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

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Message-ID: <01a5ab11-ed87-4d30-a162-987db34e6483@rivosinc.com>
Date: Mon, 2 Jun 2025 09:19:45 +0200
From: Clément Léger <cleger@...osinc.com>
To: Alexandre Ghiti <alex@...ti.fr>, 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 31/05/2025 15:32, Alexandre Ghiti wrote:
> 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?

Yep sure.

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