NFS: Prevent memory allocation failure in nfsacl_encode()

nfsacl_encode() allocates memory in certain cases.  This of course
is not guaranteed to work.

Since commit 9f06c719 "SUNRPC: New xdr_streams XDR encoder API", the
kernel's XDR encoders can't return a result indicating possibly a
failure, so a memory allocation failure in nfsacl_encode() has become
fatal (ie, the XDR code Oopses) in some cases.

However, the allocated memory is a tiny fixed amount, on the order
of 40-50 bytes.  We can easily use a stack-allocated buffer for
this, with only a wee bit of nose-holding.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

3 files changed, 30 insertions, 13 deletions

diff --git a/fs/nfs/nfs3acl.c b/fs/nfs/nfs3acl.c
index 9f88c5f4c7e2..274342771655 100644
--- a/fs/nfs/nfs3acl.c
+++ b/fs/nfs/nfs3acl.c
@@ -311,8 +311,8 @@ static int nfs3_proc_setacls(struct inode *inode, struct posix_acl *acl,
         if (!nfs_server_capable(inode, NFS_CAP_ACLS))
                 goto out;
 
-        /* We are doing this here, because XDR marshalling can only
-           return -ENOMEM. */
+        /* We are doing this here because XDR marshalling does not
+         * return any results, it BUGs. */
         status = -ENOSPC;
         if (acl != NULL && acl->a_count > NFS_ACL_MAX_ENTRIES)
                 goto out;
diff --git a/fs/nfs_common/nfsacl.c b/fs/nfs_common/nfsacl.c
index a3e78bd18679..84c27d69d421 100644
--- a/fs/nfs_common/nfsacl.c
+++ b/fs/nfs_common/nfsacl.c
@@ -42,6 +42,11 @@ struct nfsacl_encode_desc {
         gid_t gid;
 };
 
+struct nfsacl_simple_acl {
+        struct posix_acl acl;
+        struct posix_acl_entry ace[4];
+};
+
 static int
 xdr_nfsace_encode(struct xdr_array2_desc *desc, void *elem)
 {
@@ -99,17 +104,22 @@ int nfsacl_encode(struct xdr_buf *buf, unsigned int base, struct inode *inode,
                 .uid = inode->i_uid,
                 .gid = inode->i_gid,
         };
+        struct nfsacl_simple_acl aclbuf;
         int err;
-        struct posix_acl *acl2 = NULL;
 
         if (entries > NFS_ACL_MAX_ENTRIES ||
             xdr_encode_word(buf, base, entries))
                 return -EINVAL;
         if (encode_entries && acl && acl->a_count == 3) {
-                /* Fake up an ACL_MASK entry. */
-                acl2 = posix_acl_alloc(4, GFP_KERNEL);
-                if (!acl2)
-                        return -ENOMEM;
+                struct posix_acl *acl2 = &aclbuf.acl;
+
+                /* Avoid the use of posix_acl_alloc().  nfsacl_encode() is
+                 * invoked in contexts where a memory allocation failure is
+                 * fatal.  Fortunately this fake ACL is small enough to
+                 * construct on the stack. */
+                memset(acl2, 0, sizeof(acl2));
+                posix_acl_init(acl2, 4);
+
                 /* Insert entries in canonical order: other orders seem
                  to confuse Solaris VxFS. */
                 acl2->a_entries[0] = acl->a_entries[0];  /* ACL_USER_OBJ */
@@ -120,8 +130,6 @@ int nfsacl_encode(struct xdr_buf *buf, unsigned int base, struct inode *inode,
                 nfsacl_desc.acl = acl2;
         }
         err = xdr_encode_array2(buf, base + 4, &nfsacl_desc.desc);
-        if (acl2)
-                posix_acl_release(acl2);
         if (!err)
                 err = 8 + nfsacl_desc.desc.elem_size *
                           nfsacl_desc.desc.array_len;
diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 39df95a0ec25..b1cf6bf4b41d 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -22,6 +22,7 @@
 
 #include <linux/errno.h>
 
+EXPORT_SYMBOL(posix_acl_init);
 EXPORT_SYMBOL(posix_acl_alloc);
 EXPORT_SYMBOL(posix_acl_clone);
 EXPORT_SYMBOL(posix_acl_valid);
@@ -32,6 +33,16 @@ EXPORT_SYMBOL(posix_acl_chmod_masq);
 EXPORT_SYMBOL(posix_acl_permission);
 
 /*
+ * Init a fresh posix_acl
+ */
+void
+posix_acl_init(struct posix_acl *acl, int count)
+{
+        atomic_set(&acl->a_refcount, 1);
+        acl->a_count = count;
+}
+
+/*
  * Allocate a new ACL with the specified number of entries.
  */
 struct posix_acl *
@@ -40,10 +51,8 @@ posix_acl_alloc(int count, gfp_t flags)
         const size_t size = sizeof(struct posix_acl) +
                             count * sizeof(struct posix_acl_entry);
         struct posix_acl *acl = kmalloc(size, flags);
-        if (acl) {
-                atomic_set(&acl->a_refcount, 1);
-                acl->a_count = count;
-        }
+        if (acl)
+                posix_acl_init(acl, count);
         return acl;
 }