fanotify: Validate the return value of mnt_ns_from_dentry() before dereferencing

In the Linux kernel, the following vulnerability has been resolved:

fanotify: Validate the return value of mnt_ns_from_dentry() before dereferencing

The function do_fanotify_mark() does not validate if
mnt_ns_from_dentry() returns NULL before dereferencing mntns->user_ns.
This causes a NULL pointer dereference in do_fanotify_mark() if the
path is not a mount namespace object.

Fix this by checking mnt_ns_from_dentry()'s return value before
dereferencing it.

Before the patch

$ gcc fanotify_nullptr.c -o fanotify_nullptr
$ mkdir A
$ ./fanotify_nullptr
Fanotify fd: 3
fanotify_mark: Operation not permitted
$ unshare -Urm
Fanotify fd: 3
Killed

int main(void){
int ffd;
ffd = fanotify_init(FAN_CLASS_NOTIF | FAN_REPORT_MNT, 0);
if(ffd < 0){
perror("fanotify_init");
exit(EXIT_FAILURE);
}

printf("Fanotify fd: %d\n",ffd);

if(fanotify_mark(ffd, FAN_MARK_ADD | FAN_MARK_MNTNS,
FAN_MNT_ATTACH, AT_FDCWD, "A") < 0){
perror("fanotify_mark");
exit(EXIT_FAILURE);
}

return 0;
}

After the patch

$ gcc fanotify_nullptr.c -o fanotify_nullptr
$ mkdir A
$ ./fanotify_nullptr
Fanotify fd: 3
fanotify_mark: Operation not permitted
$ unshare -Urm
Fanotify fd: 3
fanotify_mark: Invalid argument

[ 25.694973] BUG: kernel NULL pointer dereference, address: 0000000000000038
[ 25.695006] #PF: supervisor read access in kernel mode
[ 25.695012] #PF: error_code(0x0000) - not-present page
[ 25.695017] PGD 109a30067 P4D 109a30067 PUD 142b46067 PMD 0
[ 25.695025] Oops: Oops: 0000 [#1] SMP NOPTI
[ 25.695032] CPU: 4 UID: 1000 PID: 1478 Comm: fanotify_nullpt Not
tainted 6.17.0-rc4 #1 PREEMPT(lazy)
[ 25.695040] Hardware name: VMware, Inc. VMware Virtual
Platform/440BX Desktop Reference Platform, BIOS 6.00 11/12/2020
[ 25.695049] RIP: 0010:do_fanotify_mark+0x817/0x950
[ 25.695066] Code: 04 00 00 e9 45 fd ff ff 48 8b 7c 24 48 4c 89 54
24 18 4c 89 5c 24 10 4c 89 0c 24 e8 b3 11 fc ff 4c 8b 54 24 18 4c 8b
5c 24 10 <48> 8b 78 38 4c 8b 0c 24 49 89 c4 e9 13 fd ff ff 8b 4c 24 28
85 c9
[ 25.695081] RSP: 0018:ffffd31c469e3c08 EFLAGS: 00010203
[ 25.695104] RAX: 0000000000000000 RBX: 0000000001000000 RCX: ffff8eb48aebd220
[ 25.695110] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8eb4835e8180
[ 25.695115] RBP: 0000000000000111 R08: 0000000000000000 R09: 0000000000000000
[ 25.695142] R10: ffff8eb48a7d56c0 R11: ffff8eb482bede00 R12: 00000000004012a7
[ 25.695148] R13: 0000000000000110 R14: 0000000000000001 R15: ffff8eb48a7d56c0
[ 25.695154] FS: 00007f8733bda740(0000) GS:ffff8eb61ce5f000(0000)
knlGS:0000000000000000
[ 25.695162] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 25.695170] CR2: 0000000000000038 CR3: 0000000136994006 CR4: 00000000003706f0
[ 25.695201] Call Trace:
[ 25.695209] <TASK>
[ 25.695215] __x64_sys_fanotify_mark+0x1f/0x30
[ 25.695222] do_syscall_64+0x82/0x2c0
...