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Message-ID: <40eae910-16f3-8c6f-6cc7-c52b77b30ccd@fb.com>
Date: Thu, 28 Feb 2019 22:49:43 +0000
From: Yonghong Song <yhs@...com>
To: Masami Hiramatsu <mhiramat@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"Shuah Khan" <shuah@...nel.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"Andy Lutomirski" <luto@...capital.net>,
Ingo Molnar <mingo@...nel.org>,
"Andrew Morton" <akpm@...ux-foundation.org>,
Changbin Du <changbin.du@...il.com>,
Jann Horn <jannh@...gle.com>,
Kees Cook <keescook@...omium.org>,
"Andy Lutomirski" <luto@...nel.org>,
Alexei Starovoitov <alexei.starovoitov@...il.com>,
Nadav Amit <namit@...are.com>,
"Peter Zijlstra" <peterz@...radead.org>,
Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v5 3/6] uaccess: Add non-pagefault user-space read
functions
On 2/28/19 8:03 AM, Masami Hiramatsu wrote:
> Add probe_user_read(), strncpy_from_unsafe_user() and
> strnlen_unsafe_user() which allows caller to access user-space
> in IRQ context.
>
> Current probe_kernel_read() and strncpy_from_unsafe() are
> not available for user-space memory, because it sets
> KERNEL_DS while accessing data. On some arch, user address
> space and kernel address space can be co-exist, but others
> can not. In that case, setting KERNEL_DS means given
Just curious. Given the list of arch's currently linux supports,
do you know which arch's fall into "user address space and
kernel address space" can co-exist, and which arch's cannot?
Thanks!
Yonghong
> address is treated as a kernel address space.
> Also strnlen_user() is only available from user context since
> it can sleep if pagefault is enabled.
>
> To access user-space memory without pagefault, we need
> these new functions which sets USER_DS while accessing
> the data.
>
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> ---
> Changes in v5:
> - Simplify probe_user_read() (Thanks, Peter!)
> - Add strnlen_unsafe_user()
> Changes in v3:
> - Use user_access_ok() for probe_user_read().
> Changes in v2:
> - Simplify strncpy_from_unsafe_user() using strncpy_from_user()
> according to Linus's suggestion.
> - Simplify probe_user_read() not using intermediate function.
> ---
> include/linux/uaccess.h | 14 +++++
> mm/maccess.c | 122 +++++++++++++++++++++++++++++++++++++++++++++--
> 2 files changed, 130 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 1afd9dfabe67..5be7f9adb418 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -258,6 +258,17 @@ extern long probe_kernel_read(void *dst, const void *src, size_t size);
> extern long __probe_kernel_read(void *dst, const void *src, size_t size);
>
> /*
> + * probe_user_read(): safely attempt to read from a location in user space
> + * @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.
> + */
> +extern long probe_user_read(void *dst, const void __user *src, size_t size);
> +
> +/*
> * probe_kernel_write(): safely attempt to write to a location
> * @dst: address to write to
> * @src: pointer to the data that shall be written
> @@ -270,6 +281,9 @@ extern long notrace probe_kernel_write(void *dst, const void *src, size_t size);
> extern long notrace __probe_kernel_write(void *dst, const void *src, size_t size);
>
> extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
> +extern long strncpy_from_unsafe_user(char *dst, const void __user *unsafe_addr,
> + long count);
> +extern long strnlen_unsafe_user(const void __user *unsafe_addr, long count);
>
> /**
> * probe_kernel_address(): safely attempt to read from a location
> diff --git a/mm/maccess.c b/mm/maccess.c
> index ec00be51a24f..d1b2ec78d9ef 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -5,8 +5,20 @@
> #include <linux/mm.h>
> #include <linux/uaccess.h>
>
> +static __always_inline long
> +probe_read_common(void *dst, const void __user *src, size_t size)
> +{
> + long ret;
> +
> + pagefault_disable();
> + ret = __copy_from_user_inatomic(dst, src, size);
> + pagefault_enable();
> +
> + return ret ? -EFAULT : 0;
> +}
> +
> /**
> - * probe_kernel_read(): safely attempt to read from a location
> + * probe_kernel_read(): safely attempt to read from a kernel-space location
> * @dst: pointer to the buffer that shall take the data
> * @src: address to read from
> * @size: size of the data chunk
> @@ -29,17 +41,45 @@ long __probe_kernel_read(void *dst, const void *src, size_t size)
> mm_segment_t old_fs = get_fs();
>
> set_fs(KERNEL_DS);
> - pagefault_disable();
> - ret = __copy_from_user_inatomic(dst,
> - (__force const void __user *)src, size);
> - pagefault_enable();
> + ret = probe_read_common(dst, (__force const void __user *)src, size);
> set_fs(old_fs);
>
> - return ret ? -EFAULT : 0;
> + return ret;
> }
> EXPORT_SYMBOL_GPL(probe_kernel_read);
>
> /**
> + * probe_user_read(): safely attempt to read from a user-space location
> + * @dst: pointer to the buffer that shall take the data
> + * @src: address to read from. This must be a user address.
> + * @size: size of the data chunk
> + *
> + * Safely read from user address @src to the buffer at @dst. If a kernel fault
> + * happens, handle that and return -EFAULT.
> + */
> +
> +long __weak probe_user_read(void *dst, const void __user *src, size_t size)
> + __attribute__((alias("__probe_user_read")));
> +
> +long __probe_user_read(void *dst, const void __user *src, size_t size)
> +{
> + long ret = -EFAULT;
> + mm_segment_t old_fs = get_fs();
> +
> + /*
> + * Since this can be called in IRQ context, we carefully set the
> + * USER_DS and use user_access_ok() which checks segment setting
> + * instead of task context.
> + */
> + set_fs(USER_DS);
> + if (user_access_ok(src, size))
> + ret = probe_read_common(dst, src, size);
> + set_fs(old_fs);
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(probe_user_read);
> +
> +/**
> * probe_kernel_write(): safely attempt to write to a location
> * @dst: address to write to
> * @src: pointer to the data that shall be written
> @@ -66,6 +106,7 @@ long __probe_kernel_write(void *dst, const void *src, size_t size)
> }
> EXPORT_SYMBOL_GPL(probe_kernel_write);
>
> +
> /**
> * strncpy_from_unsafe: - Copy a NUL terminated string from unsafe address.
> * @dst: Destination address, in kernel space. This buffer must be at
> @@ -105,3 +146,72 @@ long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count)
>
> return ret ? -EFAULT : src - unsafe_addr;
> }
> +
> +/**
> + * strncpy_from_unsafe_user: - Copy a NUL terminated string from unsafe user
> + * address.
> + * @dst: Destination address, in kernel space. This buffer must be at
> + * least @count bytes long.
> + * @unsafe_addr: Unsafe user address.
> + * @count: Maximum number of bytes to copy, including the trailing NUL.
> + *
> + * Copies a NUL-terminated string from unsafe user address to kernel buffer.
> + *
> + * On success, returns the length of the string INCLUDING the trailing NUL.
> + *
> + * If access fails, returns -EFAULT (some data may have been copied
> + * and the trailing NUL added).
> + *
> + * If @count is smaller than the length of the string, copies @count-1 bytes,
> + * sets the last byte of @dst buffer to NUL and returns @count.
> + */
> +long strncpy_from_unsafe_user(char *dst, const void __user *unsafe_addr,
> + long count)
> +{
> + mm_segment_t old_fs = get_fs();
> + long ret;
> +
> + if (unlikely(count <= 0))
> + return 0;
> +
> + set_fs(USER_DS);
> + pagefault_disable();
> + ret = strncpy_from_user(dst, unsafe_addr, count);
> + pagefault_enable();
> + set_fs(old_fs);
> + if (ret >= count) {
> + ret = count;
> + dst[ret - 1] = '\0';
> + } else if (ret > 0)
> + ret++;
> + return ret;
> +}
> +
> +/**
> + * strnlen_unsafe_user: - Get the size of a user string INCLUDING final NUL.
> + * @unsafe_addr: The string to measure.
> + * @count: Maximum count (including NUL character)
> + *
> + * Get the size of a NUL-terminated string in user space without pagefault.
> + *
> + * Returns the size of the string INCLUDING the terminating NUL.
> + *
> + * If the string is too long, returns a number larger than @count. User
> + * has to check the return value against "> count".
> + * On exception (or invalid count), returns 0.
> + *
> + * Unlike strnlen_user, this can be used from IRQ handler etc. because
> + * it disables pagefaults.
> + */
> +long strnlen_unsafe_user(const void __user *unsafe_addr, long count)
> +{
> + mm_segment_t old_fs = get_fs();
> + int ret;
> +
> + set_fs(USER_DS);
> + pagefault_disable();
> + ret = strnlen_user(unsafe_addr, count);
> + pagefault_enable();
> + set_fs(old_fs);
> + return ret;
> +}
>
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