linux-kernel - [PATCH v2] riscv: Make mmap() with PROT_WRITE imply PROT

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Date:   Thu,  8 Sep 2022 14:50:06 -0400
From:   Andrew Bresticker <abrestic@...osinc.com>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Atish Patra <atishp@...shpatra.org>,
        Celeste Liu <coelacanthus@...look.com>,
        dram <dramforever@...e.com>, Ruizhe Pan <c141028@...il.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Andrew Bresticker <abrestic@...osinc.com>
Subject: [PATCH v2] riscv: Make mmap() with PROT_WRITE imply PROT_READ

Commit 2139619bcad7 ("riscv: mmap with PROT_WRITE but no PROT_READ is
invalid") made mmap() return EINVAL if PROT_WRITE was set wihtout
PROT_READ with the justification that a write-only PTE is considered a
reserved PTE permission bit pattern in the privileged spec. This check
is unnecessary since RISC-V defines its protection_map such that PROT_WRITE
maps to the same PTE permissions as PROT_WRITE|PROT_READ, and it is
inconsistent with other architectures that don't support write-only PTEs,
creating a potential software portability issue. Just remove the check
altogether and let PROT_WRITE imply PROT_READ as is the case on other
architectures.

Note that this also allows PROT_WRITE|PROT_EXEC mappings which were
disallowed prior to the aforementioned commit; PROT_READ is implied in
such mappings as well.

Fixes: 2139619bcad7 ("riscv: mmap with PROT_WRITE but no PROT_READ is invalid")
Signed-off-by: Andrew Bresticker <abrestic@...osinc.com>
---
v1 -> v2: Update access_error() to account for write-implies-read
---
 arch/riscv/kernel/sys_riscv.c | 3 ---
 arch/riscv/mm/fault.c         | 3 ++-
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/kernel/sys_riscv.c b/arch/riscv/kernel/sys_riscv.c
index 571556bb9261..5d3f2fbeb33c 100644
--- a/arch/riscv/kernel/sys_riscv.c
+++ b/arch/riscv/kernel/sys_riscv.c
@@ -18,9 +18,6 @@ static long riscv_sys_mmap(unsigned long addr, unsigned long len,
 	if (unlikely(offset & (~PAGE_MASK >> page_shift_offset)))
 		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));
 }
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index f2fbd1400b7c..d86f7cebd4a7 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -184,7 +184,8 @@ static inline bool access_error(unsigned long cause, struct vm_area_struct *vma)
 		}
 		break;
 	case EXC_LOAD_PAGE_FAULT:
-		if (!(vma->vm_flags & VM_READ)) {
+		/* Write implies read */
+		if (!(vma->vm_flags & (VM_READ | VM_WRITE))) {
 			return true;
 		}
 		break;
-- 
2.25.1