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SubjectRe: Is ->sendmsg() allowed to change the msghdr struct it is given?
Date
Paolo Abeni <pabeni@redhat.com> wrote:

> udp_sendmsg() can set the MSG_TRUNC bit in msg->msg_flags, so I guess
> that kind of actions are sort of allowed. Still, AFAICS, the kernel
> based msghdr is not copied back to the user-space, so such change
> should be almost a no-op in practice.
>
> @David: which would be the end goal for such action?

Various places in the kernel use sock_sendmsg() - and I've added a bunch more
with the MSG_SPLICE_PAGES patches. For some of the things I've added, there's
a loop which used to call ->sendpage() and now calls sock_sendmsg(). In most
of those places, msghdr will get reset each time round the loop - but not in
all cases.

Of particular immediate interest is net/ceph/messenger_v2.c. If you go to:

https://lore.kernel.org/r/3111635.1687813501@warthog.procyon.org.uk/

and look at the resultant code:

static int do_sendmsg(struct socket *sock, struct iov_iter *it)
{
struct msghdr msg = { .msg_flags = CEPH_MSG_FLAGS };
int ret;

msg.msg_iter = *it;
while (iov_iter_count(it)) {
ret = sock_sendmsg(sock, &msg);
if (ret <= 0) {
if (ret == -EAGAIN)
ret = 0;
return ret;
}

iov_iter_advance(it, ret);
}

WARN_ON(msg_data_left(&msg));
return 1;
}

for example. It could/would malfunction if sendmsg() is allowed to modify
msghdr - or if it doesn't update msg_iter. Likewise:

static int do_try_sendpage(struct socket *sock, struct iov_iter *it)
{
struct msghdr msg = { .msg_flags = CEPH_MSG_FLAGS };
struct bio_vec bv;
int ret;

if (WARN_ON(!iov_iter_is_bvec(it)))
return -EINVAL;

while (iov_iter_count(it)) {
/* iov_iter_iovec() for ITER_BVEC */
bvec_set_page(&bv, it->bvec->bv_page,
min(iov_iter_count(it),
it->bvec->bv_len - it->iov_offset),
it->bvec->bv_offset + it->iov_offset);

/*
* MSG_SPLICE_PAGES cannot properly handle pages with
* page_count == 0, we need to fall back to sendmsg if
* that's the case.
*
* Same goes for slab pages: skb_can_coalesce() allows
* coalescing neighboring slab objects into a single frag
* which triggers one of hardened usercopy checks.
*/
if (sendpage_ok(bv.bv_page))
msg.msg_flags |= MSG_SPLICE_PAGES;
else
msg.msg_flags &= ~MSG_SPLICE_PAGES;

iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bv, 1, bv.bv_len);
ret = sock_sendmsg(sock, &msg);
if (ret <= 0) {
if (ret == -EAGAIN)
ret = 0;
return ret;
}

iov_iter_advance(it, ret);
}

return 1;
}

could be similarly affected if ->sendmsg() mucks about with msg_flags.

David

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