Re: [PATCH v2] nfs: handle failure of nfs_get_lock_context in unlock path

[Li Lingfeng <lilingfeng3@huawei.com>]

Hi Anna,

Thank you for reviewing the patch.
I agree that handling the error case first makes the code cleaner and
reduces nesting. Your suggested change looks great to me.
I'll send v3 soon.

Lingfeng

在 2025/5/13 21:50, Anna Schumaker 写道:
> Hi Li,
>
> On 5/13/25 3:42 AM, Li Lingfeng wrote:
>> When memory is insufficient, the allocation of nfs_lock_context in
>> nfs_get_lock_context() fails and returns -ENOMEM. If we mistakenly treat
>> an nfs4_unlockdata structure (whose l_ctx member has been set to -ENOMEM)
>> as valid and proceed to execute rpc_run_task(), this will trigger a NULL
>> pointer dereference in nfs4_locku_prepare. For example:
>>
>> BUG: kernel NULL pointer dereference, address: 000000000000000c
>> PGD 0 P4D 0
>> Oops: Oops: 0000 [#1] SMP PTI
>> CPU: 15 UID: 0 PID: 12 Comm: kworker/u64:0 Not tainted 6.15.0-rc2-dirty #60
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-2.fc40
>> Workqueue: rpciod rpc_async_schedule
>> RIP: 0010:nfs4_locku_prepare+0x35/0xc2
>> Code: 89 f2 48 89 fd 48 c7 c7 68 69 ef b5 53 48 8b 8e 90 00 00 00 48 89 f3
>> RSP: 0018:ffffbbafc006bdb8 EFLAGS: 00010246
>> RAX: 000000000000004b RBX: ffff9b964fc1fa00 RCX: 0000000000000000
>> RDX: 0000000000000000 RSI: fffffffffffffff4 RDI: ffff9ba53fddbf40
>> RBP: ffff9ba539934000 R08: 0000000000000000 R09: ffffbbafc006bc38
>> R10: ffffffffb6b689c8 R11: 0000000000000003 R12: ffff9ba539934030
>> R13: 0000000000000001 R14: 0000000004248060 R15: ffffffffb56d1c30
>> FS: 0000000000000000(0000) GS:ffff9ba5881f0000(0000) knlGS:00000000
>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 000000000000000c CR3: 000000093f244000 CR4: 00000000000006f0
>> Call Trace:
>>   <TASK>
>>   __rpc_execute+0xbc/0x480
>>   rpc_async_schedule+0x2f/0x40
>>   process_one_work+0x232/0x5d0
>>   worker_thread+0x1da/0x3d0
>>   ? __pfx_worker_thread+0x10/0x10
>>   kthread+0x10d/0x240
>>   ? __pfx_kthread+0x10/0x10
>>   ret_from_fork+0x34/0x50
>>   ? __pfx_kthread+0x10/0x10
>>   ret_from_fork_asm+0x1a/0x30
>>   </TASK>
>> Modules linked in:
>> CR2: 000000000000000c
>> ---[ end trace 0000000000000000 ]---
>>
>> Free the allocated nfs4_unlockdata when nfs_get_lock_context() fails and
>> return NULL to terminate subsequent rpc_run_task, preventing NULL pointer
>> dereference.
>>
>> Fixes: f30cb757f680 ("NFS: Always wait for I/O completion before unlock")
>> Link: https://lore.kernel.org/all/21817f2c-2971-4568-9ae4-1ccc25f7f1ef@huawei.com/
>> Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
>> ---
>> Changes in v2:
>>    Add a comment explaining that error handling for ctx acquisition failure
>>    is unnecessary.
> Thanks for the patch!
>
>>   fs/nfs/nfs4proc.c | 13 ++++++++++++-
>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
>> index 970f28dbf253..e52e2ac1ab39 100644
>> --- a/fs/nfs/nfs4proc.c
>> +++ b/fs/nfs/nfs4proc.c
>> @@ -7074,18 +7074,29 @@ static struct nfs4_unlockdata *nfs4_alloc_unlockdata(struct file_lock *fl,
>>   	struct nfs4_unlockdata *p;
>>   	struct nfs4_state *state = lsp->ls_state;
>>   	struct inode *inode = state->inode;
>> +	struct nfs_lock_context *l_ctx;
>>   
>>   	p = kzalloc(sizeof(*p), GFP_KERNEL);
>>   	if (p == NULL)
>>   		return NULL;
>> +	l_ctx = nfs_get_lock_context(ctx);
>> +	if (!IS_ERR(l_ctx)) {
>> +		p->l_ctx = l_ctx;
>> +	} else {
>> +		kfree(p);
>> +		return NULL;
>> +	}
> I was thinking the code would look a little neater if this if / else block
> was changed to:
>
>          if (IS_ERR(l_ctx)) {
>                  kfree(p);
>                  return NULL;
>          }
>
>>   	p->arg.fh = NFS_FH(inode);
>>   	p->arg.fl = &p->fl;
>>   	p->arg.seqid = seqid;
>>   	p->res.seqid = seqid;
>>   	p->lsp = lsp;
>>   	/* Ensure we don't close file until we're done freeing locks! */
>> +	/*
>> +	 * Since the caller holds a reference to ctx, the refcount must be non-zero.
>> +	 * Therefore, error handling for failed ctx acquisition is unnecessary here.
>> +	 */
>>   	p->ctx = get_nfs_open_context(ctx);
>> -	p->l_ctx = nfs_get_lock_context(ctx);
> And then update this to set p->l_ctx = l_ctx.
>
> What do you think?
> Anna
>
>>   	locks_init_lock(&p->fl);
>>   	locks_copy_lock(&p->fl, fl);
>>   	p->server = NFS_SERVER(inode);
>