Re: [PATCH] sched/core: check format and overflows in cgroup2 cpu.max
From: Konstantin Khlebnikov
Date: Wed Mar 06 2019 - 12:21:48 EST
On 06.03.2019 19:48, Peter Zijlstra wrote:
On Wed, Mar 06, 2019 at 08:11:54AM -0800, Tejun Heo wrote:
Hello, Konstantin.
On Tue, Mar 05, 2019 at 08:03:24PM +0300, Konstantin Khlebnikov wrote:
Ditto as the blkio patch. Unless there is a correctness problem, my
preference is towards keeping the parsing functions simple and I don't
think the kernel needs to play the role of strict input verifier here
as long as the only foot getting shot is the user's own.
IMHO non-strict interface more likely hides bugs and could cause
problems for future changes.
Here is only only one fatal bug - buffer overflow in sscanf because
%s has no limit.
Ah, indeed. Can you please post a patch to fix that problem first?
Done.
Please see [PATCH] sched/core: fix buffer overflow in cgroup2 property cpu.max
Strict validation could be done as more strict sscanf variant or
some kind of extension for format string.
I don't necessarily disagree with you; however, what often ends up
with these manually crafted parsing approach are 1. code which is
unnecessarily difficult to follow 2. different subset of validations
and parsing bugs (of course) everywhere.
Given the above, I tend to lean towards dump sscanf() parsing. If we
wanna improve the situation, I think the right thing to do is either
improving sscanf or introducing new helpers to parse these things
rather than hand-crafting each site. It is really error-prone.
I'm playing with sscanf right now.
Both problems (integer overflows and matching end of string)
are relatively easy to fix without breaking sane compatibility.
Always use a field width specifier with %s. Which is exactly what the
proposed patch did IIRC.
Maybe that's something checkpatch could warn about.
This could be done mandatory.
In-kernel sscanf always requires width for "%[...]".