Re: [PATCH v2 1/2] mm: add probe_user_read()

From: Andrew Morton <akpm@linux-foundation.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Kees Cook <keescook@chromium.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Mike Rapoport <rppt@linux.ibm.com>,
	linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-mm@kvack.org
Subject: Re: [PATCH v2 1/2] mm: add probe_user_read()
Date: Tue, 8 Jan 2019 11:48:03 -0800	[thread overview]
Message-ID: <20190108114803.583f203b86d4a368ac9796f3@linux-foundation.org> (raw)
In-Reply-To: <0b0db24e18063076e9d9f4e376994af83da05456.1546932949.git.christophe.leroy@c-s.fr>

On Tue,  8 Jan 2019 07:37:44 +0000 (UTC) Christophe Leroy <christophe.leroy@c-s.fr> wrote:

> In powerpc code, there are several places implementing safe
> access to user data. This is sometimes implemented using
> probe_kernel_address() with additional access_ok() verification,
> sometimes with get_user() enclosed in a pagefault_disable()/enable()
> pair, etc. :
>     show_user_instructions()
>     bad_stack_expansion()
>     p9_hmi_special_emu()
>     fsl_pci_mcheck_exception()
>     read_user_stack_64()
>     read_user_stack_32() on PPC64
>     read_user_stack_32() on PPC32
>     power_pmu_bhrb_to()
> 
> In the same spirit as probe_kernel_read(), this patch adds
> probe_user_read().
> 
> probe_user_read() does the same as probe_kernel_read() but
> first checks that it is really a user address.
> 
> ...
>
> --- 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
> + *
> + * Returns: 0 on success, -EFAULT on error.
> + *
> + * 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.
> + */
> +
> +#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;
> +
> +	pagefault_disable();
> +	ret = __copy_from_user_inatomic(dst, src, size);
> +	pagefault_enable();
> +
> +	return ret ? -EFAULT : 0;
> +}
> +#endif

Why was the __always_inline needed?

This function is pretty large.  Why is it inlined?