Re: [PATCH] mm/mmap: Fix memory leak in mmap_region()

[Li Zetao]

Hi,

On 2022/10/27 15:30, Liam Howlett wrote:
> * Li Zetao <lizetao1@xxxxxxxxxx> [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@xxxxxxxxxx>
> > ---
> >   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