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

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Message-ID: <1aefb8dc-9145-4cee-b262-7d4212159444@ghiti.fr>
Date: Sat, 31 May 2025 15:32:37 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Clément Léger <cleger@...osinc.com>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>
Subject: Re: [PATCH 0/2] riscv: misaligned: fix misaligned accesses handling
 in put/get_user()

On 5/30/25 22:56, Clément Léger wrote:
> 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(-)


We also need to prevent unsafe routines to trigger misaligned accesses, 
I have a patch for this here 
https://github.com/linux-riscv/linux/commit/7c172121aeb235dedeb6f5e06740527530edd6af

Clément, can you add this one to the series please?

I have just  triggered a CI with those fixes on top of my sbi 3.0 branch.

Thanks,

Alex