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[68.48.65.54]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8cc812e0413sm188819356d6.31.2026.05.27.09.30.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 27 May 2026 09:30:53 -0700 (PDT) From: Michael Bommarito To: Trond Myklebust , Anna Schumaker Cc: Jeff Layton , Tom Haynes , Peng Tao , Kees Cook , Mike Snitzer , Tigran Mkrtchyan , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: [PATCH v2 1/2] NFSv4/pNFS: reject zero-length r_addr in nfs4_decode_mp_ds_addr Date: Wed, 27 May 2026 12:30:35 -0400 Message-ID: <20260527163036.1524927-2-michael.bommarito@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260527163036.1524927-1-michael.bommarito@gmail.com> References: <20260527163036.1524927-1-michael.bommarito@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 7bit nfs4_decode_mp_ds_addr() decodes the r_netid and r_addr opaques of a netaddr4 from a GETDEVICEINFO multipath-DS body, then immediately calls strrchr(buf, '.') to locate the port separator. Both decodes use xdr_stream_decode_string_dup(), and the current code checks only "nlen < 0" / "rlen < 0" before dereferencing the returned string. When the on-wire opaque has length zero, xdr_stream_decode_opaque_inline() returns 0 and xdr_stream_decode_string_dup() falls through to its "*str = NULL; return ret" tail, leaving buf NULL with a return value of 0. The "< 0" check does not catch this, and the next line is strrchr(NULL, '.'), a kernel NULL pointer dereference reachable from any pNFS-flexfile client mounted against a malicious or compromised metadata server. Reject the zero-length cases explicitly so the decoder fails with -EBADMSG (treated as a malformed GETDEVICEINFO body) instead of panicking the client. Cc: stable@vger.kernel.org Fixes: 6b7f3cf96364 ("nfs41: pull decode_ds_addr from file layout to generic pnfs") Assisted-by: Claude:claude-opus-4-7 Signed-off-by: Michael Bommarito --- Reproduced via a malicious NFSv4.1/pNFS server returning a flexfile GETDEVICEINFO body with multipath_count >= 3 and one valid (netid, uaddr) pair. Linux 7.0-rc7 + KASAN, QEMU/KVM. Stock kernel: Oops: general protection fault [#1] SMP KASAN NOPTI KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] RIP: 0010:strrchr+0x24/0x80 Call Trace: nfs4_decode_mp_ds_addr+0xca/0x570 nfs4_ff_alloc_deviceid_node+0x357/0x1370 nfs4_find_get_deviceid+0x6b6/0xa90 nfs4_ff_layout_prepare_ds+0x3cf/0xa40 ff_layout_choose_ds_for_read+0x14c/0x350 ff_layout_pg_init_read+0x2a2/0xb90 ... nfs_file_splice_read+0xcf/0x190 do_sendfile+0x8eb/0xdf0 Kernel panic - not syncing: Fatal exception Deterministic for any multipath_count >= 3; multipath_count <= 2 does not crash because the loop ends before consuming the malformed trailing bytes. Patched kernel rejects the same crafted body and a baseline multipath_count = 1 mount + read completes normally. fs/nfs/pnfs_nfs.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/fs/nfs/pnfs_nfs.c b/fs/nfs/pnfs_nfs.c index 12632a706da88..0ff43dbcb7cd7 100644 --- a/fs/nfs/pnfs_nfs.c +++ b/fs/nfs/pnfs_nfs.c @@ -1075,14 +1075,14 @@ nfs4_decode_mp_ds_addr(struct net *net, struct xdr_stream *xdr, gfp_t gfp_flags) /* r_netid */ nlen = xdr_stream_decode_string_dup(xdr, &netid, XDR_MAX_NETOBJ, gfp_flags); - if (unlikely(nlen < 0)) + if (unlikely(nlen <= 0)) goto out_err; /* r_addr: ip/ip6addr with port in dec octets - see RFC 5665 */ /* port is ".ABC.DEF", 8 chars max */ rlen = xdr_stream_decode_string_dup(xdr, &buf, INET6_ADDRSTRLEN + IPV6_SCOPE_ID_LEN + 8, gfp_flags); - if (unlikely(rlen < 0)) + if (unlikely(rlen <= 0)) goto out_free_netid; /* replace port '.' with '-' */ -- 2.53.0