Re: [LKP] 4ce5f9c9e7 [ 1.323881] WARNING: CPU: 0 PID: 1 at mm/slab_common.c:1031 kmalloc_slab

[Eric W. Biederman]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> On Wed, Oct 10, 2018 at 05:06:52PM -0500, Eric W. Biederman wrote:
>> ebiederm@xxxxxxxxxxxx (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.

diff --git a/kernel/signal.c b/kernel/signal.c
index 2bffc5a50183..4fd431ce4f91 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2860,7 +2860,7 @@ static const struct {
        [SIGSYS]  = { NSIGSYS,  SIL_SYS },
 };
 
-static bool known_siginfo_layout(int sig, int si_code)
+static bool known_siginfo_layout(unsigned sig, int si_code)
 {
        if (si_code == SI_KERNEL)
                return true;
@@ -2879,7 +2879,7 @@ static bool known_siginfo_layout(int sig, int si_code)
        return false;
 }
 
-enum siginfo_layout siginfo_layout(int sig, int si_code)
+enum siginfo_layout siginfo_layout(unsigned sig, int si_code)
 {
        enum siginfo_layout layout = SIL_KILL;
        if ((si_code > SI_USER) && (si_code < SI_KERNEL)) {