On Thu, Sep 04, 2025 at 09:58:12AM -0300, Anderson Nascimento wrote:
> Validating the return value of mnt_ns_from_dentry() before
> dereferencing mntns->user_ns in do_fanotify_mark()
>
> 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. Tested on v6.17-rc4.
>
> 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_nullptr
> Fanotify fd: 3
> Killed
> # cat fanotify_nullptr.c
> #include <stdio.h>
> #include <stdlib.h>
> #include <fcntl.h>
> #include <sys/fanotify.h>
>
> 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_nullptr
> 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
> [ 25.695229] ? do_syscall_64+0x82/0x2c0
> [ 25.695234] ? memcg1_commit_charge+0x7a/0xa0
> [ 25.695240] ? mod_memcg_lruvec_state+0xe7/0x2e0
> [ 25.695246] ? charge_memcg+0x48/0x80
> [ 25.695251] ? blk_cgroup_congested+0x65/0x70
> [ 25.695258] ? __lruvec_stat_mod_folio+0x85/0xd0
> [ 25.695272] ? __folio_mod_stat+0x2d/0x90
> [ 25.695284] ? set_ptes.isra.0+0x36/0x80
> [ 25.695290] ? do_anonymous_page+0x100/0x520
> [ 25.695295] ? __handle_mm_fault+0x54f/0x6a0
> [ 25.695317] ? anon_inode_getfile_fmode+0x18/0x30
> [ 25.695322] ? count_memcg_events+0xd6/0x220
> [ 25.695327] ? handle_mm_fault+0x248/0x360
> [ 25.695333] ? do_user_addr_fault+0x21a/0x690
> [ 25.695339] ? clear_bhb_loop+0x50/0xa0
> [ 25.695344] ? clear_bhb_loop+0x50/0xa0
> [ 25.695348] ? clear_bhb_loop+0x50/0xa0
> [ 25.695353] entry_SYSCALL_64_after_hwframe+0x76/0x7e
> [ 25.695358] RIP: 0033:0x7f8733cd26ae
> [ 25.695373] Code: f8 48 8d 75 f8 e8 12 3c ff ff c9 48 83 f8 08 0f
> 95 c0 0f b6 c0 f7 d8 c3 0f 1f 40 00 f3 0f 1e fa 41 89 ca b8 2d 01 00
> 00 0f 05 <48> 3d 00 f0 ff ff 77 0a c3 66 0f 1f 84 00 00 00 00 00 48 8b
> 15 19
> [ 25.695613] RSP: 002b:00007ffcd6842cd8 EFLAGS: 00000206 ORIG_RAX:
> 000000000000012d
> [ 25.695820] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f8733cd26ae
> [ 25.695992] RDX: 0000000001000000 RSI: 0000000000000111 RDI: 0000000000000003
> [ 25.696141] RBP: 00007ffcd6842cf0 R08: 00000000004012a7 R09: 0000000000000000
> [ 25.696273] R10: 00000000ffffff9c R11: 0000000000000206 R12: 00007ffcd6842e18
> [ 25.696438] R13: 0000000000000001 R14: 00007f8733e15000 R15: 0000000000402e00
> [ 25.696616] </TASK>
> [ 25.696752] Modules linked in: rfkill nft_fib_inet nft_fib_ipv4
> nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6
> nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6
> nf_defrag_ipv4 nf_tables qrtr intel_rapl_msr intel_rapl_common
> intel_uncore_frequency_common intel_pmc_core pmt_telemetry
> pmt_discovery pmt_class intel_pmc_ssram_telemetry intel_vsec rapl
> vmw_balloon pcspkr i2c_piix4 i2c_smbus joydev loop vsock_loopback
> vmw_vsock_virtio_transport_common vmw_vsock_vmci_transport vsock zram
> vmw_vmci lz4hc_compress lz4_compress xfs polyval_clmulni vmwgfx
> ghash_clmulni_intel vmxnet3 nvme drm_ttm_helper ata_generic ttm
> pata_acpi nvme_tcp nvme_fabrics nvme_core nvme_keyring nvme_auth
> serio_raw sunrpc be2iscsi bnx2i cnic uio cxgb4i cxgb4 tls cxgb3i cxgb3
> mdio libcxgbi libcxgb qla4xxx iscsi_boot_sysfs iscsi_tcp libiscsi_tcp
> libiscsi scsi_transport_iscsi scsi_dh_rdac scsi_dh_emc scsi_dh_alua
> fuse i2c_dev dm_multipath nfnetlink
> [ 25.698055] CR2: 0000000000000038
> [ 25.698202] ---[ end trace 0000000000000000 ]---
> [ 25.698385] RIP: 0010:do_fanotify_mark+0x817/0x950
> [ 25.698595] 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
> [ 25.698921] RSP: 0018:ffffd31c469e3c08 EFLAGS: 00010203
> [ 25.699076] RAX: 0000000000000000 RBX: 0000000001000000 RCX: ffff8eb48aebd220
> [ 25.699232] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8eb4835e8180
> [ 25.699409] RBP: 0000000000000111 R08: 0000000000000000 R09: 0000000000000000
> [ 25.699645] R10: ffff8eb48a7d56c0 R11: ffff8eb482bede00 R12: 00000000004012a7
> [ 25.699818] R13: 0000000000000110 R14: 0000000000000001 R15: ffff8eb48a7d56c0
> [ 25.699970] FS: 00007f8733bda740(0000) GS:ffff8eb61ce5f000(0000)
> knlGS:0000000000000000
> [ 25.700125] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 25.700280] CR2: 0000000000000038 CR3: 0000000136994006 CR4: 00000000003706f0
> [ 25.700495] note: fanotify_nullpt[1478] exited with irqs disabled
>
> Fixes: 58f5fbeb367f ("fanotify: support watching filesystems and
> mounts inside userns")
> Signed-off-by: Anderson Nascimento <anderson@xxxxxxxxxxxxxxxxxx>
> ---
Reviewed-by: Christian Brauner <brauner@xxxxxxxxxx>
