Re: [PATCH 1/4] lib/vsprintf.c: handle invalid format specifiers more robustly

From: Kees Cook
Date: Mon Sep 28 2015 - 18:30:14 EST

On Fri, Sep 25, 2015 at 10:41 AM, Rasmus Villemoes
<linux@xxxxxxxxxxxxxxxxxx> wrote:
> If we meet any invalid or unsupported format specifier, 'handling' it
> by just printing it as a literal string is not safe: Presumably the
> format string and the arguments passed gcc's type checking, but that
> means something like sprintf(buf, "%n %pd", &intvar, dentry) would end
> up interpreting &intvar as a struct dentry*.
> When the offending specifier was %n it used to be at the end of the
> format string, but we can't rely on that always being the case. Also,
> gcc doesn't complain about some more or less exotic qualifiers (or
> 'length modifiers' in posix-speak) such as 'j' or 'q', but being
> unrecognized by the kernel's printf implementation, they'd be
> interpreted as unknown specifiers, and the rest of arguments would be
> interpreted wrongly.
> So let's complain about anything we don't understand, not just %n, and
> stop pretending that we'd be able to make sense of the rest of the
> format/arguments. If the offending specifier is in a printk() call we
> unfortunately only get a "BUG: recent printk recursion!", but at least
> direct users of the sprintf family will be caught.

I like it! Thanks :)

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>


Kees Cook
Chrome OS Security
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