 07, 2025 at 05:26:21PM +0200, Arnaud Lecomte wrote:
>> Ensure we have enough data in linear buffer from skb before accessing
>> initial bytes. This prevents potential out-of-bounds accesses
>> when processing short packets.
>>
>> When ppp_sync_txmung receives an incoming package with an empty
>> payload:
>> (remote) gef➤ p *(struct pppoe_hdr *) (skb->head + skb->network_header)
>> $18 = {
>> type = 0x1,
>> ver = 0x1,
>> code = 0x0,
>> sid = 0x2,
>> length = 0x0,
>> tag = 0xffff8880371cdb96
>> }
>>
>> from the skb struct (trimmed)
>> tail = 0x16,
>> end = 0x140,
>> head = 0xffff88803346f400 "4",
>> data = 0xffff88803346f416 ":\377",
>> truesize = 0x380,
>> len = 0x0,
>> data_len = 0x0,
>> mac_len = 0xe,
>> hdr_len = 0x0,
>>
>> it is not safe to access data[2].
>>
>> Reported-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=29fc8991b0ecb186cf40
>> Tested-by: syzbot+29fc8991b0ecb186cf40@syzkaller.appspotmail.com
>> Fixes: 9946eaf552b1 ("Merge tag 'hardening-v6.14-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux")
> It doesn't seem right to use a Merge commit in a fixes tag.
>
> Looking over the code, the access to data[2] seems to have existed since
> the beginning of git history, in which case I think we can use this:
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>
>> Signed-off-by: Arnaud Lecomte <contact@arnaud-lcm.com>
>> ---
>> drivers/net/ppp/ppp_synctty.c | 5 +++++
>> 1 file changed, 5 insertions(+)
>>
>> diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
>> index 644e99fc3623..520d895acc60 100644
>> --- a/drivers/net/ppp/ppp_synctty.c
>> +++ b/drivers/net/ppp/ppp_synctty.c
>> @@ -506,6 +506,11 @@ ppp_sync_txmunge(struct syncppp *ap, struct sk_buff *skb)
>> unsigned char *data;
>> int islcp;
>>
>> + /* Ensure we can safely access protocol field and LCP code */
>> + if (!skb || !pskb_may_pull(skb, 3)) {
> I doubt that skb can be NULL.
>
>> + kfree_skb(skb);
>> + return NULL;
>> + }
>> data = skb->data;
>> proto = get_unaligned_be16(data);
>>
>>
>> ---
>> base-commit: 9946eaf552b194bb352c2945b54ff98c8193b3f1
>> change-id: 20250405-bound-checking-ppp_txmung-4807c854ed85
>>
>> Best regards,
>> --
>> Arnaud Lecomte <contact@arnaud-lcm.com>
>>
>>
^ permalink raw reply [flat|nested] 3+ messages in thread
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2025-04-07 15:26 [PATCH] net: ppp: Add bound checking for skb d on ppp_sync_txmung Arnaud Lecomte
2025-04-08 15:33 ` Simon Horman
2025-04-08 15:58 ` Arnaud Lecomte
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