linux-kernel - Re: BUG: kernel NULL pointer dereference, address: RIP: 0010:shmem_getpage

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Date:   Mon, 14 Sep 2020 12:55:59 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Naresh Kamboju <naresh.kamboju@...aro.org>
Cc:     open list <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        lkft-triage@...ts.linaro.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        William Kucharski <william.kucharski@...cle.com>,
        gandalf@...ds.org, Qian Cai <cai@....pw>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Michal Hocko <mhocko@...e.com>, Yang Shi <shy828301@...il.com>,
        Shakeel Butt <shakeelb@...gle.com>
Subject: Re: BUG: kernel NULL pointer dereference, address: RIP:
 0010:shmem_getpage_gfp.isra.0+0x470/0x750

On Mon, Sep 14, 2020 at 03:49:43PM +0530, Naresh Kamboju wrote:
> While running LTP fs on qemu x86 and qemu_i386 these kernel BUGs noticed.

I actually sent the fix for this a couple of days ago [1], but I think Andrew
overlooked it while constructing the -mm tree.  Here's a fix you can
apply to the -mm tree:

[1] https://lore.kernel.org/linux-mm/20200912032042.GA6583@casper.infradead.org/

diff --git a/mm/shmem.c b/mm/shmem.c
index d2a46ef7df43..58bc9e326d0d 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1793,7 +1793,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 	struct mm_struct *charge_mm;
 	struct page *page;
 	enum sgp_type sgp_huge = sgp;
-	pgoff_t hindex;
+	pgoff_t hindex = index;
 	int error;
 	int once = 0;
 	int alloced = 0;
@@ -1822,6 +1822,8 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 		return error;
 	}
 
+	if (page)
+		hindex = page->index;
 	if (page && sgp == SGP_WRITE)
 		mark_page_accessed(page);
 
@@ -1832,6 +1834,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 		unlock_page(page);
 		put_page(page);
 		page = NULL;
+		hindex = index;
 	}
 	if (page || sgp == SGP_READ)
 		goto out;
@@ -1982,7 +1985,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 		goto unlock;
 	}
 out:
-	*pagep = page + index - page->index;
+	*pagep = page + index - hindex;
 	return 0;
 
 	/*