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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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