Re: [PATCH] mm/mmap: Fix MAP_FIXED address return on VMA merge

[David Hildenbrand]

On 18.10.22 21:17, Liam Howlett wrote:
> mmap should return the start address of newly mapped area when
> successful.  On a successful merge of a VMA, the return address was
> changed and thus was violating that expectation from userspace.

Just wondering, do we have a simple user space reproducer / test?

Do we want to add some more tests for such scenarios?

> This is a restoration of functionality provided by 309d08d9b3a3
> (mm/mmap.c: fix mmap return value when vma is merged after call_mmap()).
> For completeness of fixing MAP_FIXED, implement the comments from the
> previous discussion to never update the address and fail if the address
> changes.  Leaving the error as a WARN_ON() to avoid crashing the kernel.
>
> Cc: Liu Zixian <liuzixian4@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/all/Y06yk66SKxlrwwfb@lakrids/
> Link: https://lore.kernel.org/all/20201203085350.22624-1-liuzixian4@xxxxxxxxxx/
> Fixes: 4dd1b84140c1 (mm/mmap: use advanced maple tree API for mmap_region())
> Reported-by: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---
>   mm/mmap.c | 15 +++++++--------
>   1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 42cd2c260898..22010e13f1a1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2625,14 +2625,14 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>   		if (error)
>   			goto unmap_and_free_vma;
>   
> -		/* Can addr have changed??
> -		 *
> -		 * Answer: Yes, several device drivers can do it in their
> -		 *         f_op->mmap method. -DaveM
> +		/*
> +		 * Expansion is handled above, merging is handled below.
> +		 * Drivers should not alter the address of the VMA.
>   		 */
> -		WARN_ON_ONCE(addr != vma->vm_start);
> -
> -		addr = vma->vm_start;
> +		if (WARN_ON((addr != vma->vm_start))) {
> +			error = -EINVAL;
> +			goto close_and_free_vma;
> +		}

If this is something that user space can trigger, WARN_* is the wrong 
choice. But what I understand from the comment change is that this must 
not happen at that point unless there is a real issue.

Why not "if (WARN_ON_ONCE)" ?

--
Thanks,

David / dhildenb