[PATCH] NFSv4/flexfiles: reject zero filehandle version count

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

ff_layout_alloc_lseg() decodes the filehandle-version array count
from the flexfiles layout body. The value is used as the count for
kzalloc_objs(), and the current code only rejects NULL.

A zero count yields ZERO_SIZE_PTR, which can be stored in
dss_info->fh_versions even though later flexfiles paths assume that at
least one filehandle version exists.

Reject fh_count == 0 before the allocation, matching the existing zero
version_count validation in the flexfiles GETDEVICEINFO parser.

A QEMU/KASAN run with a malformed flexfiles layout hit:

  KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
  RIP: 0010:ff_layout_encode_ff_layoutupdate.isra.0+0x15f/0x750
  ff_layout_encode_layoutreturn+0x683/0x970
  nfs4_xdr_enc_layoutreturn+0x278/0x3a0
  Kernel panic - not syncing: Fatal exception

The patched kernel rejects the malformed layout without KASAN/oops/panic,
and a valid fh_count=1 regression still opens, reads, and unmounts cleanly.

Cc: stable@vger.kernel.org
Fixes: d67ae825a59d ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 fs/nfs/flexfilelayout/flexfilelayout.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c b/fs/nfs/flexfilelayout/flexfilelayout.c
index 8b1559171fe3..e22a8e0daf2c 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.c
+++ b/fs/nfs/flexfilelayout/flexfilelayout.c
@@ -551,6 +551,10 @@ ff_layout_alloc_lseg(struct pnfs_layout_hdr *lh,
 			if (!p)
 				goto out_err_free;
 			fh_count = be32_to_cpup(p);
+			if (fh_count == 0) {
+				rc = -EINVAL;
+				goto out_err_free;
+			}
 
 			dss_info->fh_versions =
 			    kzalloc_objs(struct nfs_fh, fh_count, gfp_flags);
-- 
2.53.0