bpf: Disable file_alloc_security hook

A use-after-free bug may be triggered by calling bpf_inode_storage_get()
in a BPF LSM program hooked to file_alloc_security. Disable the hook to
prevent this from happening.

The cause of the bug is shown in the trace below. In alloc_file(), a
file struct is first allocated through kmem_cache_alloc(). Then,
file_alloc_security hook is invoked. Since the zero initialization or
assignment of f->f_inode happen after this LSM hook, a BPF program may
get a dangeld inode pointer by walking the file struct.

alloc_file()
-> alloc_empty_file()
-> f = kmem_cache_alloc()
-> init_file()
-> security_file_alloc() // f->f_inode not init-ed yet!
-> f->f_inode = NULL;
-> file_init_path()
-> f->f_inode = path->dentry->d_inode

Reported-by: Kaiyan Mei <[e-mail]>
Reported-by: Yinhao Hu <[e-mail]>
Reported-by: Dongliang Mu <[e-mail]>
Closes: https://lore.kernel.org/bpf/[e-mail]/
Signed-off-by: Amery Hung <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/bpf_lsm.c

···
 BTF_ID(func, bpf_lsm_audit_rule_match)
 #endif
 BTF_ID(func, bpf_lsm_ismaclabel)
+BTF_ID(func, bpf_lsm_file_alloc_security)
 BTF_SET_END(bpf_lsm_disabled_hooks)
 
 /* List of LSM hooks that should operate on 'current' cgroup regardless