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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , 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 v2] bpf, sockmap: keep sk_msg copy state in sync Date: Sun, 17 May 2026 20:16:26 +0800 Message-Id: <20260517121626.406516-1-rollkingzzc@gmail.com> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 or stale in slots that no longer contain the original entry. A copied entry can therefore later occupy a slot whose copy bit is clear and be exposed as directly writable packet data. 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 every slot 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 --- v2: Sashiko-bot pointed out that bpf_msg_pull_data() could leave stale copy bits on collapsed tail entries. Clear msg->sg.copy for every entry consumed by bpf_msg_pull_data() before compacting the scatterlist ring. 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 | 66 +++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 64 insertions(+), 2 deletions(-) diff --git a/net/core/filter.c b/net/core/filter.c index 9590877b0714f..018c30a0d71fb 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -2654,6 +2654,27 @@ static void sk_msg_reset_curr(struct sk_msg *msg) } } =20 +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_set_elem_copy(struct sk_msg *msg, u32 i, bool copy) +{ + if (copy) + __set_bit(i, msg->sg.copy); + else + __clear_bit(i, msg->sg.copy); +} + +static void sk_msg_clear_copy_range(struct sk_msg *msg, u32 start, u32 end) +{ + while (start !=3D end) { + __clear_bit(start, msg->sg.copy); + sk_msg_iter_var_next(start); + } +} + static const struct bpf_func_proto bpf_msg_cork_bytes_proto =3D { .func =3D bpf_msg_cork_bytes, .gpl_only =3D false, @@ -2738,6 +2759,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u= 32, start, } while (i !=3D last_sge); =20 sg_set_page(&msg->sg.data[first_sge], page, copy, 0); + sk_msg_set_elem_copy(msg, first_sge, false); =20 /* To repair sg ring we need to shift entries. If we only * had a single entry though we can just replace it and @@ -2747,13 +2769,20 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg,= u32, start, shift =3D last_sge > first_sge ? last_sge - first_sge - 1 : NR_MSG_FRAG_IDS - first_sge + last_sge - 1; - if (!shift) + if (!shift) { + sk_msg_set_elem_copy(msg, msg->sg.end, false); goto out; + } + + i =3D first_sge; + sk_msg_iter_var_next(i); + sk_msg_clear_copy_range(msg, i, last_sge); =20 i =3D first_sge; sk_msg_iter_var_next(i); do { u32 move_from; + bool move_copy; =20 if (i + shift >=3D NR_MSG_FRAG_IDS) move_from =3D i + shift - NR_MSG_FRAG_IDS; @@ -2762,16 +2791,20 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg,= u32, start, if (move_from =3D=3D msg->sg.end) break; =20 + move_copy =3D sk_msg_elem_is_copy(msg, move_from); msg->sg.data[i] =3D msg->sg.data[move_from]; + sk_msg_set_elem_copy(msg, i, move_copy); msg->sg.data[move_from].length =3D 0; msg->sg.data[move_from].page_link =3D 0; msg->sg.data[move_from].offset =3D 0; + sk_msg_set_elem_copy(msg, move_from, false); sk_msg_iter_var_next(i); } while (1); =20 msg->sg.end =3D msg->sg.end - shift > msg->sg.end ? msg->sg.end - shift + NR_MSG_FRAG_IDS : msg->sg.end - shift; + sk_msg_set_elem_copy(msg, msg->sg.end, false); out: sk_msg_reset_curr(msg); msg->data =3D sg_virt(&msg->sg.data[first_sge]) + start - offset; @@ -2794,6 +2827,8 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg, u= 32, start, { struct scatterlist sge, nsge, nnsge, rsge =3D {0}, *psge; u32 new, i =3D 0, l =3D 0, space, copy =3D 0, offset =3D 0; + bool sge_copy =3D false, nsge_copy =3D false, nnsge_copy =3D false; + bool rsge_copy =3D false; u8 *raw, *to, *from; struct page *page; =20 @@ -2866,6 +2901,7 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg, u= 32, start, sk_msg_iter_var_prev(i); psge =3D sk_msg_elem(msg, i); rsge =3D sk_msg_elem_cpy(msg, i); + rsge_copy =3D sk_msg_elem_is_copy(msg, i); =20 psge->length =3D start - offset; rsge.length -=3D psge->length; @@ -2891,23 +2927,31 @@ BPF_CALL_4(bpf_msg_push_data, struct sk_msg *, msg,= u32, start, /* Shift one or two slots as needed */ sge =3D sk_msg_elem_cpy(msg, new); sg_unmark_end(&sge); + sge_copy =3D sk_msg_elem_is_copy(msg, new); =20 nsge =3D sk_msg_elem_cpy(msg, i); + nsge_copy =3D sk_msg_elem_is_copy(msg, i); if (rsge.length) { sk_msg_iter_var_next(i); nnsge =3D sk_msg_elem_cpy(msg, i); + nnsge_copy =3D sk_msg_elem_is_copy(msg, i); sk_msg_iter_next(msg, end); } =20 while (i !=3D msg->sg.end) { msg->sg.data[i] =3D sge; + sk_msg_set_elem_copy(msg, i, sge_copy); sge =3D nsge; + sge_copy =3D nsge_copy; sk_msg_iter_var_next(i); if (rsge.length) { nsge =3D nnsge; + nsge_copy =3D nnsge_copy; nnsge =3D sk_msg_elem_cpy(msg, i); + nnsge_copy =3D sk_msg_elem_is_copy(msg, i); } else { nsge =3D sk_msg_elem_cpy(msg, i); + nsge_copy =3D sk_msg_elem_is_copy(msg, i); } } =20 @@ -2915,13 +2959,15 @@ 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 +=3D len; - __clear_bit(new, msg->sg.copy); + sk_msg_set_elem_copy(msg, new, false); 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] =3D rsge; + sk_msg_set_elem_copy(msg, new, rsge_copy); } + sk_msg_set_elem_copy(msg, msg->sg.end, false); =20 sk_msg_reset_curr(msg); sk_msg_compute_data_pointers(msg); @@ -2945,29 +2991,41 @@ static void sk_msg_shift_left(struct sk_msg *msg, i= nt i) =20 put_page(sg_page(sge)); do { + bool copy; + prev =3D i; sk_msg_iter_var_next(i); + copy =3D sk_msg_elem_is_copy(msg, i); msg->sg.data[prev] =3D msg->sg.data[i]; + sk_msg_set_elem_copy(msg, prev, copy); } while (i !=3D msg->sg.end); =20 sk_msg_iter_prev(msg, end); + sk_msg_set_elem_copy(msg, msg->sg.end, false); } =20 static void sk_msg_shift_right(struct sk_msg *msg, int i) { struct scatterlist tmp, sge; + bool tmp_copy, sge_copy; =20 sk_msg_iter_next(msg, end); sge =3D sk_msg_elem_cpy(msg, i); + sge_copy =3D sk_msg_elem_is_copy(msg, i); sk_msg_iter_var_next(i); tmp =3D sk_msg_elem_cpy(msg, i); + tmp_copy =3D sk_msg_elem_is_copy(msg, i); =20 while (i !=3D msg->sg.end) { msg->sg.data[i] =3D sge; + sk_msg_set_elem_copy(msg, i, sge_copy); sk_msg_iter_var_next(i); sge =3D tmp; + sge_copy =3D tmp_copy; tmp =3D sk_msg_elem_cpy(msg, i); + tmp_copy =3D sk_msg_elem_is_copy(msg, i); } + sk_msg_set_elem_copy(msg, msg->sg.end, false); } =20 BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u32, start, @@ -3024,8 +3082,10 @@ BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u= 32, start, */ if (start !=3D offset) { struct scatterlist *nsge, *sge =3D sk_msg_elem(msg, i); + u32 sge_idx =3D i; int a =3D start - offset; int b =3D sge->length - pop - a; + bool sge_copy =3D sk_msg_elem_is_copy(msg, sge_idx); =20 sk_msg_iter_var_next(i); =20 @@ -3038,6 +3098,7 @@ BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u3= 2, start, sg_set_page(nsge, sg_page(sge), b, sge->offset + pop + a); + sk_msg_set_elem_copy(msg, i, sge_copy); } else { struct page *page, *orig; u8 *to, *from; @@ -3054,6 +3115,7 @@ BPF_CALL_4(bpf_msg_pop_data, struct sk_msg *, msg, u3= 2, start, memcpy(to, from, a); memcpy(to + a, from + a + pop, b); sg_set_page(sge, page, a + b, 0); + sk_msg_set_elem_copy(msg, sge_idx, false); put_page(orig); } pop =3D 0; --=20 2.43.0