x86/pkeys: Default to a restrictive init PKRU

hansendc · KAGA-KOKO · commit acd547b29880 · 2016-09-09T13:02:28.000+02:00
PKRU is the register that lets you disallow writes or all access to a given protection key. The XSAVE hardware defines an "init state" of 0 for PKRU: its most permissive state, allowing access/writes to everything. Since we start off all new processes with the init state, we start all processes off with the most permissive possible PKRU. This is unfortunate. If a thread is clone()'d [1] before a program has time to set PKRU to a restrictive value, that thread will be able to write to all data, no matter what pkey is set on it. This weakens any integrity guarantees that we want pkeys to provide. To fix this, we define a very restrictive PKRU to override the XSAVE-provided value when we create a new FPU context. We choose a value that only allows access to pkey 0, which is as restrictive as we can practically make it. This does not cause any practical problems with applications using protection keys because we require them to specify initial permissions for each key when it is allocated, which override the restrictive default. In the end, this ensures that threads which do not know how to manage their own pkey rights can not do damage to data which is pkey-protected. I would have thought this was a pretty contrived scenario, except that I heard a bug report from an MPX user who was creating threads in some very early code before main(). It may be crazy, but folks evidently _do_ it. Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-arch@vger.kernel.org Cc: Dave Hansen <dave@sr71.net> 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/20160729163021.F3C25D4A@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
@@ -1643,6 +1643,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	initrd=		[BOOT] Specify the location of the initial ramdisk
 
+	init_pkru=	[x86] Specify the default memory protection keys rights
+			register contents for all processes.  0x55555554 by
+			default (disallow access to all but pkey 0).  Can
+			override in debugfs after boot.
+
 	inport.irq=	[HW] Inport (ATI XL and Microsoft) busmouse driver
 			Format: <irq>
 
diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
@@ -100,5 +100,6 @@ extern int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
 		unsigned long init_val);
 extern int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
 		unsigned long init_val);
+extern void copy_init_pkru_to_fpregs(void);
 
 #endif /*_ASM_X86_PKEYS_H */
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
@@ -12,6 +12,7 @@
 #include <asm/traps.h>
 
 #include <linux/hardirq.h>
+#include <linux/pkeys.h>
 
 #define CREATE_TRACE_POINTS
 #include <asm/trace/fpu.h>
@@ -505,6 +506,9 @@ static inline void copy_init_fpstate_to_fpregs(void)
 		copy_kernel_to_fxregs(&init_fpstate.fxsave);
 	else
 		copy_kernel_to_fregs(&init_fpstate.fsave);
+
+	if (boot_cpu_has(X86_FEATURE_OSPKE))
+		copy_init_pkru_to_fpregs();
 }
 
 /*
diff --git a/arch/x86/mm/pkeys.c b/arch/x86/mm/pkeys.c
@@ -121,3 +121,41 @@ int __arch_override_mprotect_pkey(struct vm_area_struct *vma, int prot, int pkey
 	 */
 	return vma_pkey(vma);
 }
+
+#define PKRU_AD_KEY(pkey)	(PKRU_AD_BIT << ((pkey) * PKRU_BITS_PER_PKEY))
+
+/*
+ * Make the default PKRU value (at execve() time) as restrictive
+ * as possible.  This ensures that any threads clone()'d early
+ * in the process's lifetime will not accidentally get access
+ * to data which is pkey-protected later on.
+ */
+u32 init_pkru_value = PKRU_AD_KEY( 1) | PKRU_AD_KEY( 2) | PKRU_AD_KEY( 3) |
+		      PKRU_AD_KEY( 4) | PKRU_AD_KEY( 5) | PKRU_AD_KEY( 6) |
+		      PKRU_AD_KEY( 7) | PKRU_AD_KEY( 8) | PKRU_AD_KEY( 9) |
+		      PKRU_AD_KEY(10) | PKRU_AD_KEY(11) | PKRU_AD_KEY(12) |
+		      PKRU_AD_KEY(13) | PKRU_AD_KEY(14) | PKRU_AD_KEY(15);
+
+/*
+ * Called from the FPU code when creating a fresh set of FPU
+ * registers.  This is called from a very specific context where
+ * we know the FPU regstiers are safe for use and we can use PKRU
+ * directly.  The fact that PKRU is only available when we are
+ * using eagerfpu mode makes this possible.
+ */
+void copy_init_pkru_to_fpregs(void)
+{
+	u32 init_pkru_value_snapshot = READ_ONCE(init_pkru_value);
+	/*
+	 * Any write to PKRU takes it out of the XSAVE 'init
+	 * state' which increases context switch cost.  Avoid
+	 * writing 0 when PKRU was already 0.
+	 */
+	if (!init_pkru_value_snapshot && !read_pkru())
+		return;
+	/*
+	 * Override the PKRU state that came from 'init_fpstate'
+	 * with the baseline from the process.
+	 */
+	write_pkru(init_pkru_value_snapshot);
+}
diff --git a/include/linux/pkeys.h b/include/linux/pkeys.h
@@ -35,6 +35,10 @@ static inline int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
 	return 0;
 }
 
+static inline void copy_init_pkru_to_fpregs(void)
+{
+}
+
 #endif /* ! CONFIG_ARCH_HAS_PKEYS */
 
 #endif /* _LINUX_PKEYS_H */

Original file line number	Diff line number	Diff line change
`@@ -35,6 +35,10 @@ static inline int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,`
`35`	`35`	`return 0;`
`36`	`36`	`}`
`37`	`37`
	`38`	`+static inline void copy_init_pkru_to_fpregs(void)`
	`39`	`+{`
	`40`	`+}`
	`41`	`+`
`38`	`42`	`#endif /* ! CONFIG_ARCH_HAS_PKEYS */`
`39`	`43`
`40`	`44`	`#endif /* _LINUX_PKEYS_H */`