[PATCH] pnfs/blocklayout: reject zero chunk_size and volumes_count in GETDEVICEINFO

nfs4_block_decode_volume() in fs/nfs/blocklayout/dev.c decodes stripe
parameters from GETDEVICEINFO XDR without checking for zero values.
A malicious pNFS server returning chunk_size=0 causes a division-by-
zero panic in bl_map_stripe() via div_u64(offset, dev->chunk_size).
Separately, volumes_count=0 passes the existing upper-bound check
and causes a second division-by-zero via div_u64_rem(chunk,
dev->nr_children=0).

Impact: a malicious or compromised pNFS blocklayout server can panic an
affected Linux NFS client after the client mounts/uses the server and maps
I/O through the poisoned blocklayout. An in-kernel parser/mapper KUnit
reproducer is available privately.

Reject both zero values at decode time with -EIO.

Fixes: 5c83746a0cf2 ("pnfs/blocklayout: in-kernel GETDEVICEINFO XDR parsing")
Cc: [email protected]
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <[email protected]>
---
 fs/nfs/blocklayout/dev.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/nfs/blocklayout/dev.c b/fs/nfs/blocklayout/dev.c
index cc6327d97a91a..fc60669db3ec4 100644
--- a/fs/nfs/blocklayout/dev.c
+++ b/fs/nfs/blocklayout/dev.c
@@ -183,8 +183,11 @@ nfs4_block_decode_volume(struct xdr_stream *xdr, struct pnfs_block_volume *b)
 			return -EIO;
 
 		p = xdr_decode_hyper(p, &b->stripe.chunk_size);
+		if (!b->stripe.chunk_size)
+			return -EIO;
 		b->stripe.volumes_count = be32_to_cpup(p++);
-		if (b->stripe.volumes_count > PNFS_BLOCK_MAX_DEVICES) {
+		if (!b->stripe.volumes_count ||
+		    b->stripe.volumes_count > PNFS_BLOCK_MAX_DEVICES) {
 			dprintk("Too many volumes: %d\n", b->stripe.volumes_count);
 			return -EIO;
 		}
-- 
2.53.0