linux-kernel - Re: [PATCH] lib/iov_iter: fix to increase non slab folio refcount

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Message-ID: <Z-v2ReHKyFIXQlKs@casper.infradead.org>
Date: Tue, 1 Apr 2025 15:20:53 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Sheng Yong <shengyong2021@...il.com>
Cc: akpm@...ux-foundation.org, vbabka@...e.cz, linux-kernel@...r.kernel.org,
	linux-mm@...hiver.kernel.org, Sheng Yong <shengyong1@...omi.com>
Subject: Re: [PATCH] lib/iov_iter: fix to increase non slab folio refcount

On Tue, Apr 01, 2025 at 10:02:55PM +0800, Sheng Yong wrote:
> When testing EROFS file-backed mount over v9fs on qemu, I encounter
> a folio UAF and page sanity check reports the following call trace.
> Fix it by increasing non slab folio refcount correctly.

This report needs to say what the problem _is_, which is that pages may
be coalesced across a folio boundary.

> +++ b/lib/iov_iter.c
> @@ -1191,8 +1191,7 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
>  			return -ENOMEM;
>  		p = *pages;
>  		for (int k = 0; k < n; k++) {
> -			struct folio *folio = page_folio(page);
> -			p[k] = page + k;
> +			struct folio *folio = page_folio(p[k] = page + k);

Never write code this ugly.

-			struct folio *folio = page_folio(page);
+			struct folio *folio = page_folio(page + k);
			p[k] = page + k;

is much more readable.