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-rw-r--r--lib/strncpy_from_user.c146
1 files changed, 146 insertions, 0 deletions
diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
new file mode 100644
index 000000000..6432b8c3e
--- /dev/null
+++ b/lib/strncpy_from_user.c
@@ -0,0 +1,146 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/compiler.h>
+#include <linux/export.h>
+#include <linux/fault-inject-usercopy.h>
+#include <linux/kasan-checks.h>
+#include <linux/thread_info.h>
+#include <linux/uaccess.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/mm.h>
+
+#include <asm/byteorder.h>
+#include <asm/word-at-a-time.h>
+
+#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
+#define IS_UNALIGNED(src, dst) 0
+#else
+#define IS_UNALIGNED(src, dst) \
+ (((long) dst | (long) src) & (sizeof(long) - 1))
+#endif
+
+/*
+ * Do a strncpy, return length of string without final '\0'.
+ * 'count' is the user-supplied count (return 'count' if we
+ * hit it), 'max' is the address space maximum (and we return
+ * -EFAULT if we hit it).
+ */
+static __always_inline long do_strncpy_from_user(char *dst, const char __user *src,
+ unsigned long count, unsigned long max)
+{
+ const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
+ unsigned long res = 0;
+
+ if (IS_UNALIGNED(src, dst))
+ goto byte_at_a_time;
+
+ while (max >= sizeof(unsigned long)) {
+ unsigned long c, data, mask;
+
+ /* Fall back to byte-at-a-time if we get a page fault */
+ unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
+
+ /*
+ * Note that we mask out the bytes following the NUL. This is
+ * important to do because string oblivious code may read past
+ * the NUL. For those routines, we don't want to give them
+ * potentially random bytes after the NUL in `src`.
+ *
+ * One example of such code is BPF map keys. BPF treats map keys
+ * as an opaque set of bytes. Without the post-NUL mask, any BPF
+ * maps keyed by strings returned from strncpy_from_user() may
+ * have multiple entries for semantically identical strings.
+ */
+ if (has_zero(c, &data, &constants)) {
+ data = prep_zero_mask(c, data, &constants);
+ data = create_zero_mask(data);
+ mask = zero_bytemask(data);
+ *(unsigned long *)(dst+res) = c & mask;
+ return res + find_zero(data);
+ }
+
+ *(unsigned long *)(dst+res) = c;
+
+ res += sizeof(unsigned long);
+ max -= sizeof(unsigned long);
+ }
+
+byte_at_a_time:
+ while (max) {
+ char c;
+
+ unsafe_get_user(c,src+res, efault);
+ dst[res] = c;
+ if (!c)
+ return res;
+ res++;
+ max--;
+ }
+
+ /*
+ * Uhhuh. We hit 'max'. But was that the user-specified maximum
+ * too? If so, that's ok - we got as much as the user asked for.
+ */
+ if (res >= count)
+ return res;
+
+ /*
+ * Nope: we hit the address space limit, and we still had more
+ * characters the caller would have wanted. That's an EFAULT.
+ */
+efault:
+ return -EFAULT;
+}
+
+/**
+ * strncpy_from_user: - Copy a NUL terminated string from userspace.
+ * @dst: Destination address, in kernel space. This buffer must be at
+ * least @count bytes long.
+ * @src: Source address, in user space.
+ * @count: Maximum number of bytes to copy, including the trailing NUL.
+ *
+ * Copies a NUL-terminated string from userspace to kernel space.
+ *
+ * On success, returns the length of the string (not including the trailing
+ * NUL).
+ *
+ * If access to userspace fails, returns -EFAULT (some data may have been
+ * copied).
+ *
+ * If @count is smaller than the length of the string, copies @count bytes
+ * and returns @count.
+ */
+long strncpy_from_user(char *dst, const char __user *src, long count)
+{
+ unsigned long max_addr, src_addr;
+
+ might_fault();
+ if (should_fail_usercopy())
+ return -EFAULT;
+ if (unlikely(count <= 0))
+ return 0;
+
+ max_addr = TASK_SIZE_MAX;
+ src_addr = (unsigned long)untagged_addr(src);
+ if (likely(src_addr < max_addr)) {
+ unsigned long max = max_addr - src_addr;
+ long retval;
+
+ /*
+ * Truncate 'max' to the user-specified limit, so that
+ * we only have one limit we need to check in the loop
+ */
+ if (max > count)
+ max = count;
+
+ kasan_check_write(dst, count);
+ check_object_size(dst, count, false);
+ if (user_read_access_begin(src, max)) {
+ retval = do_strncpy_from_user(dst, src, count, max);
+ user_read_access_end();
+ return retval;
+ }
+ }
+ return -EFAULT;
+}
+EXPORT_SYMBOL(strncpy_from_user);