[PATCH v2] riscv: don't allow write but not read page mapping request in mmap

[Celeste Liu]

When xctan tries to run one of libaio's tests
(https://pagure.io/libaio/blob/1b18bfafc6a2f7b9fa2c6be77a95afed8b7be448/f/harness/cases/5.t),
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"[1], 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[2], write+exec
only is marked as invalid, but write only is not marked as invalid.

This patch refines that judgment.

[1]: https://github.com/riscv/riscv-isa-manual/releases/download/Priv-v1.12/riscv-privileged-20211203.pdf
[2]: modified in commit e0d17c842c0f824fd4df9f4688709fc6907201e1
     (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e0d17c842c0f824fd4df9f4688709fc6907201e1)

Reported-by: xctan <xc-tan@xxxxxxxxxxx>
Co-developed-by: dram <dramforever@xxxxxxxx>
Signed-off-by: dram <dramforever@xxxxxxxx>
Co-developed-by: Ruizhe Pan <c141028@xxxxxxxxx>
Signed-off-by: Ruizhe Pan <c141028@xxxxxxxxx>
Signed-off-by: Celeste Liu <coelacanthus@xxxxxxxxxxx>
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.

 arch/riscv/kernel/sys_riscv.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
index 9c0194f176fc..571556bb9261 100644
--- a/arch/riscv/kernel/sys_riscv.c
+++ b/arch/riscv/kernel/sys_riscv.c
@@ -18,9 +18,8 @@ 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;
+	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