RE: [PATCH v2] erofs: fix partially uninitialized misuse in z_erofs_onlinepage_fixup

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Hi xiang:

Hongyu reported "id != index" in z_erofs_onlinepage_fixup() with specific aarch64 environment easily, which wasn't shown before.

After digging into that, I found that high 32 bits of page->private was set to 0xaaaaaaaa rather than 0 (due to z_erofs_onlinepage_init behavior with specific compiler options). Actually we only use low
32 bits to keep the page information since page->private is only 4 bytes on most 32-bit platforms. However z_erofs_onlinepage_fixup() uses the upper 32 bits by mistake.

Tested-by: hongyu.jin@xxxxxxxxxx

It's ok.

-----Original Message-----
From: Gao Xiang [mailto:hsiangkao@xxxxxxx]
Sent: Friday, June 19, 2020 7:44 AM
To: linux-erofs@xxxxxxxxxxxxxxxx; Chao Yu <yuchao0@xxxxxxxxxx>
Cc: Chao Yu <chao@xxxxxxxxxx>; Li Guifu <bluce.liguifu@xxxxxxxxxx>; Fang Wei <fangwei1@xxxxxxxxxx>; LKML <linux-kernel@xxxxxxxxxxxxxxx>; Gao Xiang <hsiangkao@xxxxxxxxxx>; 金红宇 (Hongyu Jin) <hongyu.jin@xxxxxxxxxx>; stable@xxxxxxxxxxxxxxx
Subject: [PATCH v2] erofs: fix partially uninitialized misuse in z_erofs_onlinepage_fixup

From: Gao Xiang <hsiangkao@xxxxxxxxxx>

Hongyu reported "id != index" in z_erofs_onlinepage_fixup() with specific aarch64 environment easily, which wasn't shown before.

After digging into that, I found that high 32 bits of page->private was set to 0xaaaaaaaa rather than 0 (due to z_erofs_onlinepage_init behavior with specific compiler options). Actually we only use low
32 bits to keep the page information since page->private is only 4 bytes on most 32-bit platforms. However z_erofs_onlinepage_fixup() uses the upper 32 bits by mistake.

Let's fix it now.

Reported-by: Hongyu Jin <hongyu.jin@xxxxxxxxxx>
Fixes: 3883a79abd02 ("staging: erofs: introduce VLE decompression support")
Cc: <stable@xxxxxxxxxxxxxxx> # 4.19+
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxx>
---
change since v1:
 move .v assignment out since it doesn't need for every loop;

 fs/erofs/zdata.h | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/fs/erofs/zdata.h b/fs/erofs/zdata.h index 7824f5563a55..9b66c28b3ae9 100644
--- a/fs/erofs/zdata.h
+++ b/fs/erofs/zdata.h
@@ -144,22 +144,22 @@ static inline void z_erofs_onlinepage_init(struct page *page)  static inline void z_erofs_onlinepage_fixup(struct page *page,
 uintptr_t index, bool down)
 {
-unsigned long *p, o, v, id;
-repeat:
-p = &page_private(page);
-o = READ_ONCE(*p);
+union z_erofs_onlinepage_converter u = { .v = &page_private(page) };
+int orig, orig_index, val;

-id = o >> Z_EROFS_ONLINEPAGE_INDEX_SHIFT;
-if (id) {
+repeat:
+orig = atomic_read(u.o);
+orig_index = orig >> Z_EROFS_ONLINEPAGE_INDEX_SHIFT;
+if (orig_index) {
 if (!index)
 return;

-DBG_BUGON(id != index);
+DBG_BUGON(orig_index != index);
 }

-v = (index << Z_EROFS_ONLINEPAGE_INDEX_SHIFT) |
-((o & Z_EROFS_ONLINEPAGE_COUNT_MASK) + (unsigned int)down);
-if (cmpxchg(p, o, v) != o)
+val = (index << Z_EROFS_ONLINEPAGE_INDEX_SHIFT) |
+((orig & Z_EROFS_ONLINEPAGE_COUNT_MASK) + (unsigned int)down);
+if (atomic_cmpxchg(u.o, orig, val) != orig)
 goto repeat;
 }

--
2.24.0

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