linux-kernel - Re: [PATCH] cifs: Fix null-ptr-deref by static initializing global lock

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Message-ID: <CAH2r5mt97q_jtjtAT0DVDySU0jnk9=KeTDYha-KgOip_CAT_iw@mail.gmail.com>
Date: Wed, 6 Aug 2025 15:09:10 -0500
From: Steve French <smfrench@...il.com>
To: Yunseong Kim <ysk@...lloc.com>
Cc: Steve French <sfrench@...ba.org>, Paulo Alcantara <pc@...guebit.org>, 
	Ronnie Sahlberg <ronniesahlberg@...il.com>, Shyam Prasad N <sprasad@...rosoft.com>, 
	Tom Talpey <tom@...pey.com>, Bharath SM <bharathsm@...rosoft.com>, 
	Namjae Jeon <linkinjeon@...nel.org>, linux-cifs@...r.kernel.org, 
	samba-technical@...ts.samba.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cifs: Fix null-ptr-deref by static initializing global lock

merged into cifs-2.6.git for-next

On Wed, Aug 6, 2025 at 8:23 AM Yunseong Kim <ysk@...lloc.com> wrote:
>
> A kernel panic can be triggered by reading /proc/fs/cifs/debug_dirs.
> The crash is a null-ptr-deref inside spin_lock(), caused by the use of the
> uninitialized global spinlock cifs_tcp_ses_lock.
>
> init_cifs()
>  └── cifs_proc_init()
>       └── // User can access /proc/fs/cifs/debug_dirs here
>            └── cifs_debug_dirs_proc_show()
>                 └── spin_lock(&cifs_tcp_ses_lock); // Uninitialized!
>
> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> Mem abort info:
> ESR = 0x0000000096000005
> EC = 0x25: DABT (current EL), IL = 32 bits
> SET = 0, FnV = 0
> EA = 0, S1PTW = 0
> FSC = 0x05: level 1 translation fault
> Data abort info:
> ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
> CM = 0, WnR = 0, TnD = 0, TagAccess = 0
> GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
> [dfff800000000000] address between user and kernel address ranges
> Internal error: Oops: 0000000096000005 [#1] SMP
> Modules linked in:
> CPU: 3 UID: 0 PID: 16435 Comm: stress-ng-procf Not tainted 6.16.0-10385-g79f14b5d84c6 #37 PREEMPT
> Hardware name: QEMU KVM Virtual Machine, BIOS 2025.02-8ubuntu1 06/11/2025
> pstate: 23400005 (nzCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
> pc : do_raw_spin_lock+0x84/0x2cc
> lr : _raw_spin_lock+0x24/0x34
> sp : ffff8000966477e0
> x29: ffff800096647860 x28: ffff800096647b88 x27: ffff0001c0c22070
> x26: ffff0003eb2b60c8 x25: ffff0001c0c22018 x24: dfff800000000000
> x23: ffff0000f624e000 x22: ffff0003eb2b6020 x21: ffff0000f624e768
> x20: 0000000000000004 x19: 0000000000000000 x18: 0000000000000000
> x17: 0000000000000000 x16: ffff8000804b9600 x15: ffff700012cc8f04
> x14: 1ffff00012cc8f04 x13: 0000000000000004 x12: ffffffffffffffff
> x11: 1ffff00012cc8f00 x10: ffff80008d9af0d2 x9 : f3f3f304f1f1f1f1
> x8 : 0000000000000000 x7 : 7365733c203e6469 x6 : 20656572743c2023
> x5 : ffff0000e0ce0044 x4 : ffff80008a4deb6e x3 : ffff8000804b9718
> x2 : 0000000000000001 x1 : 0000000000000000 x0 : 0000000000000000
> Call trace:
> do_raw_spin_lock+0x84/0x2cc (P)
> _raw_spin_lock+0x24/0x34
> cifs_debug_dirs_proc_show+0x1ac/0x4c0
> seq_read_iter+0x3b0/0xc28
> proc_reg_read_iter+0x178/0x2a8
> vfs_read+0x5f8/0x88c
> ksys_read+0x120/0x210
> __arm64_sys_read+0x7c/0x90
> invoke_syscall+0x98/0x2b8
> el0_svc_common+0x130/0x23c
> do_el0_svc+0x48/0x58
> el0_svc+0x40/0x140
> el0t_64_sync_handler+0x84/0x12c
> el0t_64_sync+0x1ac/0x1b0
> Code: aa0003f3 f9000feb f2fe7e69 f8386969 (38f86908)
> ---[ end trace 0000000000000000 ]---
>
> The root cause is an initialization order problem. The lock is declared
> as a global variable and intended to be initialized during module startup.
> However, the procfs entry that uses this lock can be accessed by userspace
> before the spin_lock_init() call has run. This creates a race window where
> reading the proc file will attempt to use the lock before it is
> initialized, leading to the crash.
>
> For a global lock with a static lifetime, the correct and robust approach
> is to use compile-time initialization.
>
> Fixes: 844e5c0eb176 ("smb3 client: add way to show directory leases for improved debugging")
> Signed-off-by: Yunseong Kim <ysk@...lloc.com>
> ---
>  fs/smb/client/cifsfs.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
> index 31930b7266db..3bd85ab2deb1 100644
> --- a/fs/smb/client/cifsfs.c
> +++ b/fs/smb/client/cifsfs.c
> @@ -77,7 +77,7 @@ unsigned int global_secflags = CIFSSEC_DEF;
>  unsigned int GlobalCurrentXid; /* protected by GlobalMid_Lock */
>  unsigned int GlobalTotalActiveXid; /* prot by GlobalMid_Lock */
>  unsigned int GlobalMaxActiveXid;       /* prot by GlobalMid_Lock */
> -spinlock_t GlobalMid_Lock; /* protects above & list operations on midQ entries */
> +DEFINE_SPINLOCK(GlobalMid_Lock); /* protects above & list operations on midQ entries */
>
>  /*
>   *  Global counters, updated atomically
> @@ -97,7 +97,7 @@ atomic_t total_buf_alloc_count;
>  atomic_t total_small_buf_alloc_count;
>  #endif/* STATS2 */
>  struct list_head       cifs_tcp_ses_list;
> -spinlock_t             cifs_tcp_ses_lock;
> +DEFINE_SPINLOCK(cifs_tcp_ses_lock);
>  static const struct super_operations cifs_super_ops;
>  unsigned int CIFSMaxBufSize = CIFS_MAX_MSGSIZE;
>  module_param(CIFSMaxBufSize, uint, 0444);
> @@ -1863,8 +1863,6 @@ init_cifs(void)
>         GlobalCurrentXid = 0;
>         GlobalTotalActiveXid = 0;
>         GlobalMaxActiveXid = 0;
> -       spin_lock_init(&cifs_tcp_ses_lock);
> -       spin_lock_init(&GlobalMid_Lock);
>
>         cifs_lock_secret = get_random_u32();
>
> --
> 2.50.0
>
>


-- 
Thanks,

Steve