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c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1757527601; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Uq/ffucX0LrsPavRUjZZ8ksU3DGad+yaH3VgAUHZsZo=; b=fSKjbcwwn41cSTCRnBzJ6qlXFtFq9LHLG6tl653IFKzKSvTfkwN7rW1O8khDcFDLUh53Zq wGevrc95sB+hTNFwX10D1g0A9CwgAhLvxBUvFj6PYOM7jqGN4CrDjg/qq2BcWJgGW/tIRd vZ5fCTzhylr+H2NRzoUVUzEW2ytMFfo= X-MC-Unique: xmhrZunnO8WCF9k4-hZkpQ-1 X-Mimecast-MFC-AGG-ID: xmhrZunnO8WCF9k4-hZkpQ_1757527599 From: Stefan Hajnoczi To: qemu-devel@nongnu.org Cc: Aarushi Mehta , Fam Zheng , Stefano Garzarella , Hanna Czenczek , eblake@redhat.com, Stefan Hajnoczi , Kevin Wolf , qemu-block@nongnu.org, hibriansong@gmail.com, Stefan Weil , Paolo Bonzini Subject: [PATCH v4 01/12] aio-posix: fix race between io_uring CQE and AioHandler deletion Date: Wed, 10 Sep 2025 13:56:52 -0400 Message-ID: <20250910175703.374499-2-stefanha@redhat.com> In-Reply-To: <20250910175703.374499-1-stefanha@redhat.com> References: <20250910175703.374499-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; 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charset="utf-8" When an AioHandler is enqueued on ctx->submit_list for removal, the fill_sq_ring() function will submit an io_uring POLL_REMOVE operation to cancel the in-flight POLL_ADD operation. There is a race when another thread enqueues an AioHandler for deletion on ctx->submit_list when the POLL_ADD CQE has already appeared. In that case POLL_REMOVE is unnecessary. The code already handled this, but forgot that the AioHandler itself is still on ctx->submit_list when the POLL_ADD CQE is being processed. It's unsafe to delete the AioHandler at that point in time (use-after-free). Solve this problem by keeping the AioHandler alive but setting a flag so that it will be deleted by fill_sq_ring() when it runs. Signed-off-by: Stefan Hajnoczi Reviewed-by: Eric Blake --- util/fdmon-io_uring.c | 33 ++++++++++++++++++++++++++------- 1 file changed, 26 insertions(+), 7 deletions(-) diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c index b0d68bdc44..ad89160f31 100644 --- a/util/fdmon-io_uring.c +++ b/util/fdmon-io_uring.c @@ -52,9 +52,10 @@ enum { FDMON_IO_URING_ENTRIES =3D 128, /* sq/cq ring size */ =20 /* AioHandler::flags */ - FDMON_IO_URING_PENDING =3D (1 << 0), - FDMON_IO_URING_ADD =3D (1 << 1), - FDMON_IO_URING_REMOVE =3D (1 << 2), + FDMON_IO_URING_PENDING =3D (1 << 0), + FDMON_IO_URING_ADD =3D (1 << 1), + FDMON_IO_URING_REMOVE =3D (1 << 2), + FDMON_IO_URING_DELETE_AIO_HANDLER =3D (1 << 3), }; =20 static inline int poll_events_from_pfd(int pfd_events) @@ -218,6 +219,16 @@ static void fill_sq_ring(AioContext *ctx) if (flags & FDMON_IO_URING_REMOVE) { add_poll_remove_sqe(ctx, node); } + if (flags & FDMON_IO_URING_DELETE_AIO_HANDLER) { + /* + * process_cqe() sets this flag after ADD and REMOVE have been + * cleared. They cannot be set again, so they must be clear. + */ + assert(!(flags & FDMON_IO_URING_ADD)); + assert(!(flags & FDMON_IO_URING_REMOVE)); + + QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_d= eleted); + } } } =20 @@ -241,7 +252,12 @@ static bool process_cqe(AioContext *ctx, */ flags =3D qatomic_fetch_and(&node->flags, ~FDMON_IO_URING_REMOVE); if (flags & FDMON_IO_URING_REMOVE) { - QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_delet= ed); + if (flags & FDMON_IO_URING_PENDING) { + /* Still on ctx->submit_list, defer deletion until fill_sq_rin= g() */ + qatomic_or(&node->flags, FDMON_IO_URING_DELETE_AIO_HANDLER); + } else { + QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_d= eleted); + } return false; } =20 @@ -347,10 +363,13 @@ void fdmon_io_uring_destroy(AioContext *ctx) unsigned flags =3D qatomic_fetch_and(&node->flags, ~(FDMON_IO_URING_PENDING | FDMON_IO_URING_ADD | - FDMON_IO_URING_REMOVE)); + FDMON_IO_URING_REMOVE | + FDMON_IO_URING_DELETE_AIO_HANDLER)); =20 - if (flags & FDMON_IO_URING_REMOVE) { - QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, no= de_deleted); + if ((flags & FDMON_IO_URING_REMOVE) || + (flags & FDMON_IO_URING_DELETE_AIO_HANDLER)) { + QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, + node, node_deleted); } =20 QSLIST_REMOVE_HEAD_RCU(&ctx->submit_list, node_submitted); --=20 2.51.0