[PATCH v2] fuse: check for NULL root inode in fuse_fill_super_submount

fuse_iget() can return NULL when its inode allocation fails, but
fuse_fill_super_submount() passed the result straight to get_fuse_inode()
and decremented fi->nlookup without checking it:

        root = fuse_iget(sb, parent_fi->nodeid, ...);
        fi = get_fuse_inode(root);
        fi->nlookup--;

Inside fuse_iget() the inode allocation can fail and return NULL.  The
submount root takes the iget5_locked() path, whose alloc_inode() can fail
under memory pressure (the auto-submount branch can fail the same way in
new_inode() or fuse_alloc_submount_lookup()):

        inode = iget5_locked(sb, nodeid, fuse_inode_eq, fuse_inode_set,
                             &nodeid);
        if (!inode)
                return NULL;

A NULL root makes get_fuse_inode() a container_of() on NULL and the
nlookup decrement a write to a bogus address, oopsing the mount.  With
CONFIG_KASAN the following null pointer dereference is reported when the
root inode allocation of an auto-submount fails (e.g. under memory
pressure):

==================================================================
BUG: KASAN: null-ptr-deref in fuse_get_tree_submount+0x656/0x8b0
Read of size 8 at addr 00000000000002b0 by task ls/942
CPU: 0 PID: 942 Comm: ls Tainted: G W 6.6 #15
Call Trace:
 <TASK>
 fuse_get_tree_submount+0x656/0x8b0
 vfs_get_tree+0x48/0x140
 fc_mount+0x13/0x50
 fuse_dentry_automount+0x7a/0xb0
 __traverse_mounts+0xca/0x330
 step_into+0x339/0xac0
 path_lookupat+0xc5/0x2f0
 filename_lookup+0x163/0x2a0
 vfs_statx+0xd5/0x200
 do_statx+0x83/0xd0
 __x64_sys_statx+0xa0/0xc0
 do_syscall_64+0x37/0x90
 entry_SYSCALL_64_after_hwframe+0x78/0xe2
 </TASK>
==================================================================

Return -ENOMEM instead; the caller tears down the partially built
superblock on error, matching the other error returns in this
function.

Fixes: 1866d779d5d2 ("fuse: Allow fuse_fill_super_common() for submounts")
Signed-off-by: Baokun Li <[email protected]>
---
Changes since v1:
 * Correct the Fixes tag.

v1: https://patch.msgid.link/[email protected]

 fs/fuse/inode.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
index d975073c6029..455c7feba057 100644
--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -1639,6 +1639,8 @@ static int fuse_fill_super_submount(struct super_block *sb,
 	fuse_fill_attr_from_inode(&root_attr, parent_fi);
 	root = fuse_iget(sb, parent_fi->nodeid, 0, &root_attr, 0, 0,
 			 fuse_get_evict_ctr(fm->fc));
+	if (!root)
+		return -ENOMEM;
 	/*
 	 * This inode is just a duplicate, so it is not looked up and
 	 * its nlookup should not be incremented.  fuse_iget() does
-- 
2.43.7