Re: [External] : [PATCH] ocfs2: Fix potential ERR_PTR dereference in ocfs2_unlock_and_free_folios

[Mark Tinguely]

On 5/2/25 10:37 AM, Zhiyu Zhang wrote:
> When allocation of a folio fails (e.g., -ENOMEM), ocfs2_write_begin_nolock()
> stores an ERR_PTR in wc->w_folios[i]. ocfs2_unlock_and_free_folios() later
> walks this array but only skips NULL entries, thus passing the error pointer
> to folio_unlock(), which eventually dereferences it in const_folio_flags and
> panics with BUG: unable to handle kernel paging request in const_folio_flags.
>
> This patch fixes this issue by adding IS_ERR_OR_NULL() to filter both NULL
> and error entries before any folio operations.
>
> Fixes: 7e119cff9d0a ("ocfs2: convert w_pages to w_folios")
> Reported-by: Zhiyu Zhang <zhiyuzhang999@xxxxxxxxx>
> Closes: https://urldefense.com/v3/__https://lore.kernel.org/linux-fsdevel/CALf2hKtnFskBvmZeigK_=mqq9Vd4TWT*YOXcwfNNt1ydOY=0Yg@xxxxxxxxxxxxxx/T/*u__;KyM!!ACWV5N9M2RV99hQ!O2QZFrLv_PvhBiQH1Ak3ZDOIFOQG2RgkiOSogyy_3n0ZsLL_GXVwf0aiu7ti71LagUjWpF54NlvjjQP7IfPUT2vS0yY$
> Tested-by: Zhiyu Zhang <zhiyuzhang999@xxxxxxxxx>
> Signed-off-by: Zhiyu Zhang <zhiyuzhang999@xxxxxxxxx>
> ---
>   fs/ocfs2/aops.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ocfs2/aops.c b/fs/ocfs2/aops.c
> index 40b6bce12951..9e500c5fee38 100644
> --- a/fs/ocfs2/aops.c
> +++ b/fs/ocfs2/aops.c
> @@ -760,7 +760,7 @@ void ocfs2_unlock_and_free_folios(struct folio **folios, int num_folios)
>   	int i;
>
>   	for(i = 0; i < num_folios; i++) {
> -		if (!folios[i])
> +		if (IS_ERR_OR_NULL(folios[i]))
>   			continue;
>   		folio_unlock(folios[i]);
>   		folio_mark_accessed(folios[i]);
> --
> 2.34.1


yes, this was my bug in the folio port.
I submitted the patch with subject "ocfs2: fix panic in failed foilio 
allocation" as a solution.
In the old page allocation code, a NULL was the failed allocation. Folio 
returns an error code. the patch put the NULL back into the folio araay 
location:

 https://lore.kernel.org/ocfs2-devel/7de24670-89cc-4502-adbe-308bd5786f1d@xxxxxxxxxxxxxxxxx/T/#t