Re: [PATCH] Revert "mm, swap: avoid redundant swap device pinning"

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Hi, Kairui,

Kairui Song via B4 Relay <devnull+kasong.tencent.com@kernel.org> writes:

> From: Kairui Song <kasong@tencent.com>
>
> This reverts commit 78524b05f1a3e16a5d00cc9c6259c41a9d6003ce.
>
> While reviewing recent leaf entry changes, I noticed that commit
> 78524b05f1a3 ("mm, swap: avoid redundant swap device pinning") isn't
> correct. It's true that most all callers of __read_swap_cache_async are
> already holding a swap entry reference, so the repeated swap device
> pinning isn't needed on the same swap device, but it is possible that
> VMA readahead (swap_vma_readahead()) may encounter swap entries from a
> different swap device when there are multiple swap devices, and call
> __read_swap_cache_async without holding a reference to that swap device.
>
> So it is possible to cause a UAF if swapoff of device A raced with
> swapin on device B, and VMA readahead tries to read swap entries from
> device A. It's not easy to trigger but in theory possible to cause real
> issues. And besides, that commit made swap more vulnerable to issues
> like corrupted page tables.
>
> Just revert it. __read_swap_cache_async isn't that sensitive to
> performance after all, as it's mostly used for SSD/HDD swap devices with
> readahead. SYNCHRONOUS_IO devices may fallback onto it for swap count >
> 1 entries, but very soon we will have a new helper and routine for
> such devices, so they will never touch this helper or have redundant
> swap device reference overhead.

Is it better to add get_swap_device() in swap_vma_readahead()?  Whenever
we get a swap entry, the first thing we need to do is call
get_swap_device() to check the validity of the swap entry and prevent
the backing swap device from going under us.  This helps us to avoid
checking the validity of the swap entry in every swap function.  Does
this sound reasonable?

> Fixes: 78524b05f1a3 ("mm, swap: avoid redundant swap device pinning")
> Signed-off-by: Kairui Song <kasong@tencent.com>

[snip]

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Best Regards,
Huang, Ying