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([2405:201:682f:389d:899a:49a8:c3d1:e400]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-359c0179a67sm4263149a91.13.2026.03.07.01.00.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 07 Mar 2026 01:00:49 -0800 (PST) From: Deepanshu Kartikey To: dhowells@redhat.com, pc@manguebit.org Cc: jlayton@kernel.org, netfs@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Deepanshu Kartikey , syzbot+9c058f0d63475adc97fd@syzkaller.appspotmail.com, Deepanshu Kartikey Subject: [PATCH] netfs: Fix kernel BUG in netfs_limit_iter() for ITER_KVEC iterators Date: Sat, 7 Mar 2026 14:30:41 +0530 Message-ID: <20260307090041.359870-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 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" When a process crashes and the kernel writes a core dump to a 9P filesystem, __kernel_write() creates an ITER_KVEC iterator. This iterator reaches netfs_limit_iter() via netfs_unbuffered_write(), which only handles ITER_FOLIOQ, ITER_BVEC and ITER_XARRAY iterator types, hitting the BUG() for any other type. Fix this by adding netfs_limit_kvec() following the same pattern as netfs_limit_bvec(), since both kvec and bvec are simple segment arrays with pointer and length fields. Dispatch it from netfs_limit_iter() when the iterator type is ITER_KVEC. Fixes: cae932d3aee5 ("netfs: Add func to calculate pagecount/size-limited s= pan of an iterator") Reported-by: syzbot+9c058f0d63475adc97fd@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3D9c058f0d63475adc97fd Tested-by: syzbot+9c058f0d63475adc97fd@syzkaller.appspotmail.com Signed-off-by: Deepanshu Kartikey Tested-by: Vitaly Chikunov --- fs/netfs/iterator.c | 43 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 43 insertions(+) diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c index 72a435e5fc6d..154a14bb2d7f 100644 --- a/fs/netfs/iterator.c +++ b/fs/netfs/iterator.c @@ -142,6 +142,47 @@ static size_t netfs_limit_bvec(const struct iov_iter *= iter, size_t start_offset, return min(span, max_size); } =20 +/* + * Select the span of a kvec iterator we're going to use. Limit it by both + * maximum size and maximum number of segments. Returns the size of the s= pan + * in bytes. + */ +static size_t netfs_limit_kvec(const struct iov_iter *iter, size_t start_o= ffset, + size_t max_size, size_t max_segs) +{ + const struct kvec *kvecs =3D iter->kvec; + unsigned int nkv =3D iter->nr_segs, ix =3D 0, nsegs =3D 0; + size_t len, span =3D 0, n =3D iter->count; + size_t skip =3D iter->iov_offset + start_offset; + + if (WARN_ON(!iov_iter_is_kvec(iter)) || + WARN_ON(start_offset > n) || + n =3D=3D 0) + return 0; + + while (n && ix < nkv && skip) { + len =3D kvecs[ix].iov_len; + if (skip < len) + break; + skip -=3D len; + n -=3D len; + ix++; + } + + while (n && ix < nkv) { + len =3D min3(n, kvecs[ix].iov_len - skip, max_size); + span +=3D len; + nsegs++; + ix++; + if (span >=3D max_size || nsegs >=3D max_segs) + break; + skip =3D 0; + n -=3D len; + } + + return min(span, max_size); +} + /* * Select the span of an xarray iterator we're going to use. Limit it by = both * maximum size and maximum number of segments. It is assumed that segmen= ts @@ -245,6 +286,8 @@ size_t netfs_limit_iter(const struct iov_iter *iter, si= ze_t start_offset, return netfs_limit_bvec(iter, start_offset, max_size, max_segs); if (iov_iter_is_xarray(iter)) return netfs_limit_xarray(iter, start_offset, max_size, max_segs); + if (iov_iter_is_kvec(iter)) + return netfs_limit_kvec(iter, start_offset, max_size, max_segs); BUG(); } EXPORT_SYMBOL(netfs_limit_iter); --=20 2.43.0