linux-kernel - [PATCH v3 2/2] riscv: Allow PROT

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Date:   Fri,  9 Sep 2022 17:27:31 -0400
From:   Andrew Bresticker <abrestic@...osinc.com>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        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>,
        Atish Patra <atishp@...osinc.com>
Subject: [PATCH v3 2/2] riscv: Allow PROT_WRITE-only mmap()

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 we let VM_WRITE imply VM_READ on RISC-V, 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")
Reviewed-by: Atish Patra <atishp@...osinc.com>
Signed-off-by: Andrew Bresticker <abrestic@...osinc.com>
---
v1 -> v2: Update access_error() to account for write-implies-read
v2 -> v3: Separate into two commits
---
 arch/riscv/kernel/sys_riscv.c | 3 ---
 1 file changed, 3 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));
 }
-- 
2.25.1