Messages in this thread | | | Date | Tue, 24 Nov 2020 13:53:37 -0800 | From | Kees Cook <> | Subject | Re: Printk specifiers for __user pointers |
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On Mon, Nov 23, 2020 at 10:53:24AM +0100, Petr Mladek wrote: > On Fri 2020-11-20 13:42:42, Steven Rostedt wrote: > > On Fri, 20 Nov 2020 11:44:12 -0500 > > Alan Stern <stern@rowland.harvard.edu> wrote: > > > > > To the VSPRINTF maintainers: > > > > > > Documentation/core-api/printk-formats.rst lists a large number of format > > > specifiers for pointers of various sorts. Yet as far as I can see, > > > there is no specifier meant for use with __user pointers. > > > > > > The security implications of printing the true, unmangled value of a > > > __user pointer are minimal, since doing so does not leak any kernel > > > information. So %px would work, but tools like checkpatch.pl don't like > > > it. > > Just to be sure as I am not a security expert. Is there really that > big difference in the risk? The following scenarios come to my mind: > > 1. The address would show a well defined location in the userspace > application? Could it be used to attack the application?
Yes -- this is the primary risk in my view. Exposing addresses of any kind can be a risk. While an unprivileged user may not have direct access to dmesg, there tend to be many indirect ways to see its contents. As such, exposing a userspace address (when not then also terminating the process, as seen with the segv reporting) poses a potential exposure risk. I admit it's not a LARGE risk, but modern attacks use these kind of building blocks to construct all the steps to reaching their target.
> 2. The address shows a location that is being accessed by kernel. > Could not it be used to pass a value that might be used to attack > kernel?
This is also a risk: it provides feedback about where something may be as a target within a confused-deputy style attack. (i.e. set up one process to confuse the kernel, and exploit it from another).
> > > Should a new specifier be added? If not, should we simply use %px? > > > > There's currently no user of '%pu' (although there is a '%pus'. Perhaps we > > should have a '%pux'? > > > > I would even state that if it is used, that if makes sure that the value is > > indeed a user space pointer (goes through the same checks as accessing user > > space), before its printed, otherwise it shows "(fault)" or something. > > I have mixed feelings about this. > > One one hand, it might make sense to mark locations where userspace > address is printed. We could easily decide how to print them (hash or > value) and we could check that it is really from a userspace one. > > But I have few concerns: > > 1. The existing "%pus" has a kind of opposite meaning. It says what > address space should be used when the kernel and userspace address > space is overlapping. > > 2. There is the history with "%pk". It did not work because people did > not use it. > > 3. I am not sure about the output when the address is not from > userspace. Printing ("fault") is not much helpful. Printing > hashed value might be confusing. Well, I am still not sure > that it is really safe to print real userspace addresses > by default.
I think this should just be %px. Or better yet, not printed at all. See Linus's prior comments: https://www.kernel.org/doc/html/latest/process/deprecated.html#p-format-specifier
-- Kees Cook
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