Messages in this thread | | | From | Yonghong Song <> | Subject | Re: [PATCH v5 3/6] uaccess: Add non-pagefault user-space read functions | Date | Thu, 28 Feb 2019 22:49:43 +0000 |
| |
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@kernel.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; > +} >
| |