linux-kernel - Re: [PATCH v3 1/2] mm: add probe_user

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Date:   Thu, 07 Feb 2019 16:04:07 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Murilo Opsfelder Araujo <muriloo@...ux.ibm.com>,
        Christophe Leroy <christophe.leroy@....fr>
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Mike Rapoport <rppt@...ux.ibm.com>, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] mm: add probe_user_read()

Murilo Opsfelder Araujo <muriloo@...ux.ibm.com> writes:
>> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
>> index 37b226e8df13..ef99edd63da3 100644
>> --- a/include/linux/uaccess.h
>> +++ b/include/linux/uaccess.h
>> @@ -263,6 +263,40 @@ extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
>>  #define probe_kernel_address(addr, retval)		\
>>  	probe_kernel_read(&retval, addr, sizeof(retval))
>>
>> +/**
>> + * probe_user_read(): safely attempt to read from a user location
>> + * @dst: pointer to the buffer that shall take the data
>> + * @src: address to read from
>> + * @size: size of the data chunk
>> + *
>> + * Safely read from address @src to the buffer at @dst.  If a kernel fault
>> + * happens, handle that and return -EFAULT.
>> + *
>> + * We ensure that the copy_from_user is executed in atomic context so that
>> + * do_page_fault() doesn't attempt to take mmap_sem.  This makes
>> + * probe_user_read() suitable for use within regions where the caller
>> + * already holds mmap_sem, or other locks which nest inside mmap_sem.
>> + *
>> + * Returns: 0 on success, -EFAULT on error.
>> + */
>> +
>> +#ifndef probe_user_read
>> +static __always_inline long probe_user_read(void *dst, const void __user *src,
>> +					    size_t size)
>> +{
>> +	long ret;
>> +
>> +	if (!access_ok(src, size))
>> +		return -EFAULT;
>
> Hopefully, there is still time for a minor comment.
>
> Do we need to differentiate the returned error here, e.g.: return
> -EACCES?
>
> I wonder if there will be situations where callers need to know why
> probe_user_read() failed.

It's pretty standard to return EFAULT when an access_ok() check fails,
so I think using EFAULT here is the safest option.

If we used EACCES we'd need to be more careful about converting code to
use this helper, as doing so would potentially cause the error value to
change, which in some cases is not OK.

cheers