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Date: Sat, 13 Feb 2016 19:51:04 +0300
From: Alexander Cherepanov <ch3root@...nwall.com>
To: oss-security@...ts.openwall.com
Subject: Re: snprintf return value misuse in a lot of projects
On 2016-02-13 17:11, Yuriy M. Kaminskiy wrote:
> I noticed dangerous pattern in a lot of projects, where snprintf(3)
> return value is used without checking, with potentially disasterous
> consequences:
It's kinda a known. E.g., some such patterns are listed in
https://sourceware.org/ml/libc-alpha/2013-10/msg00686.html .
The same problem is with strlcpy.
> And there are yet another very common pattern:
>
> p += snprintf(p, end-p,[....]);
> p += snprintf(p, end-p,[....]);
> p += snprintf(p, end-p,[....]);
> ...
>
> which may be 'barely safe' by posix (if you'd read `man 3posix snprintf`,
> you'd expect 2nd line is [somewhat] safe (end-p is negative, then
> casted to size_t and produce value larger than (size_t)INT_MAX, that
> should result in error EOVERFLOW), and third and following will dance
> around last byte, likely remaining safe), but it is TOTALLY
> broken on glibc, as glibc's snprintf DOES NOT follow posix, and accepts
> *any* size.
For a glibc discussion please see
https://sourceware.org/bugzilla/show_bug.cgi?id=14771 .
As for POSIX, the requirement of EOVERFLOW for a big second parameter is
a (rejected) bug in POSIX -- http://austingroupbugs.net/view.php?id=761
. A closely related bug -- http://austingroupbugs.net/view.php?id=1020 .
ISO C describes the size parameter of snprintf as a limit to the number
of output characters written, without any connections to the size of the
buffer. Thus, the following examples are valid in ISO C:
char s[10];
snprintf(s, 20, "abc");
snprintf(s, SIZE_MAX, "%s", "abc");
OTOH POSIX describes the size parameter as the actual size of the buffer
(bug 1020) and requires to reject buffers of size larger than INT_MAX
(bug 761).
Even though POSIX contradicts ISO C in this question (while formally
deferring to ISO C) there is a sentiment that the POSIX approach is
better for safety/security. (E.g., it was expressed during the recent
discussion about strlcpy/strlcat in the glibc mailing list.)
As it turned out, the same problem affects the fread function, with the
Linux kernel instead of POSIX contradicting ISO C. See
https://sourceware.org/bugzilla/show_bug.cgi?id=19165 and
https://sourceware.org/ml/libc-alpha/2016-02/msg00274.html .
Perhaps this is a topic that will benefit from input from a wider community.
--
Alexander Cherepanov
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