[bug?] Access was denied by memory protection keys in execute-only address

[Ram Pai]

On Thu, Mar 08, 2018 at 11:19:12PM +1100, Michael Ellerman wrote:
> Li Wang <liwang at redhat.com> writes:
> > Hi,
> >
> > ltp/mprotect04[1] crashed by SEGV_PKUERR on ppc64(LPAR on P730, Power 8
> > 8247-22L) with kernel-v4.16.0-rc4.
> >
> > 10000000-10020000 r-xp 00000000 fd:00 167223           mprotect04
> > 10020000-10030000 r--p 00010000 fd:00 167223           mprotect04
> > 10030000-10040000 rw-p 00020000 fd:00 167223           mprotect04
> > 1001a380000-1001a3b0000 rw-p 00000000 00:00 0          [heap]
> > 7fffa6c60000-7fffa6c80000 --xp 00000000 00:00 0 ​
> >
> > ​&exec_func = 0x10030170​
> >
> > ​&func = 0x7fffa6c60170​
> >
> > ​While perform ​
> > "(*func)();" we get the
> > ​segmentation fault.
> > ​
> >
> > ​strace log:​
> >
> > -------------------
> > ​mprotect(0x7fffaed00000, 131072, PROT_EXEC) = 0
> > rt_sigprocmask(SIG_BLOCK, NULL, [], 8)  = 0
> > --- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_PKUERR, si_addr=0x7fffaed00170}
> > ---​
> 
> Looks like a bug to me.
> 
> Please Cc linuxppc-dev on powerpc bugs.
> 
> I also can't reproduce this failure on my machine.
> Not sure what's going on?

I could reproduce it on a power7 lpar.  But not on a power8 lpar.

The problem seems to be that the cpu generates a key exception if
the page with Read/Write-disable-but-execute-enable key is executed
on power7. If I enable read on that key, the exception disappears.

BTW: the testcase executes mprotect(..,PROT_EXEC).
The mprotect(, PROT_EXEC) system call internally generates a
execute-only key and associates it with the pages in the address-range.

Now since Li Wang claims that he can reproduce it on power8 as well, i
am wondering if the slightly different cpu behavior is dependent on the
version of the firmware/microcode?


RP