[PATCH 6.1.y] NFSv4/flexfiles: reject zero filehandle version count

[Sasha Levin <sashal@kernel.org>]

From: Michael Bommarito <michael.bommarito@gmail.com>

[ Upstream commit 2c6bb3c40bc24f6aa8dfbe6fe98c3ad6389203f2 ]

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>
Signed-off-by: Anna Schumaker <anna.schumaker@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 e84ac71bdc18f8..01378fb1958451 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.c
+++ b/fs/nfs/flexfilelayout/flexfilelayout.c
@@ -460,6 +460,10 @@ 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;
+		}
 
 		fls->mirror_array[i]->fh_versions =
 			kcalloc(fh_count, sizeof(struct nfs_fh),
-- 
2.53.0