linux-kernel - Re: [PATCH] mm/mmap: Fix memory leak in mmap

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Date:   Fri, 28 Oct 2022 10:17:52 +0800
From:   Li Zetao <lizetao1@...wei.com>
To:     Liam Howlett <liam.howlett@...cle.com>
CC:     "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "willy@...radead.org" <willy@...radead.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "lizetao1@...wei.com" <lizetao1@...wei.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "cmllamas@...gle.com" <cmllamas@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/mmap: Fix memory leak in mmap_region()

Hi,

On 2022/10/27 15:30, Liam Howlett wrote:
> * Li Zetao <lizetao1@...wei.com> [221026 21:55]:
>> There is a memory leak reported by kmemleak:
>>
>>    unreferenced object 0xffff88817231ce40 (size 224):
>>      comm "mount.cifs", pid 19308, jiffies 4295917571 (age 405.880s)
>>      hex dump (first 32 bytes):
>>        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>        60 c0 b2 00 81 88 ff ff 98 83 01 42 81 88 ff ff  `..........B....
>>      backtrace:
>>        [<ffffffff81936171>] __alloc_file+0x21/0x250
>>        [<ffffffff81937051>] alloc_empty_file+0x41/0xf0
>>        [<ffffffff81937159>] alloc_file+0x59/0x710
>>        [<ffffffff81937964>] alloc_file_pseudo+0x154/0x210
>>        [<ffffffff81741dbf>] __shmem_file_setup+0xff/0x2a0
>>        [<ffffffff817502cd>] shmem_zero_setup+0x8d/0x160
>>        [<ffffffff817cc1d5>] mmap_region+0x1075/0x19d0
>>        [<ffffffff817cd257>] do_mmap+0x727/0x1110
>>        [<ffffffff817518b2>] vm_mmap_pgoff+0x112/0x1e0
>>        [<ffffffff83adf955>] do_syscall_64+0x35/0x80
>>        [<ffffffff83c0006a>] entry_SYSCALL_64_after_hwframe+0x46/0xb0
>>
>> The root cause was traced to an error handing path in mmap_region()
>> when arch_validate_flags() or mas_preallocate() fails. In the shared
>> anonymous mapping sence, vma will be setuped and mapped with a new
>> shared anonymous file via shmem_zero_setup(). So in this case, the
>> file resource needs to be released.
>>
>> Fix it by calling fput(vma->vm_file) when arch_validate_flags() or
>> mas_preallocate() returns an error. And for the beauty of the code,
>> put fput() under mapping_unmap_writable().
> It does look like the unrolling is in the wrong order in that section.
>
>> Fixes: d4af56c5c7c6 ("mm: start tracking VMAs with maple tree")
>> Fixes: c462ac288f2c ("mm: Introduce arch_validate_flags()")
>> Signed-off-by: Li Zetao <lizetao1@...wei.com>
>> ---
>>   mm/mmap.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index e270057ed04e..8530195b3ec5 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -2674,6 +2674,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>>   		error = -EINVAL;
>>   		if (file)
>>   			goto close_and_free_vma;I will modify it in the second version
>> +		else if (vm_flags & VM_SHARED)
>> +			goto put_vma_file;
>>   		else
>>   			goto free_vma;
>>   	}
>> @@ -2682,6 +2684,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>>   		error = -ENOMEM;
>>   		if (file)
>>   			goto close_and_free_vma;
>> +		else if (vm_flags & VM_SHARED)
>> +			goto put_vma_file;
>>   		else
>>   			goto free_vma;
>>   	}
> I am not happy about this getting more complex as it already duplicates
> too much code.

It's true that my modification would cause the code to further 
complicate. I will rework it in the

next version.

>> @@ -2746,13 +2750,13 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>>   	if (vma->vm_ops && vma->vm_ops->close)
>>   		vma->vm_ops->close(vma);
>>   unmap_and_free_vma:
>> -	fput(vma->vm_file);
>> -	vma->vm_file = NULL;
>> -
>>   	/* Undo any partial mapping done by a device driver. */
>>   	unmap_region(mm, mas.tree, vma, prev, next, vma->vm_start, vma->vm_end);
>>   	if (vm_flags & VM_SHARED)
> Could we change the above if to "if (file && vm_flags & VM_SHARED)" and
> jump to unmap_and_free_vma "if (vma->vm_file)"?  We could then drop your
> new goto label. I still think the reodering is correct and worth while.
>
> Or am I missing something?
Thanks for your constructive comments.
>>   		mapping_unmap_writable(file->f_mapping);
>> +put_vma_file:
>> +	fput(vma->vm_file);
>> +	vma->vm_file = NULL;
>>   free_vma:
>>   	vm_area_free(vma);
>>   unacct_error:
>> -- 
>> 2.31.1

Regards,

Li Zetao