Re: [RFC v5 38/38] Documentation: PowerPC specific updates to memory protection keys
From: Ram Pai
Date: Thu Jul 13 2017 - 15:56:52 EST
On Tue, Jul 11, 2017 at 11:23:29AM -0700, Dave Hansen wrote:
> On 07/05/2017 02:22 PM, Ram Pai wrote:
> > Add documentation updates that capture PowerPC specific changes.
> > Signed-off-by: Ram Pai <linuxram@xxxxxxxxxx>
> > ---
> > Documentation/vm/protection-keys.txt | 85 ++++++++++++++++++++++++++--------
> > 1 files changed, 65 insertions(+), 20 deletions(-)
> > diff --git a/Documentation/vm/protection-keys.txt b/Documentation/vm/protection-keys.txt
> > index b643045..d50b6ab 100644
> > --- a/Documentation/vm/protection-keys.txt
> > +++ b/Documentation/vm/protection-keys.txt
> > @@ -1,21 +1,46 @@
> > -Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
> > -which will be found on future Intel CPUs.
> > +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature found in
> > +new generation of intel CPUs and on PowerPC 7 and higher CPUs.
> Please try not to change the wording here. I really did mean to
> literally put "future Intel CPUs." Also, you broke my nice wrapping. :)
> I'm also thinking that this needs to be more generic. The ppc _CPU_
> feature is *NOT* for userspace-only, right?
It can be used for protecting the kernel aswell with the help of the
hypervisor. But the current implementation is towards "Protection keys
for Userspace" only; not yet "Protection keys for Kernel". Hence will
not talk about it yet :).
> > Memory Protection Keys provides a mechanism for enforcing page-based
> > -protections, but without requiring modification of the page tables
> > -when an application changes protection domains. It works by
> > -dedicating 4 previously ignored bits in each page table entry to a
> > -"protection key", giving 16 possible keys.
> > -
> > -There is also a new user-accessible register (PKRU) with two separate
> > -bits (Access Disable and Write Disable) for each key. Being a CPU
> > -register, PKRU is inherently thread-local, potentially giving each
> > -thread a different set of protections from every other thread.
> > -
> > -There are two new instructions (RDPKRU/WRPKRU) for reading and writing
> > -to the new register. The feature is only available in 64-bit mode,
> > -even though there is theoretically space in the PAE PTEs. These
> > -permissions are enforced on data access only and have no effect on
> > +protections, but without requiring modification of the page tables when an
> > +application changes protection domains.
> > +
> > +
> > +On Intel:
> > +
> > + It works by dedicating 4 previously ignored bits in each page table
> > + entry to a "protection key", giving 16 possible keys.
> > +
> > + There is also a new user-accessible register (PKRU) with two separate
> > + bits (Access Disable and Write Disable) for each key. Being a CPU
> > + register, PKRU is inherently thread-local, potentially giving each
> > + thread a different set of protections from every other thread.
> > +
> > + There are two new instructions (RDPKRU/WRPKRU) for reading and writing
> > + to the new register. The feature is only available in 64-bit mode,
> > + even though there is theoretically space in the PAE PTEs. These
> > + permissions are enforced on data access only and have no effect on
> > + instruction fetches.
> > +
> > +
> > +On PowerPC:
> > +
> > + It works by dedicating 5 page table entry bits to a "protection key",
> > + giving 32 possible keys.
> > +
> > + There is a user-accessible register (AMR) with two separate bits;
> > + Access Disable and Write Disable, for each key. Being a CPU
> > + register, AMR is inherently thread-local, potentially giving each
> > + thread a different set of protections from every other thread. NOTE:
> > + Disabling read permission does not disable write and vice-versa.
> > +
> > + The feature is available on 64-bit HPTE mode only.
> > + 'mtspr 0xd, mem' reads the AMR register
> > + 'mfspr mem, 0xd' writes into the AMR register.
> The whole "being a CPU register" bits seem pretty common. Should it be
> in the leading paragraph that is shared?
> > +Permissions are enforced on data access only and have no effect on
> > instruction fetches.
> Shouldn't we mention the ppc support for execute-disable here too?
yes. have reformated the structure to capture all that information. Will
be in my v6 patch version.
> Also, *does* this apply to ppc? You have it both in this common area
> and in the x86 portion.
> > =========================== Syscalls ===========================
> > @@ -28,9 +53,9 @@ There are 3 system calls which directly interact with pkeys:
> > unsigned long prot, int pkey);
> > Before a pkey can be used, it must first be allocated with
> > -pkey_alloc(). An application calls the WRPKRU instruction
> > +pkey_alloc(). An application calls the WRPKRU/AMR instruction
> > directly in order to change access permissions to memory covered
> > -with a key. In this example WRPKRU is wrapped by a C function
> > +with a key. In this example WRPKRU/AMR is wrapped by a C function
> > called pkey_set().
> > int real_prot = PROT_READ|PROT_WRITE;
> > @@ -52,11 +77,11 @@ is no longer in use:
> > munmap(ptr, PAGE_SIZE);
> > pkey_free(pkey);
> > -(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
> > +(Note: pkey_set() is a wrapper for the RDPKRU,WRPKRU or AMR instructions.
> > An example implementation can be found in
> > tools/testing/selftests/x86/protection_keys.c)
> > -=========================== Behavior ===========================
> > +=========================== Behavior =================================
> > The kernel attempts to make protection keys consistent with the
> > behavior of a plain mprotect(). For instance if you do this:
> > @@ -83,3 +108,23 @@ with a read():
> > The kernel will send a SIGSEGV in both cases, but si_code will be set
> > to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when
> > the plain mprotect() permissions are violated.
> > +
> > +
> > +====================================================================
> > + Semantic differences
> > +
> > +The following semantic differences exist between x86 and power.
> > +
> > +a) powerpc allows creation of a key with execute-disabled. The following
> > + is allowed on powerpc.
> > + pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_ACCESS |
> > + PKEY_DISABLE_EXECUTE);
> > + x86 disallows PKEY_DISABLE_EXECUTE during key creation.
> It isn't that powerpc supports *creation* of the key. It doesn't
> support setting PKEY_DISABLE_EXECUTE, period, which implies that you
> can't set it at pkey_alloc(). That's a pretty important distinction, IMNHO.
ok. will the following wording capture the subtle distinction?
+a) powerpc *also* allows creation of a key with execute-disabled.
+ The following is allowed on powerpc.
+ pkey = pkey_alloc(0, PKEY_DISABLE_EXECUTE);
> > +b) changing the permission bits of a key from a signal handler does not
> > + persist on x86. The PKRU specific fpregs entry needs to be modified
> > + for it to persist. On powerpc the permission bits of the key can be
> > + modified by programming the AMR register from the signal handler.
> > + The changes persists across signal boundaries.
> ^"changes persist", not "persists".