From c74fe3940848c6afea83bfbda64a9baf9da547c8 Mon Sep 17 00:00:00 2001 From: Dave Hansen Date: Fri, 29 Jul 2016 09:30:20 -0700 Subject: pkeys: Add details of system call use to Documentation/ This spells out all of the pkey-related system calls that we have and provides some example code fragments to demonstrate how we expect them to be used. Signed-off-by: Dave Hansen Cc: linux-arch@vger.kernel.org Cc: Dave Hansen Cc: mgorman@techsingularity.net Cc: arnd@arndb.de Cc: linux-api@vger.kernel.org Cc: linux-mm@kvack.org Cc: luto@kernel.org Cc: akpm@linux-foundation.org Cc: torvalds@linux-foundation.org Link: http://lkml.kernel.org/r/20160729163020.59350E33@viggo.jf.intel.com Signed-off-by: Thomas Gleixner --- Documentation/x86/protection-keys.txt | 62 +++++++++++++++++++++++++++++++++++ 1 file changed, 62 insertions(+) (limited to 'Documentation/x86') diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt index c281ded..6da7689 100644 --- a/Documentation/x86/protection-keys.txt +++ b/Documentation/x86/protection-keys.txt @@ -18,6 +18,68 @@ 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. +=========================== Syscalls =========================== + +There are 2 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); + +=========================== 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. + =========================== Config Option =========================== This config option adds approximately 1.5kb of text. and 50 bytes of -- cgit v1.1 From 6679dac513fd612f34d3a3d99d7b84ed6d5eb5cc Mon Sep 17 00:00:00 2001 From: Dave Hansen Date: Tue, 4 Oct 2016 09:38:57 -0700 Subject: x86/pkeys: Update documentation There are a few items that have gotten stale in the protection keys documentation. The config option description only applied to the execute-only support and is not accurate for the current code. There was also a typo with the number of system calls. I also wanted to call out that pkey_set() is not a kernel-provided facility, and where to find an implementation. Signed-off-by: Dave Hansen Cc: Dave Hansen Cc: linux-doc@vger.kernel.org Cc: corbet@lwn.net Link: http://lkml.kernel.org/r/20161004163857.71E0D6F6@viggo.jf.intel.com Signed-off-by: Thomas Gleixner --- Documentation/x86/protection-keys.txt | 14 +++++--------- 1 file changed, 5 insertions(+), 9 deletions(-) (limited to 'Documentation/x86') diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt index 6da7689..b643045 100644 --- a/Documentation/x86/protection-keys.txt +++ b/Documentation/x86/protection-keys.txt @@ -20,7 +20,7 @@ instruction fetches. =========================== Syscalls =========================== -There are 2 system calls which directly interact with pkeys: +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); @@ -52,6 +52,10 @@ 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 @@ -79,11 +83,3 @@ 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. - -=========================== Config Option =========================== - -This config option adds approximately 1.5kb of text. and 50 bytes of -data to the executable. A workload which does large O_DIRECT reads -of holes in XFS files was run to exercise get_user_pages_fast(). No -performance delta was observed with the config option -enabled or disabled. -- cgit v1.1