Messages in this thread | | | From | Kees Cook <> | Date | Wed, 8 Nov 2017 17:08:22 -0800 | Subject | Re: [RFC 4/6] lib/vsprintf.c: add fmtcheck utility |
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On Wed, Nov 8, 2017 at 2:30 PM, Rasmus Villemoes <linux@rasmusvillemoes.dk> wrote: > We have a few places in the kernel where a *printf function is used with > a non-constant format string, making the ordinary static type checking > done by gcc et al. impossible. Some things can still be caught at build > time with appropriate instrumentation (I'm sure one can do much better > than the format_template plugin), but that still leaves a number of > places unchecked. So this patch adds a function for doing run-time > verification of a given format string against a template. > > The fmtcheck() function takes two format string arguments and checks > whether they contain the same printf specifiers. If they do, the > first (the string-to-be-checked) string is returned. If not, the > second (the template) is returned. Regardless of which string is > returned at run-time, the __format_arg attribute allows the compiler to > do type-checking if the fmtcheck() function is used inside a *printf > call, e.g. > > sprintf(buf, fmtcheck(what->ever, "%d %lx", 0), i, m)
Cool, I like this. I wonder if there are any "hot paths" that would actually make this runtime checking expensive? Seems like anything that hot shouldn't be using sprintf anyway...
> > We actually make fmtcheck() a macro that tries very hard to ensure the > template argument is a string literal - partly to help avoid mixing up > the two "const char*" arguments, partly because much of the point of > this sanity checking vanishes if the template is not a literal (e.g., > the __format_arg annotation becomes useless).
I wonder how much work it would be to instrument vsnprintf() to warn about all non-const format strings that are being processed so we could find all the places where fmtcheck() (and the struct annotation) are needed.
> We don't treat "%*.*s" and "%d %d %s" as equivalent, despite them > taking the same vararg types, since they're morally very distinct. In > fact, at least for now, we don't even treat "%d" and "%u" as > equivalent. We can relax that, possibly via FMTCHECK_* flags, but let's > first see which users there might be and what they'd want. > > If either string contains a %p, we really should check the following > alphanumerics to see which (if any) extension is used and check that > they match as well. For now, just complain loudly, partly because I'm > lazy, partly because I don't know any in-tree code that might use > fmtcheck() with a %p in the template, and I can't really imagine > anyone would use a %pXX extension in a non-constant format string.
Yeah, seems reasonable for the first pass at this.
> I don't know if WARN is too violent; maybe just pr_warn would be ok.
I think WARN gets noticed much more by build and runtime testing tools, so I think that's the right thing to do here. A mismatch really should be noticed.
-Kees
-- Kees Cook Pixel Security
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