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vsprintf: ignore %n again

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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 <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: David Miller <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Kees Cook 2013-11-14 14:31:58 -08:00 • committed by Linus Torvalds
commit 9196436ab2
1 changed files with 9 additions and 11 deletions
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20
lib/vsprintf.c
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@ -1712,18 +1712,16 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
break;
case FORMAT_TYPE_NRCHARS: {
u8 qualifier = spec.qualifier;
/*
* Since %n poses a greater security risk than
* utility, ignore %n and skip its argument.
*/
void *skip_arg;
if (qualifier == 'l') {
long *ip = va_arg(args, long *);
*ip = (str - buf);
} else if (_tolower(qualifier) == 'z') {
size_t *ip = va_arg(args, size_t *);
*ip = (str - buf);
} else {
int *ip = va_arg(args, int *);
*ip = (str - buf);
}
WARN_ONCE(1, "Please remove ignored %%n in '%s'\n",
old_fmt);
skip_arg = va_arg(args, void *);
break;
}