[PATCH 1/2] NFSv4.1/pnfs: bound notification bitmap length in decode_getdeviceinfo

[Michael Bommarito]

decode_getdeviceinfo() reads a server-supplied notification bitmap
length into a u32 and scales it by four for xdr_inline_decode(). The
multiply is 32-bit and wraps to 0 for len >= 0x40000000, so the !p
bounds check passes on an empty request and the verify loop reads up to
~2^30 words past the inline XDR buffer. A malicious or compromised pNFS
server, or a man in the middle on an unprotected mount, can drive this
out-of-bounds read while the client decodes a GETDEVICEINFO reply.

RFC 8881 defines only the CHANGE and DELETE deviceid notification bits,
both in word 0, so a conformant reply uses a single word. Bound len to
NFS4_GETDEVICEINFO_NOTIFY_MAXWORDS before scaling, which cannot wrap and
still tolerates trailing zero words the decoder keeps verifying.

Reproduced under KASAN on QEMU via a KUnit case driving the real
decode_getdeviceinfo(); the out-of-bounds read is gone after this change.

Fixes: b1f69b754ee3 ("NFSv4.1: pnfs: add LAYOUTGET and GETDEVICEINFO infrastructure")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@xxxxxxxxx>
---

Reproduction (v7.1-rc4, x86_64 QEMU/KVM, KASAN):

  BUG: KASAN: slab-out-of-bounds in decode_getdeviceinfo

A KUnit case (patch 2) drives the real decode_getdeviceinfo() with a
0x40000000 notification length placed at a page edge; the wrapped 4*len
multiply over-reads the receive buffer on stock and faults. Patched, the
length is rejected with -EIO. Two benign in-bounds controls pass on both
stock and patched. fs/nfs has no existing kselftest for this decoder.

 fs/nfs/nfs4xdr.c     | 6 +++++-
 include/linux/nfs4.h | 3 +++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/nfs4xdr.c b/fs/nfs/nfs4xdr.c
index c23c2eee1b5c4..ca84d0c872a6c 100644
--- a/fs/nfs/nfs4xdr.c
+++ b/fs/nfs/nfs4xdr.c
@@ -6089,7 +6089,11 @@ static int decode_getdeviceinfo(struct xdr_stream *xdr,
 	if (len) {
 		uint32_t i;
 
-		p = xdr_inline_decode(xdr, 4 * len);
+		/* Bound len so len << 2 cannot wrap and defeat the check below. */
+		if (len > NFS4_GETDEVICEINFO_NOTIFY_MAXWORDS)
+			return -EIO;
+
+		p = xdr_inline_decode(xdr, len << 2);
 		if (unlikely(!p))
 			return -EIO;
 
diff --git a/include/linux/nfs4.h b/include/linux/nfs4.h
index d87be1f25273a..d6a84c4545864 100644
--- a/include/linux/nfs4.h
+++ b/include/linux/nfs4.h
@@ -752,6 +752,9 @@ enum pnfs_notify_deviceid_type4 {
 	NOTIFY_DEVICEID4_DELETE = 1 << 2,
 };
 
+/* All defined deviceid notification bits live in word 0; bound the decoder. */
+#define NFS4_GETDEVICEINFO_NOTIFY_MAXWORDS	4
+
 enum pnfs_block_volume_type {
 	PNFS_BLOCK_VOLUME_SIMPLE	= 0,
 	PNFS_BLOCK_VOLUME_SLICE		= 1,
-- 
2.53.0