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594cc251fd
Originally, the rule used to be that you'd have to do access_ok() separately, and then user_access_begin() before actually doing the direct (optimized) user access. But experience has shown that people then decide not to do access_ok() at all, and instead rely on it being implied by other operations or similar. Which makes it very hard to verify that the access has actually been range-checked. If you use the unsafe direct user accesses, hardware features (either SMAP - Supervisor Mode Access Protection - on x86, or PAN - Privileged Access Never - on ARM) do force you to use user_access_begin(). But nothing really forces the range check. By putting the range check into user_access_begin(), we actually force people to do the right thing (tm), and the range check vill be visible near the actual accesses. We have way too long a history of people trying to avoid them. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
126 lines
3.2 KiB
C
126 lines
3.2 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <linux/compiler.h>
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#include <linux/export.h>
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#include <linux/kasan-checks.h>
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#include <linux/thread_info.h>
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#include <linux/uaccess.h>
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#include <linux/kernel.h>
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#include <linux/errno.h>
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#include <asm/byteorder.h>
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#include <asm/word-at-a-time.h>
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#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
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#define IS_UNALIGNED(src, dst) 0
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#else
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#define IS_UNALIGNED(src, dst) \
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(((long) dst | (long) src) & (sizeof(long) - 1))
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#endif
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/*
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* Do a strncpy, return length of string without final '\0'.
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* 'count' is the user-supplied count (return 'count' if we
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* hit it), 'max' is the address space maximum (and we return
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* -EFAULT if we hit it).
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*/
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static inline long do_strncpy_from_user(char *dst, const char __user *src, long count, unsigned long max)
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{
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const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
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long res = 0;
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/*
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* Truncate 'max' to the user-specified limit, so that
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* we only have one limit we need to check in the loop
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*/
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if (max > count)
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max = count;
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if (IS_UNALIGNED(src, dst))
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goto byte_at_a_time;
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while (max >= sizeof(unsigned long)) {
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unsigned long c, data;
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/* Fall back to byte-at-a-time if we get a page fault */
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unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
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*(unsigned long *)(dst+res) = c;
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if (has_zero(c, &data, &constants)) {
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data = prep_zero_mask(c, data, &constants);
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data = create_zero_mask(data);
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return res + find_zero(data);
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}
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res += sizeof(unsigned long);
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max -= sizeof(unsigned long);
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}
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byte_at_a_time:
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while (max) {
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char c;
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unsafe_get_user(c,src+res, efault);
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dst[res] = c;
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if (!c)
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return res;
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res++;
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max--;
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}
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/*
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* Uhhuh. We hit 'max'. But was that the user-specified maximum
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* too? If so, that's ok - we got as much as the user asked for.
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*/
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if (res >= count)
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return res;
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/*
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* Nope: we hit the address space limit, and we still had more
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* characters the caller would have wanted. That's an EFAULT.
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*/
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efault:
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return -EFAULT;
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}
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/**
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* strncpy_from_user: - Copy a NUL terminated string from userspace.
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* @dst: Destination address, in kernel space. This buffer must be at
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* least @count bytes long.
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* @src: Source address, in user space.
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* @count: Maximum number of bytes to copy, including the trailing NUL.
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*
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* Copies a NUL-terminated string from userspace to kernel space.
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*
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* On success, returns the length of the string (not including the trailing
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* NUL).
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*
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* If access to userspace fails, returns -EFAULT (some data may have been
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* copied).
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*
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* If @count is smaller than the length of the string, copies @count bytes
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* and returns @count.
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*/
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long strncpy_from_user(char *dst, const char __user *src, long count)
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{
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unsigned long max_addr, src_addr;
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if (unlikely(count <= 0))
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return 0;
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max_addr = user_addr_max();
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src_addr = (unsigned long)src;
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if (likely(src_addr < max_addr)) {
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unsigned long max = max_addr - src_addr;
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long retval;
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kasan_check_write(dst, count);
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check_object_size(dst, count, false);
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if (user_access_begin(src, max)) {
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retval = do_strncpy_from_user(dst, src, count, max);
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user_access_end();
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return retval;
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}
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}
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return -EFAULT;
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}
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EXPORT_SYMBOL(strncpy_from_user);
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