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Subject[PATCH v3 3/5] uaccess: Add non-pagefault user-space read functions
Date
Add probe_user_read() and strncpy_from_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.

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 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 | 13 +++++++
mm/maccess.c | 94 ++++++++++++++++++++++++++++++++++++++++++++---
2 files changed, 101 insertions(+), 6 deletions(-)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 1afd9dfabe67..598a96210d04 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 +281,8 @@ 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);

/**
* probe_kernel_address(): safely attempt to read from a location
diff --git a/mm/maccess.c b/mm/maccess.c
index ec00be51a24f..b24df3e731c8 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 +41,47 @@ 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;
+ 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 = -EFAULT;
+ else
+ 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 +108,7 @@ 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 +148,42 @@ 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 *src, 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, src, count);
+ pagefault_enable();
+ set_fs(old_fs);
+ if (ret >= count) {
+ ret = count;
+ dst[ret - 1] = '\0';
+ } else if (ret > 0)
+ ret++;
+ return ret;
+}
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