From 2566866fc30965d915d0b52b5c3323b362619f0e Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?J=C3=A9r=C3=A9my=20Jean?= Date: Tue, 15 Sep 2026 12:48:07 +0000 Subject: [PATCH] net: gue: reject invalid REMCSUM offsets MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The REMCSUM option carries an absolute checksum start and checksum field offset. gue_remcsum() passes them to skb_remcsum_process(), whose partial path stores offset - start in the u16 skb->csum_offset variable. If offset is less than start, this underflows. A forwarded packet can retain CHECKSUM_PARTIAL and reach a NETIF_F_HW_CSUM driver which trusts the metadata, leading skb_copy_and_csum_dev() to write two bytes about 64 KiB beyond the destination buffer. Reject reversed tuples in validate_gue_flags(), after the existing length validation, so all GUE parsers enforce the ordering in one place. Fixes: fe881ef11cf0 ("gue: Use checksum partial with remote checksum offload") Signed-off-by: Jérémy Jean Link: https://patch.msgid.link/20260915124806.2852293-2-Jeremy.Jean@oss.cyber.gouv.fr Signed-off-by: Jakub Kicinski --- include/net/gue.h | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-) diff --git a/include/net/gue.h b/include/net/gue.h index caefd6da8693..d377155fd0b3 100644 --- a/include/net/gue.h +++ b/include/net/gue.h @@ -84,8 +84,9 @@ static inline size_t guehdr_priv_flags_len(__be32 flags) } /* Validate standard and private flags. Returns non-zero (meaning invalid) - * if there is an unknown standard or private flags, or the options length for - * the flags exceeds the options length specific in hlen of the GUE header. + * if there is an unknown standard or private flags, if the options length for + * the flags exceeds the options length specified in hlen of the GUE header, or + * if a private option contains invalid data. */ static inline int validate_gue_flags(struct guehdr *guehdr, size_t optlen) { @@ -103,8 +104,8 @@ static inline int validate_gue_flags(struct guehdr *guehdr, size_t optlen) /* Private flags are last four bytes accounted in * guehdr_flags_len */ - __be32 pflags = *(__be32 *)((void *)&guehdr[1] + - len - GUE_LEN_PRIV); + void *data = (void *)&guehdr[1] + len; + __be32 pflags = *(__be32 *)(data - GUE_LEN_PRIV); if (pflags & ~GUE_PFLAGS_ALL) return 1; @@ -112,6 +113,16 @@ static inline int validate_gue_flags(struct guehdr *guehdr, size_t optlen) len += guehdr_priv_flags_len(pflags); if (len > optlen) return 1; + + if (pflags & GUE_PFLAG_REMCSUM) { + __be16 *pd = data; + + /* The field offset pd[1] must not be less + * than the start pd[0]. + */ + if (ntohs(pd[1]) < ntohs(pd[0])) + return 1; + } } return 0;