[SRU H/I/OEM-5.14] NFSD: Fix exposure in nfsd4_decode_bitmap()

[Thadeu Lima de Souza Cascardo]

From: Chuck Lever <chuck.lever at oracle.com>

rtm at csail.mit.edu reports:
> nfsd4_decode_bitmap4() will write beyond bmval[bmlen-1] if the RPC
> directs it to do so. This can cause nfsd4_decode_state_protect4_a()
> to write client-supplied data beyond the end of
> nfsd4_exchange_id.spo_must_allow[] when called by
> nfsd4_decode_exchange_id().

Rewrite the loops so nfsd4_decode_bitmap() cannot iterate beyond
@bmlen.

Reported by: rtm at csail.mit.edu
Fixes: d1c263a031e8 ("NFSD: Replace READ* macros in nfsd4_decode_fattr()")
Signed-off-by: Chuck Lever <chuck.lever at oracle.com>
Signed-off-by: J. Bruce Fields <bfields at redhat.com>
(cherry picked from commit c0019b7db1d7ac62c711cda6b357a659d46428fe)
CVE-2021-4090
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 fs/nfsd/nfs4xdr.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index be64d3f5e411..0017da5bf23f 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -288,11 +288,8 @@ nfsd4_decode_bitmap4(struct nfsd4_compoundargs *argp, u32 *bmval, u32 bmlen)
 	p = xdr_inline_decode(argp->xdr, count << 2);
 	if (!p)
 		return nfserr_bad_xdr;
-	i = 0;
-	while (i < count)
-		bmval[i++] = be32_to_cpup(p++);
-	while (i < bmlen)
-		bmval[i++] = 0;
+	for (i = 0; i < bmlen; i++)
+		bmval[i] = (i < count) ? be32_to_cpup(p++) : 0;
 
 	return nfs_ok;
 }
-- 
2.32.0