[PATCH 0/2] vringh: fix infinite loop on cyclic top-level indirect descriptor

[Fang Xieyan]

__vringh_iov()'s F_INDIRECT branch does `continue` before the descriptor
accounting block, so a top-level indirect descriptor is never charged to
`count`.  When such a descriptor also carries NEXT pointing to itself,
returning from the indirect table resumes at the same uncharged descriptor;
`count` stays flat and `indirect_count` is reset to 0 on every return, so
neither bound in the loop check can ever trip.  A guest can keep the host
vringh worker spinning inside __vringh_iov(), causing a host DoS.

Patch 1/2 moves the accounting block above the INDIRECT switch so each
top-level descriptor, including INDIRECT ones, is charged one traversal step
before the walk descends.  Legitimate chains and indirect descriptors stay
within vring.num; a cyclic indirect descriptor is now rejected with -ELOOP.
Patch 1 carries the Fixes tag and is Cc'd to stable.

Patch 2/2 appends a regression test to tools/virtio/vringh_test.c.  It builds
the offending ring: top-level desc[1].flags = VRING_DESC_F_INDIRECT |
VRING_DESC_F_NEXT and desc[1].next = 1, with a single-entry indirect table,
and asserts that vringh_getdesc_user() returns -ELOOP.

Verified against v7.3-rc3: both patches pass git apply --check.  With the fix,
vringh_test --indirect observes -ELOOP and exits 0.  With the fix reverted, the
walk re-enters the same descriptor without making forward progress and never
returns -ELOOP, so the assertion does not hold and the regression is caught
either way.

Fang Xieyan (2):
  vringh: bound top-level re-entry into indirect tables
  vringh: add regression test for cyclic indirect descriptor

 drivers/vhost/vringh.c     | 22 ++++++++++----------
 tools/virtio/vringh_test.c | 41 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 11 deletions(-)

-- 
2.50.1