NFSv4.2: fix nfs4_listxattr kernel BUG at mm/usercopy.c:102

A call to listxattr() with a buffer size = 0 returns the actual
size of the buffer needed for a subsequent call. When size > 0,
nfs4_listxattr() does not return an error because either
generic_listxattr() or nfs4_listxattr_nfs4_label() consumes
exactly all the bytes then size is 0 when calling
nfs4_listxattr_nfs4_user() which then triggers the following
kernel BUG:

  [   99.403778] kernel BUG at mm/usercopy.c:102!
  [   99.404063] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
  [   99.408463] CPU: 0 PID: 3310 Comm: python3 Not tainted 6.6.0-61.fc40.aarch64 #1
  [   99.415827] Call trace:
  [   99.415985]  usercopy_abort+0x70/0xa0
  [   99.416227]  __check_heap_object+0x134/0x158
  [   99.416505]  check_heap_object+0x150/0x188
  [   99.416696]  __check_object_size.part.0+0x78/0x168
  [   99.416886]  __check_object_size+0x28/0x40
  [   99.417078]  listxattr+0x8c/0x120
  [   99.417252]  path_listxattr+0x78/0xe0
  [   99.417476]  __arm64_sys_listxattr+0x28/0x40
  [   99.417723]  invoke_syscall+0x78/0x100
  [   99.417929]  el0_svc_common.constprop.0+0x48/0xf0
  [   99.418186]  do_el0_svc+0x24/0x38
  [   99.418376]  el0_svc+0x3c/0x110
  [   99.418554]  el0t_64_sync_handler+0x120/0x130
  [   99.418788]  el0t_64_sync+0x194/0x198
  [   99.418994] Code: aa0003e3 d000a3e0 91310000 97f49bdb (d4210000)

Issue is reproduced when generic_listxattr() returns 'system.nfs4_acl',
thus calling lisxattr() with size = 16 will trigger the bug.

Add check on nfs4_listxattr() to return ERANGE error when it is
called with size > 0 and the return value is greater than size.

Fixes: 012a211a ("NFSv4.2: hook in the user extended attribute handlers")
Signed-off-by: Jorge Mora <mora@netapp.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs4proc.c

@@ -10615,29 +10615,33 @@ const struct nfs4_minor_version_ops *nfs_v4_minor_ops[] = {
 static ssize_t nfs4_listxattr(struct dentry *dentry, char *list, size_t size)
 {
 	ssize_t error, error2, error3;
+	size_t left = size;
 
-	error = generic_listxattr(dentry, list, size);
+	error = generic_listxattr(dentry, list, left);
 	if (error < 0)
 		return error;
 	if (list) {
 		list += error;
-		size -= error;
+		left -= error;
 	}
 
-	error2 = nfs4_listxattr_nfs4_label(d_inode(dentry), list, size);
+	error2 = nfs4_listxattr_nfs4_label(d_inode(dentry), list, left);
 	if (error2 < 0)
 		return error2;
 
 	if (list) {
 		list += error2;
-		size -= error2;
+		left -= error2;
 	}
 
-	error3 = nfs4_listxattr_nfs4_user(d_inode(dentry), list, size);
+	error3 = nfs4_listxattr_nfs4_user(d_inode(dentry), list, left);
 	if (error3 < 0)
 		return error3;
 
-	return error + error2 + error3;
+	error += error2 + error3;
+	if (size && error > size)
+		return -ERANGE;
+	return error;
 }
 
 static void nfs4_enable_swap(struct inode *inode)