In the Linux kernel, the following vulnerability has been resolved:
nfsd: defer vfree of compound ops to fix rpc_status UAF
The rpc_status netlink dumpit walks every in-flight svc_rqst under
rcu_read_lock and, for NFSv4 requests, reads opnums out of
args->ops[]. But args->ops is a separate vmalloc buffer freed
synchronously by vfree() in nfsd4_release_compoundargs() at the end
of every compound. The dumpit’s rcu_read_lock pins the svc_rqst
struct itself (freed via kfree_rcu), but nothing defers the vfree
of the ops buffer across the RCU grace period. A concurrent compound
completion can therefore free the buffer while the dumpit is reading
it — a use-after-free on vmalloc memory.
The trailing seqcount recheck (smp_load_acquire of rq_status_counter)
cannot undo a load that already retired against freed memory.
Fix by replacing vfree(args->ops) with kvfree_rcu_mightsleep(), which
defers the free until after an RCU grace period. This makes the
existing rcu_read_lock in the dumpit sufficient to protect the read.
The tradeoff is that completed compound ops buffers (up to
200 * sizeof(struct nfsd4_op)) persist in memory slightly longer,
across one grace period, before being reclaimed.
