1:14 a.m.
ebiederm(a)xmission.com (Eric W. Biederman) writes:
Sean Christopherson <sean.j.christopherson(a)intel.com> writes:
> On Wed, Oct 10, 2018 at 05:06:52PM -0500, Eric W. Biederman wrote:
>> ebiederm(a)xmission.com (Eric W. Biederman) writes:
>>
>> > So I am flummoxed. I am reading through the code and I don't see
>> > anything that could trigger this, and when I ran the supplied reproducer
>> > it did not reproduce for me.
>>
>> Even more so. With my tool chain the line that reports the failing
>> address is impossible.
>>
>> [ 73.034423] RIP: 0010:copy_siginfo_from_user+0x4d/0xd0
>>
>> With the supplied configureation my tool chain only has 0x30 bytes for
>> all of copy_siginfo_from_user. So I can't even begin to guess where
>> in that function things are failing.
>>
>> Any additional information that you can provide would be a real help
>> in tracking down this strange failure.
>
> I don't have the exact toolchain, but I was able to get somewhat close
> and may have found a smoking gun. 0x4d in my build is in the general
> vicinity of "sig_sicodes[sig].limit" in known_siginfo_layout(). This
> lines up with the register state from the log, e.g. RDI=0500104d8,
> which is the mask generated by sig_specific_sicodes. From what I can
> tell, @sig is never bounds checked. If the compiler generated an AND
> instruction to compare against sig_specific_sicodes then that could
> resolve true with any arbitrary value that happened to collide with
> sig_specific_sicodes and result in an out-of-bounds access to
> @sig_sicodes. siginfo_layout() for example explicitly checks @sig
> before indexing @sig_sicode, e.g. "sig < ARRAY_SIZE(sig_sicodes)".
>
> Maybe this?
But sig is bounds checked. Even better sig is checked to see if it
is one of the values in the array.
> From include/linux/signal.h
#define SIG_SPECIFIC_SICODES_MASK (\
rt_sigmask(SIGILL) | rt_sigmask(SIGFPE) | \
rt_sigmask(SIGSEGV) | rt_sigmask(SIGBUS) | \
rt_sigmask(SIGTRAP) | rt_sigmask(SIGCHLD) | \
rt_sigmask(SIGPOLL) | rt_sigmask(SIGSYS) | \
SIGEMT_MASK )
#define siginmask(sig, mask) \
((sig) < SIGRTMIN && (rt_sigmask(sig) & (mask)))
#define sig_specific_sicodes(sig) siginmask(sig, SIG_SPECIFIC_SICODES_MASK)
Hmm. I wonder if something is passing in a negative signal number.
There is not a bounds check for that. A sufficiently large signal
number might be the problem here. Yes. I can get an oops with
a sufficiently large negative signal number.
The code will later call valid_signal in check_permissions and
that will cause the system call to fail, so the issue is just that
the signal number is not being validated early enough.
On the output path (copy_siginfo_to_user and copy_siginfo_to_user32) the
signal number should be validated before it ever reaches userspace
which is why I expect trinity never triggered anything.
There is copy_siginfo_from_user32 and that does call siginfo_layout with
a possibly negative signal number. Which has the same potential issues.
So I am going to go with the fix below. That fixes things in my testing
and by being unsigned should fix keep negative numbers from being a
problem.
Sean thank you very much for putting me on the right path to track this
failing test down.
Eric