[PATCH] pnfs/flexfiles: validate ds_versions_cnt is non-zero

[Jenny Guanni Qu <qguanni@gmail.com>]

nfs4_ff_alloc_deviceid_node() reads version_count from XDR without
checking it is non-zero. When a malicious NFS server sends a pNFS
LAYOUTGET response with version_count=0, kcalloc(0, ...) returns
ZERO_SIZE_PTR (0x10). The subsequent ds_versions[0] access in
nfs4_ff_layout_ds_version() and other callers dereferences this
invalid pointer, causing an out-of-bounds read.

Add a check for version_count == 0 after parsing it from XDR, before
the allocation.

The OOB read was confirmed with KASAN: null-ptr-deref in range
[0x0000000000000010-0x0000000000000017] from accessing ZERO_SIZE_PTR.

Fixes: d67ae825a59d ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Reported-by: Klaudia Kloc <klaudia@vidocsecurity.com>
Reported-by: Dawid Moczadło <dawid@vidocsecurity.com>
Tested-by: Jenny Guanni Qu <qguanni@gmail.com>
Signed-off-by: Jenny Guanni Qu <qguanni@gmail.com>
---
 fs/nfs/flexfilelayout/flexfilelayoutdev.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/nfs/flexfilelayout/flexfilelayoutdev.c b/fs/nfs/flexfilelayout/flexfilelayoutdev.c
index e58bedfb1dcc..ee48920208e2 100644
--- a/fs/nfs/flexfilelayout/flexfilelayoutdev.c
+++ b/fs/nfs/flexfilelayout/flexfilelayoutdev.c
@@ -97,6 +97,11 @@ nfs4_ff_alloc_deviceid_node(struct nfs_server *server, struct pnfs_device *pdev,
 	if (unlikely(!p))
 		goto out_err_drain_dsaddrs;
 	version_count = be32_to_cpup(p);
+
+	if (version_count == 0) {
+		ret = -EINVAL;
+		goto out_err_drain_dsaddrs;
+	}
 	dprintk("%s: version count %d\n", __func__, version_count);
 
 	ds_versions = kcalloc(version_count,
-- 
2.34.1