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Date:   Wed, 29 Mar 2023 14:53:04 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Ivan Orlov <ivan.orlov0322@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        himadrispandya@gmail.com, skhan@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
Subject: Re: [PATCH] mm: khugepaged: Fix kernel BUG in
 hpage_collapse_scan_file
Message-Id: <20230329145304.66add47ba9b9fafb71b1e13d@linux-foundation.org>
In-Reply-To: <20230329145330.23191-1-ivan.orlov0322@gmail.com>
References: <20230329145330.23191-1-ivan.orlov0322@gmail.com>
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On Wed, 29 Mar 2023 18:53:30 +0400 Ivan Orlov <ivan.orlov0322@gmail.com> wrote:

> Syzkaller reported the following issue:
> 
> ...
> 
> The 'xas_store' call during page cache scanning can potentially
> translate 'xas' into the error state (with the reproducer provided
> by the syzkaller the error code is -ENOMEM). However, there are no
> further checks after the 'xas_store', and the next call of 'xas_next'
> at the start of the scanning cycle doesn't increase the xa_index,
> and the issue occurs.
> 
> This patch will add the xarray state error checking after the
> 'xas_store' and the corresponding result error code.
> 
> Tested via syzbot.
> 
> Reported-by: syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
> Link: https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc
> Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
> ---
>  mm/khugepaged.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 92e6f56a932d..4d9850d9ea7f 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -55,6 +55,7 @@ enum scan_result {
>  	SCAN_CGROUP_CHARGE_FAIL,
>  	SCAN_TRUNCATED,
>  	SCAN_PAGE_HAS_PRIVATE,
> +	SCAN_STORE_FAILED,
>  };
>  
>  #define CREATE_TRACE_POINTS
> @@ -1840,6 +1841,15 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>  					goto xa_locked;
>  				}
>  				xas_store(&xas, hpage);
> +				if (xas_error(&xas)) {
> +					/* revert shmem_charge performed
> +					 * in the previous condition
> +					 */
> +					mapping->nrpages--;
> +					shmem_uncharge(mapping->host, 1);
> +					result = SCAN_STORE_FAILED;
> +					goto xa_locked;
> +				}
>  				nr_none++;
>  				continue;
>  			}

Needs this, I assume.

--- a/include/trace/events/huge_memory.h~mm-khugepaged-fix-kernel-bug-in-hpage_collapse_scan_file-fix
+++ a/include/trace/events/huge_memory.h
@@ -36,7 +36,8 @@
 	EM( SCAN_ALLOC_HUGE_PAGE_FAIL,	"alloc_huge_page_failed")	\
 	EM( SCAN_CGROUP_CHARGE_FAIL,	"ccgroup_charge_failed")	\
 	EM( SCAN_TRUNCATED,		"truncated")			\
-	EMe(SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
+	EM( SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
+	EMe(SCAN_STORE_FAILED,		"store_failed")
 
 #undef EM
 #undef EMe
_