From: Ram Pai <linuxram@us.ibm.com>
To: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
benh@kernel.crashing.org, paulus@samba.org, mpe@ellerman.id.au,
aneesh.kumar@linux.vnet.ibm.com, bsingharora@gmail.com,
dave.hansen@intel.com, hbabu@us.ibm.com
Subject: Re: [RFC v2 11/12]Documentation: Documentation updates.
Date: Tue, 20 Jun 2017 17:04:36 -0700 [thread overview]
Message-ID: <20170621000436.GN17588@ram.oc3035372033.ibm.com> (raw)
In-Reply-To: <eb2c2bf1-4881-aeb8-1a01-8ba11f69f18a@linux.vnet.ibm.com>
On Tue, Jun 20, 2017 at 11:48:23AM +0530, Anshuman Khandual wrote:
> On 06/17/2017 09:22 AM, Ram Pai wrote:
> > The Documentaton file is moved from x86 into the generic area,
> > since this feature is now supported by more than one archs.
> >
> > Signed-off-by: Ram Pai <linuxram@us.ibm.com>
> > ---
> > Documentation/vm/protection-keys.txt | 110 ++++++++++++++++++++++++++++++++++
> > Documentation/x86/protection-keys.txt | 85 --------------------------
>
> I am not sure whether this is a good idea. There might be
> specifics for each architecture which need to be detailed
> again in this new generic one.
>
> > 2 files changed, 110 insertions(+), 85 deletions(-)
> > create mode 100644 Documentation/vm/protection-keys.txt
> > delete mode 100644 Documentation/x86/protection-keys.txt
> >
> > diff --git a/Documentation/vm/protection-keys.txt b/Documentation/vm/protection-keys.txt
> > new file mode 100644
> > index 0000000..b49e6bb
> > --- /dev/null
> > +++ b/Documentation/vm/protection-keys.txt
> > @@ -0,0 +1,110 @@
> > +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
> > +found in new generation of intel CPUs on PowerPC CPUs.
> > +
> > +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.
>
> Does resultant access through protection keys should be a
> subset of the protection bits enabled through original PTE
> PROT format ? Does the semantics exactly the same on x86
> and powerpc ?
The protection key takes precedence over protection done through
mprotect.
Yes both on x86 and powerpc we maintain the same semantics.
>
> > +
> > +
> > +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 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.
>
> Small nit. Space needed here.
>
> > +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.
> > +
> > +Permissions are enforced on data access only and have no effect on
> > +instruction fetches.
> > +
> > +=========================== Syscalls ===========================
> > +
> > +There are 3 system calls which directly interact with pkeys:
> > +
> > + int pkey_alloc(unsigned long flags, unsigned long init_access_rights)
> > + int pkey_free(int pkey);
> > + int pkey_mprotect(unsigned long start, size_t len,
> > + 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
> > +directly in order to change access permissions to memory covered
> > +with a key. In this example WRPKRU is wrapped by a C function
> > +called pkey_set().
> > +
> > + int real_prot = PROT_READ|PROT_WRITE;
> > + pkey = pkey_alloc(0, PKEY_DENY_WRITE);
> > + ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
> > + ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey);
> > + ... application runs here
> > +
> > +Now, if the application needs to update the data at 'ptr', it can
> > +gain access, do the update, then remove its write access:
> > +
> > + pkey_set(pkey, 0); // clear PKEY_DENY_WRITE
> > + *ptr = foo; // assign something
> > + pkey_set(pkey, PKEY_DENY_WRITE); // set PKEY_DENY_WRITE again
> > +
> > +Now when it frees the memory, it will also free the pkey since it
> > +is no longer in use:
> > +
> > + munmap(ptr, PAGE_SIZE);
> > + pkey_free(pkey);
> > +
> > +(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
> > + An example implementation can be found in
> > + tools/testing/selftests/x86/protection_keys.c)
> > +
> > +=========================== Behavior ===========================
> > +
> > +The kernel attempts to make protection keys consistent with the
> > +behavior of a plain mprotect(). For instance if you do this:
> > +
> > + mprotect(ptr, size, PROT_NONE);
> > + something(ptr);
> > +
> > +you can expect the same effects with protection keys when doing this:
> > +
> > + pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ);
> > + pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey);
> > + something(ptr);
> > +
> > +That should be true whether something() is a direct access to 'ptr'
> > +like:
> > +
> > + *ptr = foo;
> > +
> > +or when the kernel does the access on the application's behalf like
> > +with a read():
> > +
> > + read(fd, ptr, 1);
> > +
> > +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.
>
> I guess the right thing would be to have three files
>
> * Documentation/vm/protection-keys.txt
>
> - Generic interface, system calls
> - Signal handling, error codes
> - Semantics of programming with an example
>
> * Documentation/x86/protection-keys.txt
>
> - Number of active protections keys inside an address space
> - X86 protection key instruction details
> - PTE protection bits placement details
> - Page fault handling
> - Implementation details a bit ?
>
> * Documentation/powerpc/protection-keys.txt
>
> - Number of active protections keys inside an address space
> - Powerpc instructions details
> - PTE protection bits placement details
> - Page fault handling
> - Implementation details a bit ?
I see the value of your suggestion. This is something that will touch
atleast two architectures. Want to hear some more inputs before I do the
changes.
Dave Hansen: would like to hear your ideas.
RP
next prev parent reply other threads:[~2017-06-21 0:04 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-17 3:52 [RFC v2 00/12] powerpc: Memory Protection Keys Ram Pai
2017-06-17 3:52 ` [RFC v2 01/12] powerpc: Free up four 64K PTE bits in 4K backed hpte pages Ram Pai
2017-06-20 10:20 ` Anshuman Khandual
2017-06-20 23:23 ` Ram Pai
2017-06-21 5:35 ` Anshuman Khandual
2017-06-21 6:34 ` Ram Pai
2017-06-21 6:41 ` Aneesh Kumar K.V
2017-06-21 9:30 ` Ram Pai
2017-06-22 9:07 ` Anshuman Khandual
2017-06-22 16:20 ` Ram Pai
2017-06-17 3:52 ` [RFC v2 02/12] powerpc: Free up four 64K PTE bits in 64K " Ram Pai
2017-06-20 10:51 ` Anshuman Khandual
2017-06-20 23:25 ` Ram Pai
2017-06-21 6:50 ` Aneesh Kumar K.V
2017-06-21 6:54 ` Aneesh Kumar K.V
2017-06-21 20:14 ` Ram Pai
2017-06-17 3:52 ` [RFC v2 03/12] powerpc: Implement sys_pkey_alloc and sys_pkey_free system call Ram Pai
2017-06-19 12:18 ` Michael Ellerman
2017-06-20 22:45 ` Ram Pai
2017-06-17 3:52 ` [RFC v2 04/12] powerpc: store and restore the pkey state across context switches Ram Pai
2017-06-17 3:52 ` [RFC v2 05/12] powerpc: Implementation for sys_mprotect_pkey() system call Ram Pai
2017-06-21 7:16 ` Aneesh Kumar K.V
2017-06-17 3:52 ` [RFC v2 06/12] powerpc: Program HPTE key protection bits Ram Pai
2017-06-20 8:21 ` Anshuman Khandual
2017-06-20 23:26 ` Ram Pai
2017-06-17 3:52 ` [RFC v2 07/12] powerpc: Macro the mask used for checking DSI exception Ram Pai
2017-06-20 8:14 ` Anshuman Khandual
2017-06-20 23:28 ` Ram Pai
2017-06-21 7:25 ` Aneesh Kumar K.V
2017-06-21 9:17 ` Ram Pai
2017-06-17 3:52 ` [RFC v2 08/12] powerpc: Handle exceptions caused by violation of pkey protection Ram Pai
2017-06-20 7:24 ` Anshuman Khandual
2017-06-20 23:43 ` Ram Pai
2017-06-21 3:54 ` Anshuman Khandual
2017-06-21 6:26 ` Ram Pai
2017-06-17 3:52 ` [RFC v2 09/12] powerpc: Deliver SEGV signal on pkey violation Ram Pai
2017-06-20 6:54 ` Anshuman Khandual
2017-06-20 23:56 ` Ram Pai
2017-06-21 3:18 ` Anshuman Khandual
2017-06-21 6:10 ` Ram Pai
2017-06-17 3:52 ` [RFC v2 10/12] powerpc: Read AMR only if pkey-violation caused the exception Ram Pai
2017-06-19 11:06 ` Michael Ellerman
2017-06-19 17:59 ` Ram Pai
2017-06-20 6:46 ` Anshuman Khandual
2017-06-20 23:58 ` Ram Pai
2017-06-20 23:56 ` Ram Pai
2017-06-17 3:52 ` [RFC v2 11/12]Documentation: Documentation updates Ram Pai
2017-06-20 6:18 ` Anshuman Khandual
2017-06-21 0:04 ` Ram Pai [this message]
2017-06-17 3:52 ` [RFC v2 12/12]selftest: Updated protection key selftest Ram Pai
2017-06-19 11:04 ` Michael Ellerman
2017-06-20 6:26 ` Anshuman Khandual
2017-06-21 0:10 ` Ram Pai
2017-06-20 5:10 ` [RFC v2 00/12] powerpc: Memory Protection Keys Balbir Singh
2017-06-20 6:05 ` Anshuman Khandual
2017-06-20 9:56 ` Benjamin Herrenschmidt
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