[PATCH] NFSv4.2: fix nfs4_listxattr NULL pointer dereference

A call to listxattr() with a buffer size = 0 returns the actual
size of the buffer needed for a subsequent call. On an NFSv4.2
mount this triggers the following oops:

  [  399.768687] BUG: kernel NULL pointer dereference, address: 0000000000000000
  [  399.768705] RIP: 0010:_copy_from_pages+0x44/0xe0
  [  399.768722] Call Trace:
  [  399.768723]  nfs4_xattr_alloc_entry+0x1bf/0x1e0
  [  399.768730]  nfs4_xattr_cache_set_list+0x43/0x1f0
  [  399.768731]  nfs4_listxattr+0x21f/0x250
  [  399.768733]  vfs_listxattr+0x55/0xa0
  [  399.768736]  listxattr+0x23/0x160
  [  399.768737]  path_listxattrat+0xba/0x1e0
  [  399.768739]  do_syscall_64+0xe2/0x680

security_inode_listsecurity() now decrements the remaining size
even when the buffer is NULL, so in the size-query case 'left'
underflows to a huge size_t value and nfs4_listxattr_nfs4_user()
treats the NULL buffer as real, ending in a NULL dereference in
_copy_from_pages().

Declare 'left' as ssize_t and pass a zero length to
nfs4_listxattr_nfs4_user() when the buffer is NULL.

Fixes: f71ece9712b7 ("security,fs,nfs,net: update security_inode_listsecurity() interface")
Signed-off-by: Achilles Gaikwad <[email protected]>
---
 fs/nfs/nfs4proc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 1360409d8de9..4859c2c96c78 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -10585,7 +10585,7 @@ 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;
+	ssize_t left = size;
 
 	error = generic_listxattr(dentry, list, left);
 	if (error < 0)
@@ -10600,7 +10600,8 @@ static ssize_t nfs4_listxattr(struct dentry *dentry, char *list, size_t size)
 		return error2;
 	error2 = size - error - left;
 
-	error3 = nfs4_listxattr_nfs4_user(d_inode(dentry), list, left);
+	error3 = nfs4_listxattr_nfs4_user(d_inode(dentry), list,
+					  list ? left : 0);
 	if (error3 < 0)
 		return error3;
 

base-commit: 6eb8711ece2ce27e52e327a5b7a628ed39b97f45
-- 
2.55.0