linux-kernel - [PATCH 0/2] riscv: misaligned: fix misaligned accesses handling in put/get

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Message-ID: <20250530205658.624195-1-cleger@rivosinc.com>
Date: Fri, 30 May 2025 22:56:56 +0200
From: Clément Léger <cleger@...osinc.com>
To: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Cc: Clément Léger <cleger@...osinc.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Alexandre Ghiti <alex@...ti.fr>
Subject: [PATCH 0/2] riscv: misaligned: fix misaligned accesses handling in put/get_user()

While debugging a few problems with the misaligned access kselftest,
Alexandre discovered some crash with the current code. Indeed, some
misaligned access was done by the kernel using put_user(). This
was resulting in trap and a kernel crash since. The path was the
following:
user -> kernel -> access to user memory -> misaligned trap -> trap ->
kernel -> misaligned handling -> memcpy -> crash due to failed page fault
while in interrupt disabled section.

Last discussion about kernel misaligned handling and interrupt reenabling
were actually not to reenable interrupt when handling misaligned access
being done by kernel. The best solution being not to do any misaligned
accesses to userspace memory, we considered a few options:

- Remove any call to put/get_user() potientally doing misaligned
  accesses
- Do not do any misaligned accesses in put/get_user() itself

The second solution was the one chosen as there are too many callsite to
put/get_user() that could potentially do misaligned accesses. We tried
two approaches for that, either split access in two aligned accesses
(and do RMW for put_user()) or call copy_from/to_user() which does not
do any misaligned accesses. The later one was the simpler to implement
(although the performances are probably lower than split aligned
accesses but still way better than doing misaligned access emulation)
and allows to support what we wanted.

These commits are based on top of Alex dev/alex/get_user_misaligned_v1
branch.

Clément Léger (2):
  riscv: process: use unsigned int instead of unsigned long for
    put_user()
  riscv: uaccess: do not do misaligned accesses in get/put_user()

 arch/riscv/include/asm/uaccess.h | 28 ++++++++++++++++++++++------
 arch/riscv/kernel/process.c      |  2 +-
 2 files changed, 23 insertions(+), 7 deletions(-)

-- 
2.49.0