CVE-2021-47107

In the Linux kernel, the following vulnerability has been resolved: NFSD:
Fix READDIR buffer overflow If a client sends a READDIR count argument that
is too small (say, zero), then the buffer size calculation in the new
init_dirlist helper functions results in an underflow, allowing the XDR
stream functions to write beyond the actual buffer. This calculation has
always been suspect. NFSD has never sanity- checked the READDIR count
argument, but the old entry encoders managed the problem correctly. With
the commits below, entry encoding changed, exposing the underflow to the
pointer arithmetic in xdr_reserve_space(). Modern NFS clients attempt to
retrieve as much data as possible for each READDIR request. Also, we have
no unit tests that exercise the behavior of READDIR at the lower bound of
@count values. Thus this case was missed during testing.

Bugs

• <https://bugzilla.redhat.com/show_bug.cgi?id=2267911&gt;
• <https://bugzilla.suse.com/show_bug.cgi?id=1220965&gt;