[PATCH v4] riscv: don't allow write but not read page mapping request in mmap

[Celeste Liu]

When Xiongchuan Tan tries to run oe of libaio's tests[1], it encounters
a strange behavior: for the same PROT_WRITE only mapping, there was a
discrepancy in whether it could be read before and after writing
(readable before writing, unreadable after writing).
After some investigation, I found that mmap allows write only mapping,
an undefined behavior, on RISC-V.

As mentioned in Table 4.5 in RISC-V spec Volume 2 Section 4.3 version
"20211203 Privileged Architecture v1.12, Ratified"[2], the PTE permission
bit combination of "write+!read" is "Reserved for future use.". Hence,
don't allow such mapping request in mmap call.
In the current code[3], write+exec only is marked as invalid,
but write only is not marked as invalid.

This patch refines that judgment.

[1]: https://pagure.io/libaio/blob/1b18bfafc6a2f7b9fa2c6be77a95afed8b7be448/f/harness/cases/5.t
[2]: https://github.com/riscv/riscv-isa-manual/releases/download/Priv-v1.12/riscv-privileged-20211203.pdf
[3]: commit e0d17c842c0f ("RISC-V: Don't allow write+exec only page mapping request in mmap")

Reported-by: Xiongchuan Tan <xc-tan@xxxxxxxxxxx>
Co-developed-by: Wang Ruikang <dramforever@xxxxxxxx>
Signed-off-by: Wang Ruikang <dramforever@xxxxxxxx>
Co-developed-by: Ruizhe Pan <c141028@xxxxxxxxx>
Signed-off-by: Ruizhe Pan <c141028@xxxxxxxxx>
Signed-off-by: Celeste Liu <coelacanthus@xxxxxxxxxxx>
Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>
Cc: linux-riscv@xxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Guo Ren <guoren@xxxxxxxxxx>
Cc: Yash Shah <yash.shah@xxxxxxxxxx>
---
v2: This version adds a link to the referenced spec, and reference of the 
previous related modification.
v3: fix DCO signoff name and add comment for reason
v4: fix typo in changelog and comment

 arch/riscv/kernel/sys_riscv.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
index 9c0194f176fc..f29456cfe68a 100644
--- a/arch/riscv/kernel/sys_riscv.c
+++ b/arch/riscv/kernel/sys_riscv.c
@@ -18,9 +18,14 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
 	if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
 		return -EINVAL;
 
-	if ((prot & PROT_WRITE) && (prot & PROT_EXEC))
-		if (unlikely(!(prot & PROT_READ)))
-			return -EINVAL;
+	/*
+	 * As mentioned in Table 4.5 in RISC-V spec (20211203) Volume 2
+	 * Section 4.3 the PTE permission bit combination of "write+!read"
+	 * is "Reserved for future use.". Hence, don't allow such mapping
+	 * request in mmap call.
+	 */
+	if (unlikely((prot & PROT_WRITE) && !(prot & PROT_READ)))
+		return -EINVAL;
 
 	return ksys_mmap_pgoff(addr, len, prot, flags, fd,
 			       offset >> (PAGE_SHIFT - page_shift_offset));
-- 
2.36.1