linux-kernel - [PATCH v4 0/2] Make mmap() with PROT_WRITE imply PROT

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Message-Id: <20220915193702.2201018-1-abrestic@rivosinc.com>
Date:   Thu, 15 Sep 2022 15:37:00 -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>,
        Conor.Dooley@...rochip.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Andrew Bresticker <abrestic@...osinc.com>
Subject: [PATCH v4 0/2] Make mmap() with PROT_WRITE imply PROT_READ

Commit 2139619bcad7 ("riscv: mmap with PROT_WRITE but no PROT_READ is
invalid") made mmap() reject mappings with only PROT_WRITE set in an
attempt to fix an observed inconsistency in behavior when attempting
to read from a PROT_WRITE-only mapping. The root cause of this behavior
was actually that while RISC-V's protection_map maps VM_WRITE to
readable PTE permissions (since write-only PTEs are considered reserved
by the privileged spec), the page fault handler considered loads from
VM_WRITE-only VMAs illegal accesses. Fix the underlying cause by
handling faults in VM_WRITE-only VMAs (patch 1) and then re-enable
use of mmap(PROT_WRITE) (patch 2), making RISC-V's behavior consistent
with all other architectures that don't support write-only PTEs.

Both patches are tagged as fixes for the aforementioned commit since that
commit made a userspace visible change that will break any software relying
on mmap(PROT_WRITE). (Also cc: stable since the offending commit was
itself backported to stable).

v1 -> v2: Allow handling of load faults in VM_WRITE VMAs
v2 -> v3: Split into two pathces
v3 -> v4: Fixes tags (+ this cover letter)

Andrew Bresticker (2):
  riscv: Make VM_WRITE imply VM_READ
  riscv: Allow PROT_WRITE-only mmap()

 arch/riscv/kernel/sys_riscv.c | 3 ---
 arch/riscv/mm/fault.c         | 3 ++-
 2 files changed, 2 insertions(+), 4 deletions(-)

-- 
2.25.1