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) .... > > > +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". done. -- Ram Pai -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
