Re: [PATCH mm-hotfixes] mm/rmap: clear vma->anon_vma on error

[Jiakai Xu <jiakaipeanut@gmail.com>]

> Commit 542eda1a8329 ("mm/rmap: improve anon_vma_clone(), unlink_anon_vmas()
> comments, add asserts") alters the way errors are handled, but overlooked
> one important aspect of clean up.
> 
> When a VMA encounters an error state in anon_vma_clone() (that is, on
> attempted allocation of anon_vma_chain objects), it cleans up partially
> established state in cleanup_partial_anon_vmas(), before returning an
> error.
> 
> However, this occurs prior to anon_vma->num_active_vmas being incremented,
> and it also fails to clear the VMA's vma->anon_vma field, which remains in
> place.
> 
> This is immediately an inconsistent state, because
> anon_vma->num_active_vmas is supposed to track the number of VMAs whose
> vma->anon_vma field references that anon_vma, and now that count is
> off-by-negative-1 for each VMA for which this error state has occurred.
> 
> When VMAs are unlinked from this anon_vma, unlink_anon_vmas() will
> eventually underflow anon_vma->num_active_vmas, which will trigger a
> warning.
> 
> This will always eventually happen, as we unlink anon_vma's at process
> teardown.
> 
> It could also cause maybe_reuse_anon_vma() to incorrectly permit the reuse
> of an anon_vma which has active VMAs attached, which will lead to a
> persistently invalid state.
> 
> The solution is to clear the VMA's anon_vma field when we clean up partial
> state, as the fact we are doing so indicates clearly that the VMA is not
> correctly integrated into the anon_vma tree and thus this field is invalid.
> 
> Reported-by: Sasha Levin <sashal@kernel.org>
> Closes: https://lore.kernel.org/linux-mm/20260302151547.2389070-1-sashal@kernel.org/
> Reported-by: Jiakai Xu <jiakaipeanut@gmail.com>
> Closes: https://lore.kernel.org/linux-mm/CAFb8wJvRhatRD-9DVmr5v5pixTMPEr3UKjYBJjCd09OfH55CKg@mail.gmail.com/
> Fixes: 542eda1a8329 ("mm/rmap: improve anon_vma_clone(), unlink_anon_vmas() comments, add asserts")
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>

Tested-by: Jiakai Xu <jiakaiPeanut@gmail.com>

Thanks!

> ---
>  mm/rmap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 6398d7eef393..abe4712a220c 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -457,6 +457,13 @@ static void cleanup_partial_anon_vmas(struct vm_area_struct *vma)
>  		list_del(&avc->same_vma);
>  		anon_vma_chain_free(avc);
>  	}
> +
> +	/*
> +	 * The anon_vma assigned to this VMA is no longer valid, as we were not
> +	 * able to correctly clone AVC state. Avoid inconsistent anon_vma tree
> +	 * state by resetting.
> +	 */
> +	vma->anon_vma = NULL;
>  }
> 
>  /**
> --
> 2.53.0