2022-07-27 07:04:35

by Yongqiang Liu

Subject: [PATCH] mm/folio-compact: fix potential NULL pointer in pagecache_get_page

When __filemap_get_folio() failed and returned NULL, we would
get a NULL pointer dereference in pagecache_get_page.

Fixes: 3f0c6a07fee6 ("mm/filemap: Add filemap_get_folio")
Signed-off-by: Yongqiang Liu <[email protected]>
Cc: <[email protected]> # 5.16
---
mm/folio-compat.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/folio-compat.c b/mm/folio-compat.c
index 20bc15b57d93..7b21393480e0 100644
--- a/mm/folio-compat.c
+++ b/mm/folio-compat.c
@@ -124,7 +124,9 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
struct folio *folio;

folio = __filemap_get_folio(mapping, index, fgp_flags, gfp);
- if ((fgp_flags & FGP_HEAD) || !folio || xa_is_value(folio))
+ if (!folio)
+ return NULL;
+ if ((fgp_flags & FGP_HEAD) || xa_is_value(folio))
return &folio->page;
return folio_file_page(folio, index);
}
--
2.25.1

2022-07-27 12:17:35

by William Kucharski

Subject: Re: [PATCH] mm/folio-compact: fix potential NULL pointer in pagecache_get_page

This isn't a NULL pointer dereference; returning &(0->page) is completely legal
as was discussed regarding this exact code back in April:

https://lore.kernel.org/lkml/[email protected]/

> On Jul 27, 2022, at 12:46 AM, Yongqiang Liu <[email protected]> wrote:
>
> When __filemap_get_folio() failed and returned NULL, we would
> get a NULL pointer dereference in pagecache_get_page.
>
> Fixes: 3f0c6a07fee6 ("mm/filemap: Add filemap_get_folio")
> Signed-off-by: Yongqiang Liu <[email protected]>
> Cc: <[email protected]> # 5.16
> ---
> mm/folio-compat.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/mm/folio-compat.c b/mm/folio-compat.c
> index 20bc15b57d93..7b21393480e0 100644
> --- a/mm/folio-compat.c
> +++ b/mm/folio-compat.c
> @@ -124,7 +124,9 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
> struct folio *folio;
>
> folio = __filemap_get_folio(mapping, index, fgp_flags, gfp);
> - if ((fgp_flags & FGP_HEAD) || !folio || xa_is_value(folio))
> + if (!folio)
> + return NULL;
> + if ((fgp_flags & FGP_HEAD) || xa_is_value(folio))
> return &folio->page;
> return folio_file_page(folio, index);
> }
> --
> 2.25.1
>
>

2022-07-27 13:45:53

by Matthew Wilcox

Subject: Re: [PATCH] mm/folio-compact: fix potential NULL pointer in pagecache_get_page

On Wed, Jul 27, 2022 at 06:46:21AM +0000, Yongqiang Liu wrote:
> When __filemap_get_folio() failed and returned NULL, we would
> get a NULL pointer dereference in pagecache_get_page.

No we wouldn't.

2022-07-28 01:41:50

by Yongqiang Liu

Subject: Re: [PATCH] mm/folio-compact: fix potential NULL pointer in pagecache_get_page

Understood, thanks for your explanation :)

?? 2022/7/27 19:40, William Kucharski д??:
> This isn't a NULL pointer dereference; returning &(0->page) is completely legal
> as was discussed regarding this exact code back in April:
>
> https://lore.kernel.org/lkml/[email protected]/
>
>> On Jul 27, 2022, at 12:46 AM, Yongqiang Liu <[email protected]> wrote:
>>
>> When __filemap_get_folio() failed and returned NULL, we would
>> get a NULL pointer dereference in pagecache_get_page.
>>
>> Fixes: 3f0c6a07fee6 ("mm/filemap: Add filemap_get_folio")
>> Signed-off-by: Yongqiang Liu <[email protected]>
>> Cc: <[email protected]> # 5.16
>> ---
>> mm/folio-compat.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/folio-compat.c b/mm/folio-compat.c
>> index 20bc15b57d93..7b21393480e0 100644
>> --- a/mm/folio-compat.c
>> +++ b/mm/folio-compat.c
>> @@ -124,7 +124,9 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
>> struct folio *folio;
>>
>> folio = __filemap_get_folio(mapping, index, fgp_flags, gfp);
>> - if ((fgp_flags & FGP_HEAD) || !folio || xa_is_value(folio))
>> + if (!folio)
>> + return NULL;
>> + if ((fgp_flags & FGP_HEAD) || xa_is_value(folio))
>> return &folio->page;
>> return folio_file_page(folio, index);
>> }
>> --
>> 2.25.1
>>
>>
> .