From 34fc8b9076254513e00607134792a1b67483d2cf Mon Sep 17 00:00:00 2001 From: Rasmus Villemoes Date: Fri, 15 Jan 2016 16:58:34 -0800 Subject: [PATCH] lib/vsprintf.c: eliminate potential race in string() If the string corresponding to a %s specifier can change under us, we might end up copying a \0 byte to the output buffer. There might be callers who expect the output buffer to contain a genuine C string whose length is exactly the snprintf return value (assuming truncation hasn't happened or has been checked for). We can avoid this by only passing over the source string once, stopping the first time we meet a nul byte (or when we reach the given precision), and then letting widen_string() handle left/right space padding. As a small bonus, this code reuse also makes the generated code slightly smaller. Signed-off-by: Rasmus Villemoes Cc: Al Viro Cc: Andy Shevchenko Cc: Ingo Molnar Cc: Joe Perches Cc: Kees Cook Cc: Maurizio Lombardi Cc: Tejun Heo Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- lib/vsprintf.c | 28 +++++++++------------------- 1 file changed, 9 insertions(+), 19 deletions(-) diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 854d597ba23c..c537a73a62e8 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -560,32 +560,22 @@ char *widen_string(char *buf, int n, char *end, struct printf_spec spec) static noinline_for_stack char *string(char *buf, char *end, const char *s, struct printf_spec spec) { - int len, i; + int len = 0; + size_t lim = spec.precision; if ((unsigned long)s < PAGE_SIZE) s = "(null)"; - len = strnlen(s, spec.precision); - - if (!(spec.flags & LEFT)) { - while (len < spec.field_width--) { - if (buf < end) - *buf = ' '; - ++buf; - } - } - for (i = 0; i < len; ++i) { - if (buf < end) - *buf = *s; - ++buf; ++s; - } - while (len < spec.field_width--) { + while (lim--) { + char c = *s++; + if (!c) + break; if (buf < end) - *buf = ' '; + *buf = c; ++buf; + ++len; } - - return buf; + return widen_string(buf, len, end, spec); } static noinline_for_stack -- 2.20.1