In the Linux kernel, the following vulnerability has been resolved: nfsd: add missing read barrier to rpc_status_get dumpit seqcount retry

In the Linux kernel, the following vulnerability has been resolved:

nfsd: add missing read barrier to rpc_status_get dumpit seqcount retry

The hand-rolled seqcount-like protocol in nfsd_nl_rpc_status_get_dumpit()
is missing a read memory barrier (smp_rmb) before its second counter
check. The standard kernel read_seqcount_retry() includes smp_rmb()
to ensure that all data reads complete before the counter is re-checked.

Without this barrier, on weakly-ordered architectures (ARM, POWER),
the CPU may reorder field reads past the second counter check, making
the retry logic ineffective: it could observe a consistent counter pair
while reading fields that have been concurrently modified by the writer.

Add smp_rmb() before the second counter check to order the field reads
ahead of it, matching the barrier semantics of the standard seqcount
read-side. The begin-side smp_load_acquire() already pairs with the
smp_store_release() in nfsd_dispatch(); with the smp_rmb() now ordering
the field reads, the retry check no longer needs acquire semantics and
reads the counter with a plain READ_ONCE(), as read_seqcount_retry()
does.

[ cel: Use READ_ONCE instead of smp_load_acquire() ]