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Message-Id: <20190301112915.f00e5d5c894f73da50746bcf@kernel.org>
Date: Fri, 1 Mar 2019 11:29:15 +0900
From: Masami Hiramatsu <mhiramat@...nel.org>
To: Yonghong Song <yhs@...com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"Shuah Khan" <shuah@...nel.org>,
"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
Hi Yonghong,
On Thu, 28 Feb 2019 22:49:43 +0000
Yonghong Song <yhs@...com> wrote:
>
>
> 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?
As far as I can heard, (and based on probe_kernel_read() failure)
sparc32 (and sparc64?), arm64, and s390 will not work.
x86 works, but if user patch the 4G/4G, it shouldn't work.
Thank you,
>
> 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;
> > +}
> >
--
Masami Hiramatsu <mhiramat@...nel.org>
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