[PATCH v2] nfsd: prevent integer overflow in decode_cb_compound4res()

[Dan Carpenter <dan.carpenter@linaro.org>]

If "length" is >= U32_MAX - 3 then the "length + 4" addition can result
in an integer overflow.  The impact of this bug is not totally clear to
me, but it's safer to not allow the integer overflow.

Check that "length" is valid right away before doing any math.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
---
v2: Check that "len" is valid instead of just checking for integer
    overflows.

 fs/nfsd/nfs4callback.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index 43b8320c8255..0f5b7b6fba74 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -317,6 +317,8 @@ static int decode_cb_compound4res(struct xdr_stream *xdr,
 	hdr->status = be32_to_cpup(p++);
 	/* Ignore the tag */
 	length = be32_to_cpup(p++);
+	if (unlikely(length > xdr->buf->len))
+		goto out_overflow;
 	p = xdr_inline_decode(xdr, length + 4);
 	if (unlikely(p == NULL))
 		goto out_overflow;
-- 
2.34.1