[PATCH net v6 1/3] net: remove CAP_SYS_RAWIO zero-padding in dev_validate_header

[Qihang <q.h.hack.winter@gmail.com>]

From: Qihang Tang <q.h.hack.winter@gmail.com>

dev_validate_header() reads dev->hard_header_len directly when
zero-padding short link layer headers for CAP_SYS_RAWIO holders:

    if (capable(CAP_SYS_RAWIO)) {
        memset(ll_header + len, 0, dev->hard_header_len - len);
        return true;
    }

Packet send paths call dev_validate_header() on skbs whose headroom was
allocated from an earlier hard_header_len read. If the device is
reconfigured so that dev->hard_header_len increases before validation,
the memset writes past the reserved buffer, an out-of-bounds write.

This out-of-bounds write is masked in some SOCK_RAW paths today because
the same concurrent increase can first make skb_push() exceed the
reserved headroom and trigger skb_under_panic(). Remove the zero-padding
branch before making those hard_header_len reads consistent, so the
snapshot fixes do not turn a loud panic into a silent overwrite.

This path is only reached for variable length L2 protocols, where
len < hard_header_len but len >= min_header_len. No remaining in-tree
variable length L2 protocol implements header_ops->validate, and the
CAP_SYS_RAWIO bypass that zero-pads and accepts short headers has no
real value beyond allowing testing of intentionally malformed input.

Drop the CAP_SYS_RAWIO branch. The remaining reads of
dev->hard_header_len in dev_validate_header() are comparisons only and
have no memory safety impact.

Suggested-by: Willem de Bruijn <willemb@google.com>
Fixes: 2793a23aacbd ("net: validate variable length ll headers")
Cc: stable@vger.kernel.org
Signed-off-by: Qihang Tang <q.h.hack.winter@gmail.com>
---
 include/linux/netdevice.h | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 9981d637f8b5..9a770eb823ce 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3531,11 +3531,6 @@ static inline bool dev_validate_header(const struct net_device *dev,
 	if (len < dev->min_header_len)
 		return false;
 
-	if (capable(CAP_SYS_RAWIO)) {
-		memset(ll_header + len, 0, dev->hard_header_len - len);
-		return true;
-	}
-
 	if (dev->header_ops && dev->header_ops->validate)
 		return dev->header_ops->validate(ll_header, len);
 
-- 
2.50.1 (Apple Git-155)