From f898439ace7f89922e853b43bcbfcd26ea2e0a98 Mon Sep 17 00:00:00 2001 From: Fang Xieyan Date: Tue, 22 Sep 2026 20:29:54 +0800 Subject: [PATCH] vringh: bound top-level re-entry into indirect tables In __vringh_iov(), the F_INDIRECT branch descends into the indirect table and continues before the descriptor accounting runs. A top-level indirect descriptor is therefore never charged to count. If such a descriptor sets NEXT to point back at itself, returning from the indirect table resumes at the same top-level descriptor, which is again not counted, so the walk never makes forward progress. Because count stays flat and indirect_count is reset to 0 on every return to the top-level table, neither bound in the loop check trips. A guest can spin the vringh worker at 100% CPU inside __vringh_iov(), an uninterruptible host DoS. Move the descriptor accounting above the indirect switch so that a top-level indirect descriptor is charged one top-level traversal step before the walk descends into its table, bringing this re-entry under the existing vrh->vring.num bound. Each top-level descriptor is still charged at most one step and indirect_count still bounds a single table, so legitimate chains (including the multiple-indirect case in tools/virtio/vringh_test.c) stay within vring.num, while a cyclic indirect descriptor is now rejected with -ELOOP. Fixes: dbd29e075228 ("vringh: Fix loop descriptors check in the indirect cases") Cc: Xie Yongji Cc: stable@vger.kernel.org Assisted-by: Hawkeye:GLM-5.3-flash Assisted-by: Qoder:Qwen3.8-Max Signed-off-by: Fang Xieyan Signed-off-by: Michael S. Tsirkin Message-ID: <20260922122955.69433-2-fangxy@xiaopeng.com> --- drivers/vhost/vringh.c | 22 +++++++++++----------- 1 file changed, 11 insertions(+), 11 deletions(-) diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c index 51edd54f7107..e242ae0c1e16 100644 --- a/drivers/vhost/vringh.c +++ b/drivers/vhost/vringh.c @@ -333,6 +333,17 @@ __vringh_iov(struct vringh *vrh, u16 i, if (unlikely(err)) goto fail; + if (up_next == -1) + count++; + else + indirect_count++; + + if (count > vrh->vring.num || indirect_count > desc_max) { + vringh_bad("Descriptor loop in %p", descs); + err = -ELOOP; + goto fail; + } + if (unlikely(desc.flags & cpu_to_vringh16(vrh, VRING_DESC_F_INDIRECT))) { u64 a = vringh64_to_cpu(vrh, desc.addr); @@ -358,17 +369,6 @@ __vringh_iov(struct vringh *vrh, u16 i, continue; } - if (up_next == -1) - count++; - else - indirect_count++; - - if (count > vrh->vring.num || indirect_count > desc_max) { - vringh_bad("Descriptor loop in %p", descs); - err = -ELOOP; - goto fail; - } - if (desc.flags & cpu_to_vringh16(vrh, VRING_DESC_F_WRITE)) iov = wiov; else {