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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
3.5 KiB
C
126 lines
3.5 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <linux/kernel.h>
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#include <linux/export.h>
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#include <linux/uaccess.h>
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#include <asm/word-at-a-time.h>
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/* Set bits in the first 'n' bytes when loaded from memory */
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#ifdef __LITTLE_ENDIAN
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# define aligned_byte_mask(n) ((1ul << 8*(n))-1)
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#else
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# define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
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#endif
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/*
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* Do a strnlen, return length of string *with* final '\0'.
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* 'count' is the user-supplied count, while 'max' is the
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* address space maximum.
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*
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* Return 0 for exceptions (which includes hitting the address
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* space maximum), or 'count+1' if hitting the user-supplied
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* maximum count.
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*
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* NOTE! We can sometimes overshoot the user-supplied maximum
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* if it fits in a aligned 'long'. The caller needs to check
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* the return value against "> max".
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*/
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static inline long do_strnlen_user(const char __user *src, unsigned 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 align, res = 0;
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unsigned long c;
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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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/*
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* Do everything aligned. But that means that we
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* need to also expand the maximum..
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*/
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align = (sizeof(long) - 1) & (unsigned long)src;
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src -= align;
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max += align;
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unsafe_get_user(c, (unsigned long __user *)src, efault);
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c |= aligned_byte_mask(align);
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for (;;) {
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unsigned long data;
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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) + 1 - align;
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}
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res += sizeof(unsigned long);
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/* We already handled 'unsigned long' bytes. Did we do it all ? */
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if (unlikely(max <= sizeof(unsigned long)))
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break;
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max -= sizeof(unsigned long);
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unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
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}
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res -= align;
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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, return the marker for "too long".
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*/
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if (res >= count)
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return count+1;
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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 0.
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*/
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efault:
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return 0;
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}
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/**
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* strnlen_user: - Get the size of a user string INCLUDING final NUL.
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* @str: The string to measure.
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* @count: Maximum count (including NUL character)
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*
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* Context: User context only. This function may sleep if pagefaults are
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* enabled.
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*
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* Get the size of a NUL-terminated string in user space.
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*
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* Returns the size of the string INCLUDING the terminating NUL.
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* If the string is too long, returns a number larger than @count. User
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* has to check the return value against "> count".
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* On exception (or invalid count), returns 0.
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*
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* NOTE! You should basically never use this function. There is
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* almost never any valid case for using the length of a user space
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* string, since the string can be changed at any time by other
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* threads. Use "strncpy_from_user()" instead to get a stable copy
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* of the string.
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*/
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long strnlen_user(const char __user *str, 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)str;
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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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if (user_access_begin(str, max)) {
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retval = do_strnlen_user(str, 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 0;
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}
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EXPORT_SYMBOL(strnlen_user);
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