linux-kernel - Re: [REGRESSION] BUG: KFENCE: memory corruption in drm_gem_put

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Date:   Thu, 5 Oct 2023 14:05:27 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Oleksandr Natalenko <oleksandr@...alenko.name>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        Linux Regressions <regressions@...ts.linux.dev>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Christian König <christian.koenig@....com>,
        linaro-mm-sig@...ts.linaro.org, linux-mm@...ck.org,
        Maxime Ripard <mripard@...nel.org>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        linux-media@...r.kernel.org
Subject: Re: [REGRESSION] BUG: KFENCE: memory corruption in
 drm_gem_put_pages+0x186/0x250

On Thu, Oct 05, 2023 at 02:30:55PM +0200, Oleksandr Natalenko wrote:
> No-no, sorry for possible confusion. Let me explain again:
> 
> 1. we had an issue with i915, which was introduced by 0b62af28f249, and later was fixed by 863a8eb3f270
> 2. now I've discovered another issue, which looks very similar to 1., but in a VM with Cirrus VGA, and it happens even while having 863a8eb3f270 applied
> 3. I've tried reverting 3291e09a4638, after which I cannot reproduce the issue with Cirrus VGA, but clearly there was no fix for it discussed
> 
> IOW, 863a8eb3f270 is the fix for 0b62af28f249, but not for 3291e09a4638. It looks like 3291e09a4638 requires a separate fix.

Thank you!  Sorry about the misunderstanding.  Try this:

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 6129b89bb366..44a948b80ee1 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -540,7 +540,7 @@ struct page **drm_gem_get_pages(struct drm_gem_object *obj)
 	struct page **pages;
 	struct folio *folio;
 	struct folio_batch fbatch;
-	int i, j, npages;
+	long i, j, npages;
 
 	if (WARN_ON(!obj->filp))
 		return ERR_PTR(-EINVAL);
@@ -564,11 +564,13 @@ struct page **drm_gem_get_pages(struct drm_gem_object *obj)
 
 	i = 0;
 	while (i < npages) {
+		long nr;
 		folio = shmem_read_folio_gfp(mapping, i,
 				mapping_gfp_mask(mapping));
 		if (IS_ERR(folio))
 			goto fail;
-		for (j = 0; j < folio_nr_pages(folio); j++, i++)
+		nr = min(npages - i, folio_nr_pages(folio));
+		for (j = 0; j < nr; j++, i++)
 			pages[i] = folio_file_page(folio, i);
 
 		/* Make sure shmem keeps __GFP_DMA32 allocated pages in the