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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, zerocling0077@gmail.com, 2045gemini@gmail.com, Zhang Cen , stable@vger.kernel.org Subject: [PATCH v3] bpf, sockmap: keep sk_msg copy state in sync Date: Wed, 20 May 2026 18:27:15 +0800 Message-Id: <20260520102715.3033936-1-rollkingzzc@gmail.com> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit SK_MSG uses msg->sg.copy as per-scatterlist-entry provenance. Entries with this bit set are copied before data/data_end are exposed to SK_MSG BPF programs for direct packet access. bpf_msg_pull_data(), bpf_msg_push_data(), and bpf_msg_pop_data() rewrite the sk_msg scatterlist ring by collapsing, splitting, and shifting entries. These operations move msg->sg.data[] entries, but the parallel copy bitmap can be left behind on the old slot. A copied entry can then return to msg->sg.start with its copy bit clear and be exposed as directly writable packet data. This corruption path requires an attached SK_MSG BPF program that calls the mutating helpers; ordinary sockmap/TLS traffic that never runs push/pop/pull helper sequences is not affected. Keep msg->sg.copy synchronized with scatterlist entry moves, preserve the copy bit when an entry is split, clear it when a helper replaces an entry with a private page, and clear slots vacated by pull-data compaction. Fixes: 015632bb30da ("bpf: sk_msg program helper bpf_sk_msg_pull_data") Fixes: 6fff607e2f14 ("bpf: sk_msg program helper bpf_msg_push_data") Fixes: 7246d8ed4dcc ("bpf: helper to pop data from messages") Cc: stable@vger.kernel.org Co-developed-by: Han Guidong <2045gemini@gmail.com> Signed-off-by: Han Guidong <2045gemini@gmail.com> Signed-off-by: Zhang Cen --- v3: - Refactor copy-bit helpers per John Fastabend's review: encapsulate scatterlist element moves alongside their copy state into a unified helper, and streamline bit-clearing operations. - Clarify in commit log that only programs using push/pop/pull are affected. - Note: The additional edge cases reported by the Sashiko-bot bot and the addition of BPF selftests will be addressed in a separate follow-up patch series to expedite this core fix. v2: Address Sashiko-bot's feedback by clearing msg->sg.copy for every entry consumed by bpf_msg_pull_data() before compacting the scatterlist ring, preventing stale copy bits on collapsed tail entries. v1: While researching recent page cache bugs, we discovered this bug. We confirmed it allows overwriting the page cache of read-only files via splice(). We haven't attempted to write an exploit, but the corruption primitive is verified. PoC available upon request. Recommend fixing ASAP. --- net/core/filter.c | 88 ++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 83 insertions(+), 5 deletions(-) diff --git a/net/core/filter.c b/net/core/filter.c index 78b548158fb05..1be8fc750a1a1 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -2650,6 +2650,38 @@ static void sk_msg_reset_curr(struct sk_msg *msg) } } +static bool sk_msg_elem_is_copy(const struct sk_msg *msg, u32 i) +{ + return test_bit(i, msg->sg.copy); +} + +static void sk_msg_clear_elem_copy(struct sk_msg *msg, u32 i) +{ + __clear_bit(i, msg->sg.copy); +} + +static void sk_msg_set_elem_copy(struct sk_msg *msg, u32 i) +{ + __set_bit(i, msg->sg.copy); +} + +static void sk_msg_clear_copy_range(struct sk_msg *msg, u32 start, u32 end) +{ + while (start != end) { + sk_msg_clear_elem_copy(msg, start); + sk_msg_iter_var_next(start); + } +} + +static void sk_msg_sg_move(struct sk_msg *msg, u32 dst, u32 src) +{ + msg->sg.data[dst] = msg->sg.data[src]; + if (sk_msg_elem_is_copy(msg, src)) + sk_msg_set_elem_copy(msg, dst); + else + sk_msg_clear_elem_copy(msg, dst); +} + static const struct bpf_func_proto bpf_msg_cork_bytes_proto = { .func = bpf_msg_cork_bytes, .gpl_only = false, @@ -2688,7 +2720,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start, * account for the headroom. */ bytes_sg_total = start - offset + bytes; - if (!test_bit(i, msg->sg.copy) && bytes_sg_total <= len) + if (!sk_msg_elem_is_copy(msg, i) && bytes_sg_total <= len) goto out; /* At this point we need to linearize multiple scatterlist @@ -2734,6 +2766,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start, } while (i != last_sge); sg_set_page(&msg->sg.data[first_sge], page, copy, 0); + sk_msg_clear_elem_copy(msg, first_sge); /* To repair sg ring we need to shift entries. If we only * had a single entry though we can just replace it and @@ -2743,8 +2776,14 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start, shift = last_sge > first_sge ? last_sge - first_sge - 1 : NR_MSG_FRAG_IDS - first_sge + last_sge - 1; - if (!shift) + if (!shift) { + sk_msg_clear_elem_copy(msg, msg->sg.end); goto out; + } + + i = first_sge; + sk_msg_iter_var_next(i); + sk_msg_clear_copy_range(msg, i, last_sge); i = first_sge; sk_msg_iter_var_next(i); @@ -2758,16 +2797,18 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start, if (move_from == msg->sg.end) break; - msg->sg.data[i] = msg->sg.data[move_from]; + sk_msg_sg_move(msg, i, move_from); msg->sg.data[move_from].length = 0; msg->sg.data[move_from].page_link = 0; msg->sg.data[move_from].offset = 0; + sk_msg_clear_elem_copy(msg, move_from); sk_msg_iter_var_next(i); } while (1); msg->sg.end = msg->sg.end - shift > msg->sg.end ? msg->sg.end - shift + NR_MSG_FRAG_IDS : msg->sg.end - shift; + sk_msg_clear_elem_copy(msg, msg->sg.end); out: sk_msg_reset_curr(msg); msg->data = sg_virt(&msg->sg.data[first_sge]) + start - offset; @@ -2790,6 +2831,8 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg, u32, start, { struct scatterlist sge, nsge, nnsge, rsge = {0}, *psge; u32 new, i = 0, l = 0, space, copy = 0, offset = 0; + bool sge_copy = false, nsge_copy = false, nnsge_copy = false; + bool rsge_copy = false; u8 *raw, *to, *from; struct page *page; @@ -2862,6 +2905,7 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg, u32, start, sk_msg_iter_var_prev(i); psge = sk_msg_elem(msg, i); rsge = sk_msg_elem_cpy(msg, i); + rsge_copy = sk_msg_elem_is_copy(msg, i); psge->length = start - offset; rsge.length -= psge->length; @@ -2887,23 +2931,34 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg, u32, start, /* Shift one or two slots as needed */ sge = sk_msg_elem_cpy(msg, new); sg_unmark_end(&sge); + sge_copy = sk_msg_elem_is_copy(msg, new); nsge = sk_msg_elem_cpy(msg, i); + nsge_copy = sk_msg_elem_is_copy(msg, i); if (rsge.length) { sk_msg_iter_var_next(i); nnsge = sk_msg_elem_cpy(msg, i); + nnsge_copy = sk_msg_elem_is_copy(msg, i); sk_msg_iter_next(msg, end); } while (i != msg->sg.end) { msg->sg.data[i] = sge; + if (sge_copy) + sk_msg_set_elem_copy(msg, i); + else + sk_msg_clear_elem_copy(msg, i); sge = nsge; + sge_copy = nsge_copy; sk_msg_iter_var_next(i); if (rsge.length) { nsge = nnsge; + nsge_copy = nnsge_copy; nnsge = sk_msg_elem_cpy(msg, i); + nnsge_copy = sk_msg_elem_is_copy(msg, i); } else { nsge = sk_msg_elem_cpy(msg, i); + nsge_copy = sk_msg_elem_is_copy(msg, i); } } @@ -2911,13 +2966,18 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg, u32, start, /* Place newly allocated data buffer */ sk_mem_charge(msg->sk, len); msg->sg.size += len; - __clear_bit(new, msg->sg.copy); + sk_msg_clear_elem_copy(msg, new); sg_set_page(&msg->sg.data[new], page, len + copy, 0); if (rsge.length) { get_page(sg_page(&rsge)); sk_msg_iter_var_next(new); msg->sg.data[new] = rsge; + if (rsge_copy) + sk_msg_set_elem_copy(msg, new); + else + sk_msg_clear_elem_copy(msg, new); } + sk_msg_clear_elem_copy(msg, msg->sg.end); sk_msg_reset_curr(msg); sk_msg_compute_data_pointers(msg); @@ -2943,27 +3003,38 @@ static void sk_msg_shift_left(struct sk_msg *msg, int i) do { prev = i; sk_msg_iter_var_next(i); - msg->sg.data[prev] = msg->sg.data[i]; + sk_msg_sg_move(msg, prev, i); } while (i != msg->sg.end); sk_msg_iter_prev(msg, end); + sk_msg_clear_elem_copy(msg, msg->sg.end); } static void sk_msg_shift_right(struct sk_msg *msg, int i) { struct scatterlist tmp, sge; + bool tmp_copy, sge_copy; sk_msg_iter_next(msg, end); sge = sk_msg_elem_cpy(msg, i); + sge_copy = sk_msg_elem_is_copy(msg, i); sk_msg_iter_var_next(i); tmp = sk_msg_elem_cpy(msg, i); + tmp_copy = sk_msg_elem_is_copy(msg, i); while (i != msg->sg.end) { msg->sg.data[i] = sge; + if (sge_copy) + sk_msg_set_elem_copy(msg, i); + else + sk_msg_clear_elem_copy(msg, i); sk_msg_iter_var_next(i); sge = tmp; + sge_copy = tmp_copy; tmp = sk_msg_elem_cpy(msg, i); + tmp_copy = sk_msg_elem_is_copy(msg, i); } + sk_msg_clear_elem_copy(msg, msg->sg.end); } BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u32, start, @@ -3020,8 +3091,10 @@ BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u32, start, */ if (start != offset) { struct scatterlist *nsge, *sge = sk_msg_elem(msg, i); + u32 sge_idx = i; int a = start - offset; int b = sge->length - pop - a; + bool sge_copy = sk_msg_elem_is_copy(msg, sge_idx); sk_msg_iter_var_next(i); @@ -3034,6 +3107,10 @@ BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u32, start, sg_set_page(nsge, sg_page(sge), b, sge->offset + pop + a); + if (sge_copy) + sk_msg_set_elem_copy(msg, i); + else + sk_msg_clear_elem_copy(msg, i); } else { struct page *page, *orig; u8 *to, *from; @@ -3050,6 +3127,7 @@ BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u32, start, memcpy(to, from, a); memcpy(to + a, from + a + pop, b); sg_set_page(sge, page, a + b, 0); + sk_msg_clear_elem_copy(msg, sge_idx); put_page(orig); } pop = 0; -- 2.43.0