[PATCH] f2fs: fix to avoid NULL pointer dereference

[Chao Yu]

From: Yi Chen <chenyi77@xxxxxxxxxx>

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
pc : f2fs_put_page+0x1c/0x26c
lr : __revoke_inmem_pages+0x544/0x75c
f2fs_put_page+0x1c/0x26c
__revoke_inmem_pages+0x544/0x75c
__f2fs_commit_inmem_pages+0x364/0x3c0
f2fs_commit_inmem_pages+0xc8/0x1a0
f2fs_ioc_commit_atomic_write+0xa4/0x15c
f2fs_ioctl+0x5b0/0x1574
file_ioctl+0x154/0x320
do_vfs_ioctl+0x164/0x740
__arm64_sys_ioctl+0x78/0xa4
el0_svc_common+0xbc/0x1d0
el0_svc_handler+0x74/0x98
el0_svc+0x8/0xc

In f2fs_put_page, we access page->mapping is NULL.
The root cause is:
In some cases, the page refcount and ATOMIC_WRITTEN_PAGE
flag miss set for page-priavte flag has been set.
We add f2fs_bug_on like this:

f2fs_register_inmem_page()
{
	...
	f2fs_set_page_private(page, ATOMIC_WRITTEN_PAGE);

	f2fs_bug_on(F2FS_I_SB(inode), !IS_ATOMIC_WRITTEN_PAGE(page));
	...
}

The bug on stack follow link this:
PC is at f2fs_register_inmem_page+0x238/0x2b4
LR is at f2fs_register_inmem_page+0x2a8/0x2b4
f2fs_register_inmem_page+0x238/0x2b4
f2fs_set_data_page_dirty+0x104/0x164
set_page_dirty+0x78/0xc8
f2fs_write_end+0x1b4/0x444
generic_perform_write+0x144/0x1cc
__generic_file_write_iter+0xc4/0x174
f2fs_file_write_iter+0x2c0/0x350
__vfs_write+0x104/0x134
vfs_write+0xe8/0x19c
SyS_pwrite64+0x78/0xb8

To fix this issue, let's add page refcount add page-priavte flag.
The page-private flag is not cleared and needs further analysis.

Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
Signed-off-by: Ge Qiu <qiuge@xxxxxxxxxx>
Signed-off-by: Dehe Gu <gudehe@xxxxxxxxxx>
Signed-off-by: Yi Chen <chenyi77@xxxxxxxxxx>
---
 fs/f2fs/segment.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 0cb1ca88d4aa..d6c6c13feb43 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -186,7 +186,10 @@ void f2fs_register_inmem_page(struct inode *inode, struct page *page)
 {
 	struct inmem_pages *new;
 
-	f2fs_set_page_private(page, ATOMIC_WRITTEN_PAGE);
+	if (PagePrivate(page))
+		set_page_private(page, (unsigned long)ATOMIC_WRITTEN_PAGE);
+	else
+		f2fs_set_page_private(page, ATOMIC_WRITTEN_PAGE);
 
 	new = f2fs_kmem_cache_alloc(inmem_entry_slab, GFP_NOFS);
 
-- 
2.29.2