[PATCH] NFSv4.1/pnfs: bound layout-type count in decode_pnfs_layout_types

[Michael Bommarito <michael.bommarito@gmail.com>]

decode_pnfs_layout_types() reads a server-controlled u32 count and then
reserves 4 * count bytes:

	fsinfo->nlayouttypes = be32_to_cpup(p);
	...
	p = xdr_inline_decode(xdr, fsinfo->nlayouttypes * 4);

The multiplication is u32, so any count >= 0x40000000 wraps to a small
value and xdr_inline_decode() reserves too few bytes. The NFS_MAX_LAYOUT
cap is applied only afterwards, so the subsequent reads run past the
short reservation. array_size() is not a safe guard here either:
xdr_inline_decode() runs its nbytes argument through
XDR_QUADLEN(((l) + 3) >> 2), which wraps SIZE_MAX to a zero-word
reservation instead of failing.

Reject counts that cannot fit in the u32 multiplication before the
reservation, and use sizeof(__be32) so the size arithmetic is explicit.

A malicious NFSv4.1+ server returning a crafted FATTR4_FS_LAYOUT_TYPES
attribute triggers this on the client during FSINFO decode.

This is the decode_pnfs_layout_types companion to the same XDR-wrap class
fixed in decode_getdeviceinfo (NFSv4.1/pnfs: bound notification bitmap
length in decode_getdeviceinfo).

Fixes: ca440c383a58 ("pnfs: add a new mechanism to select a layout driver according to an ordered list")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
Reproduced on a UML KASAN build with a KUnit case that feeds
decode_pnfs_layout_types() an xdr buffer whose nlayouttypes is 0x40000001:
nlayouttypes * 4 wraps, xdr_inline_decode() reserves a short buffer, and
the decode reads past xdr->end (KASAN slab-out-of-bounds read). With this
patch the count is rejected (-EIO) before the reservation. Benign control:
a small nlayouttypes decodes correctly on both stock and patched. The
KUnit test can ride as a separate patch (matching the decode_getdeviceinfo
series). Before/after logs available on request.

 fs/nfs/nfs4xdr.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/nfs4xdr.c b/fs/nfs/nfs4xdr.c
index c23c2eee1b5c4..8b7994f61d303 100644
--- a/fs/nfs/nfs4xdr.c
+++ b/fs/nfs/nfs4xdr.c
@@ -4903,8 +4903,16 @@ static int decode_pnfs_layout_types(struct xdr_stream *xdr,
 	if (fsinfo->nlayouttypes == 0)
 		return 0;
 
+	/* Reject counts that would overflow the u32 multiplication below.
+	 * array_size() is not sufficient here: xdr_inline_decode() passes
+	 * nbytes through XDR_QUADLEN(((l)+3)>>2), which wraps SIZE_MAX to
+	 * a zero-word reservation rather than failing.
+	 */
+	if (fsinfo->nlayouttypes > U32_MAX / sizeof(__be32))
+		return -EIO;
+
 	/* Decode and set first layout type, move xdr->p past unused types */
-	p = xdr_inline_decode(xdr, fsinfo->nlayouttypes * 4);
+	p = xdr_inline_decode(xdr, fsinfo->nlayouttypes * sizeof(__be32));
 	if (unlikely(!p))
 		return -EIO;
 

base-commit: ef0c9f75a19532d7675384708fc8621e10850104
-- 
2.53.0