From f438d1786d657d57790c5d138d6db3fc9fdac392 Mon Sep 17 00:00:00 2001 From: Florian Westphal Date: Tue, 19 May 2026 22:52:07 +0200 Subject: [PATCH] netfilter: ebtables: fix OOB read in compat_mtw_from_user Luxiao Xu says: The function compat_mtw_from_user() converts ebtables extensions from 32-bit user structures to kernel native structures. However, it lacks proper validation of the user-supplied match_size/target_size. When certain extensions are processed, the kernel-side translation logic may perform memory accesses based on the extension's expected size. If the user provides a size smaller than what the extension requires, it results in an out-of-bounds read as reported by KASAN. This fix introduces a check to ensure match_size is at least as large as the extension's required compatsize. This covers matches, watchers, and targets, while maintaining compatibility with standard targets. AFAIU this is relevant for matches that need to go though match->compat_from_user() call. Those that use plain memcpy with the user-provided size are ok because the caller checks that size vs the start of the next rule entry offset (which itself is checked vs. total size copied from userspace). The ->compat_from_user() callbacks assume they can read compatsize bytes, so they need this extra check. Based on an earlier patch from Luxiao Xu. Fixes: 81e675c227ec ("netfilter: ebtables: add CONFIG_COMPAT support") Reported-by: Yuan Tan Reported-by: Yifan Wu Reported-by: Juefei Pu Reported-by: Xin Liu Signed-off-by: Luxiao Xu Signed-off-by: Ren Wei Reviewed-by: Fernando Fernandez Mancera Signed-off-by: Florian Westphal --- net/bridge/netfilter/ebtables.c | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c index b9f4daac09af..8a6a069329d2 100644 --- a/net/bridge/netfilter/ebtables.c +++ b/net/bridge/netfilter/ebtables.c @@ -1956,6 +1956,25 @@ enum compat_mwt { EBT_COMPAT_TARGET, }; +static bool match_size_ok(const struct xt_match *match, unsigned int match_size) +{ + u16 csize; + + if (match->matchsize == -1) /* cannot validate ebt_among */ + return true; + + csize = match->compatsize ? : match->matchsize; + + return match_size >= csize; +} + +static bool tgt_size_ok(const struct xt_target *tgt, unsigned int tgt_size) +{ + u16 csize = tgt->compatsize ? : tgt->targetsize; + + return tgt_size >= csize; +} + static int compat_mtw_from_user(const struct compat_ebt_entry_mwt *mwt, enum compat_mwt compat_mwt, struct ebt_entries_buf_state *state, @@ -1981,6 +2000,11 @@ static int compat_mtw_from_user(const struct compat_ebt_entry_mwt *mwt, if (IS_ERR(match)) return PTR_ERR(match); + if (!match_size_ok(match, match_size)) { + module_put(match->me); + return -EINVAL; + } + off = ebt_compat_match_offset(match, match_size); if (dst) { if (match->compat_from_user) @@ -2000,6 +2024,12 @@ static int compat_mtw_from_user(const struct compat_ebt_entry_mwt *mwt, mwt->u.revision); if (IS_ERR(wt)) return PTR_ERR(wt); + + if (!tgt_size_ok(wt, match_size)) { + module_put(wt->me); + return -EINVAL; + } + off = xt_compat_target_offset(wt); if (dst) {