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From:   Lichao Liu <liulichao@loongson.cn>
To:     tsbogend@alpha.franken.de, yuanjunqing@loongson.cn,
        chenhc@lemote.com, jiaxun.yang@flygoat.com
Cc:     linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org,
        Lichao Liu <liulichao@loongson.cn>
Subject: [PATCH] MIPS: Grant pte read permission, even if vma only have VM_WRITE permission.
Date:   Tue, 30 Jun 2020 08:58:45 +0800
Message-Id: <20200630005845.1239974-1-liulichao@loongson.cn>
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Background:
a cpu have RIXI feature.

Now, if a vma only have VM_WRITE permission, the vma->vm_page_prot will
set _PAGE_NO_READ. In general case, someone read the vma will trigger
RI exception, then do_page_fault will handle it.

But in the following scene, program will hang.

example scene(a trinity test case):
futex_wake_op() will read uaddr, which is passed from user space.
If a program mmap a vma, which only have VM_WRITE permission,
then call futex, and use an address belonging to the vma as uaddr
argument. futex_wake_op() will read the address after disable
pagefault and set correct __ex_table(return -14 directly),
do_page_fault will find the correct __ex_table, and then return -14.
Then futex_wake_op() will try to fixup this error by call
fault_in_user_writeable(), because the pte have write permission,
so handle_mm_fault will do nothing, and return success.
But the RI bit in pte and tlb entry still exsits.
The program will deadloop:
do_page_fault -> find __ex_table success -> return -14;
futex_wake_op -> call fault_in_user_writeable() to fix the error -> retry;
do_page_fault -> find __ex_table success -> return -14;
futex_wake_op -> call fault_in_user_writeable() to fix the error -> retry;
.....

The first perspective of root cause:
Futex think a pte have write permission will have read permission.
When page fault, it only try to fixup with FAULT_FLAG_WRITE.

The second perspective of root cause:
MIPS platform doesn't grant pte read permission, if vma only have
VM_WRITE permission.But X86 and arm64 will.

Most of the architecture will grant pte read permission, even if
the vma only have VM_WRITE permission.
And if the cpu doesn't have RIXI feature, MIPS platform will
grant pte read permission by set _PAGE_READ.
So I think we should fixup thix problem by grant pte read permission,
even if vma only have VM_WRITE permission.

Signed-off-by: Lichao Liu <liulichao@loongson.cn>
---
 arch/mips/mm/cache.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/mips/mm/cache.c b/arch/mips/mm/cache.c
index ad6df1cea866..72b60c44a962 100644
--- a/arch/mips/mm/cache.c
+++ b/arch/mips/mm/cache.c
@@ -160,7 +160,7 @@ static inline void setup_protection_map(void)
 	if (cpu_has_rixi) {
 		protection_map[0]  = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_NO_READ);
 		protection_map[1]  = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC);
-		protection_map[2]  = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_NO_READ);
+		protection_map[2]  = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC);
 		protection_map[3]  = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC);
 		protection_map[4]  = __pgprot(_page_cachable_default | _PAGE_PRESENT);
 		protection_map[5]  = __pgprot(_page_cachable_default | _PAGE_PRESENT);
@@ -169,7 +169,7 @@ static inline void setup_protection_map(void)
 
 		protection_map[8]  = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_NO_READ);
 		protection_map[9]  = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC);
-		protection_map[10] = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_WRITE | _PAGE_NO_READ);
+		protection_map[10] = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_WRITE);
 		protection_map[11] = __pgprot(_page_cachable_default | _PAGE_PRESENT | _PAGE_NO_EXEC | _PAGE_WRITE);
 		protection_map[12] = __pgprot(_page_cachable_default | _PAGE_PRESENT);
 		protection_map[13] = __pgprot(_page_cachable_default | _PAGE_PRESENT);
-- 
2.25.1