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nfsd: don't replace page in rq_pages if it's a continuation of last page

Jeffrey Layton requested to merge jtlayton/centos-stream-9:bz2179342 into main

Bugzilla: https://bugzilla.redhat.com/2179342

commit 27c934dd8832dd40fd34776f916dc201e18b319b
Author: Jeff Layton <jlayton@kernel.org>
Date:   Fri Mar 17 13:13:08 2023 -0400

    nfsd: don't replace page in rq_pages if it's a continuation of last page

    The splice read calls nfsd_splice_actor to put the pages containing file
    data into the svc_rqst->rq_pages array. It's possible however to get a
    splice result that only has a partial page at the end, if (e.g.) the
    filesystem hands back a short read that doesn't cover the whole page.

    nfsd_splice_actor will plop the partial page into its rq_pages array and
    return. Then later, when nfsd_splice_actor is called again, the
    remainder of the page may end up being filled out. At this point,
    nfsd_splice_actor will put the page into the array _again_ corrupting
    the reply. If this is done enough times, rq_next_page will overrun the
    array and corrupt the trailing fields -- the rq_respages and
    rq_next_page pointers themselves.

    If we've already added the page to the array in the last pass, don't add
    it to the array a second time when dealing with a splice continuation.
    This was originally handled properly in nfsd_splice_actor, but commit
    91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()") removed the check
    for it.

    Fixes: 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()")
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Reported-by: Dario Lesca <d.lesca@solinos.it>
    Tested-by: David Critch <dcritch@redhat.com>
    Link: https://bugzilla.redhat.com/show_bug.cgi?id=2150630
    Signed-off-by: Jeff Layton <jlayton@kernel.org>
    Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Signed-off-by: Jeffrey Layton jlayton@redhat.com

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