[PATCH v4 2/2] riscv: Allow PROT_WRITE-only mmap()

From: Andrew Bresticker
Date: Thu Sep 15 2022 - 15:41:59 EST

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

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")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.19+
Reviewed-by: Atish Patra <atishp@xxxxxxxxxxxx>
Signed-off-by: Andrew Bresticker <abrestic@xxxxxxxxxxxx>
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));