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Subject[PATCH -tip v9 2/6] uaccess: Add non-pagefault user-space read functions
Date
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
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>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
Changes in v8:
- Fix style issues and typos according to Ingo's comment (Thanks!)
Changes in v6:
- Remove user_access_ok()
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 5a43ef7db492..9c435c3f2105 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -242,6 +242,17 @@ static inline unsigned long __copy_from_user_inatomic_nocache(void *to,
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
@@ -255,6 +266,9 @@ 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..19c8c3dc14df 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
@@ -29,16 +41,40 @@ 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();
+
+ set_fs(USER_DS);
+ if (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
@@ -66,6 +102,7 @@ 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 +142,76 @@ 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)
+ *
+ * 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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