9p: add missing locking around taking dentry fid list

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: 9p: add missing locking around taking dentry fid list Fix a use-after-free on dentry's d_fsdata fid list when a thread looks up a fid through dentry while another thread unlinks it: UAF thread: refcount_t: addition on 0; use-after-free. p9_fid_get linux/./include/net/9p/client.h:262 v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129 v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181 v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314 v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400 vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248 Freed by: p9_fid_destroy (inlined) p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456 p9_fid_put linux/./include/net/9p/client.h:278 v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55 v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518 vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335 The problem is that d_fsdata was not accessed under d_lock, because d_release() normally is only called once the dentry is otherwise no longer accessible but since we also call it explicitly in v9fs_remove that lock is required: move the hlist out of the dentry under lock then unref its fids once they are no longer accessible.

References

• https://git.kernel.org/stable/c/3bb6763a8319170c2d41c4232c8e7e4c37dcacfb
• https://git.kernel.org/stable/c/cb299cdba09f46f090b843d78ba26b667d50a456
• https://git.kernel.org/stable/c/f0c5c944c6d8614c19e6e9a97fd2011dcd30e8f5
• https://git.kernel.org/stable/c/fe17ebf22feb4ad7094d597526d558a49aac92b4
• https://git.kernel.org/stable/c/c898afdc15645efb555acb6d85b484eb40a45409
• https://www.zerodayinitiative.com/advisories/ZDI-24-1194/