Re: [PATCH 3/4] security/selinux: decrement sizeof size in strncmp

[Julia Lawall]

On Fri, 13 Nov 2009, Valdis.Kletnieks@xxxxxx wrote:

> On Fri, 13 Nov 2009 22:26:20 +0100, Julia Lawall said:
> > On Fri, 13 Nov 2009, Valdis.Kletnieks@xxxxxx wrote:
> > > Julia, is there a way to use coccinelle to detect unsafe changes like that?  Or
> > > is expressing those semantics too difficult?
> > 
> > Could you give a concrete example of something that would be a problem?
> > If something like alias analysis is required, to know what strings a 
> > variable might be bound to, that might be difficult.  Coccinelle works 
> > better when there is some concrete codeto match against.
> 
> Here's a concrete example of how a previously audited strcmp() can go bad...
> 
> struct foo {
>     char[16] a; /* old code allows 15 chars and 1 more for the \0 */
>     int b;
>     int c;
> }
> 
> bzero(foo,sizeof(foo));
> 
> Now code can pretty safely mess with the first 15 bytes of foo->a and
> we know we're OK if we call strcmp(foo->a,....) because that bzero()
> nuked a[15] for us. It's safe to strncpy(foo->a,bar,15); and not worry
> about the fact that if bar is 15 chars long, a trailing \0 won't be put in.
> 
> Now somebody comes along and does:
> 
> struct foo {
>     char *a; /* we need more than 15 chars for some oddball hardware */
>     int b;
>     int c;
> }
> 
> bzero(foo,sizeof(foo));
> foo->a = kmalloc(32);  /* whoops should have been kzmalloc */
> 
> Now suddenly, strncpy(foo->a,bar,31); *isn't* safe....
> 
> (Yes, I know there's plenty of blame to go around in this example - the failure
> to use kzmalloc, the use of strncpy() without an explicit \0 being assigned
> someplace, the use of strcmp() rather than strncmp()... But our tendency to
> intentionally omit several steps of this to produce more efficient code means
> it's easier to shoot ourselves in the foot...)

Thanks for the example.  Coccinelle only finds patterns of code in one 
version, while this would require considering two versions at once.  Such 
a thing could be interesting though.

julia
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