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

[Ram Pai]

On Fri, Mar 09, 2018 at 11:43:00AM +0800, Li Wang wrote:
>    On Fri, Mar 9, 2018 at 12:45 AM, Ram Pai <[1]linuxram at us.ibm.com> wrote:
> 
>      On Thu, Mar 08, 2018 at 11:19:12PM +1100, Michael Ellerman wrote:
>      > Li Wang <[2]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.
> 
>    After adding read permission on that key, reproducer get PASS on my power8
>    machine too.​
>    ​(​mprotect(..,PROT_READ | PROT_EXEC))​
>     
> 
>      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?
> 
>    ​I also run this reproducer on series ppc kvm machines, but none of them
>    get the FAIL.
>    If you need some more HW info, pls let me know.​

Hi Li,

   Can you try the following patch and see if it solves your problem.


diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
index c269817..184a10a 100644
--- a/arch/powerpc/mm/pkeys.c
+++ b/arch/powerpc/mm/pkeys.c
@@ -421,7 +421,7 @@ int __arch_override_mprotect_pkey(struct vm_area_struct *vma, int prot,
 	 * The requested protection is execute-only. Hence let's use an
 	 * execute-only pkey.
 	 */
-	if (prot == PROT_EXEC) {
+	if (prot == PROT_EXEC && pkey_execute_disable_supported) {
 		pkey = execute_only_pkey(vma->vm_mm);
 		if (pkey > 0)
 			return pkey;


Thanks
RP