vsprintf: ignore %n again
This ignores %n in printf again, as was originally documented. Implementing %n poses a greater security risk than utility, so it should stay ignored. To help anyone attempting to use %n, a warning will be emitted if it is encountered. Based on an earlier patch by Joe Perches. Because %n was designed to write to pointers on the stack, it has been frequently used as an attack vector when bugs are found that leak user-controlled strings into functions that ultimately process format strings. While this class of bug can still be turned into an information leak, removing %n eliminates the common method of elevating such a bug into an arbitrary kernel memory writing primitive, significantly reducing the danger of this class of bug. For seq_file users that need to know the length of a written string for padding, please see seq_setwidth() and seq_pad() instead. Signed-off-by: Kees Cook <keescook@chromium.org> Cc: Joe Perches <joe@perches.com> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Cc: David Miller <davem@davemloft.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -1712,18 +1712,16 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
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break;
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case FORMAT_TYPE_NRCHARS: {
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u8 qualifier = spec.qualifier;
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/*
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* Since %n poses a greater security risk than
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* utility, ignore %n and skip its argument.
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*/
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void *skip_arg;
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if (qualifier == 'l') {
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long *ip = va_arg(args, long *);
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*ip = (str - buf);
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} else if (_tolower(qualifier) == 'z') {
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size_t *ip = va_arg(args, size_t *);
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*ip = (str - buf);
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} else {
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int *ip = va_arg(args, int *);
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*ip = (str - buf);
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}
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WARN_ONCE(1, "Please remove ignored %%n in '%s'\n",
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old_fmt);
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skip_arg = va_arg(args, void *);
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break;
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}
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