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authorJonathan Austin <jonathan.austin@arm.com>2013-04-18 18:37:24 +0100
committerJonathan Austin <jonathan.austin@arm.com>2013-06-17 15:13:18 +0100
commit9dfc28b6308096e48b54c28259825a1200f60742 (patch)
tree44e8b6cbe0935b20767bc79099780244d4c6722e
parent801bb21c600161c8626d7d9cb649be869ee4dbbe (diff)
ARM: mpu: protect the vectors page with an MPU region
Without an MMU it is possible for userspace programs to start executing code in places that they have no business executing. The MPU allows some level of protection against this. This patch protects the vectors page from access by userspace processes. Userspace tasks that dereference a null pointer are already protected by an svc at 0x0 that kills them. However when tasks use an offset from a null pointer (eg a function in a null struct) they miss this carefully placed svc and enter the exception vectors in user mode, ending up in the kernel. This patch causes programs that do this to receive a SEGV instead of happily entering the kernel in user-mode, and hence avoid a 'Bad Mode' panic. As part of this change it is necessary to make sigreturn happen via the stack when there is not an sa_restorer function. This change is invisible to userspace, and irrelevant to code compiled using a uClibc toolchain, which always uses an sa_restorer function. Because we don't get to remap the vectors in !MMU kuser_helpers are not in a defined location, and hence aren't usable. This means we don't need to worry about keeping them accessible from PL0 Signed-off-by: Jonathan Austin <jonathan.austin@arm.com> Reviewed-by: Will Deacon <will.deacon@arm.com> CC: Nicolas Pitre <nico@linaro.org> CC: Catalin Marinas <catalin.marinas@arm.com>
-rw-r--r--arch/arm/include/asm/mpu.h1
-rw-r--r--arch/arm/kernel/head-nommu.S16
-rw-r--r--arch/arm/kernel/signal.c9
3 files changed, 24 insertions, 2 deletions
diff --git a/arch/arm/include/asm/mpu.h b/arch/arm/include/asm/mpu.h
index 001483465aa4..c3247cc2fe08 100644
--- a/arch/arm/include/asm/mpu.h
+++ b/arch/arm/include/asm/mpu.h
@@ -46,6 +46,7 @@
#define MPU_PROBE_REGION 0
#define MPU_BG_REGION 1
#define MPU_RAM_REGION 2
+#define MPU_VECTORS_REGION 3
/* Maximum number of regions Linux is interested in */
#define MPU_MAX_REGIONS 16
diff --git a/arch/arm/kernel/head-nommu.S b/arch/arm/kernel/head-nommu.S
index 13741d004de5..75f14cc3e073 100644
--- a/arch/arm/kernel/head-nommu.S
+++ b/arch/arm/kernel/head-nommu.S
@@ -22,6 +22,7 @@
#include <asm/thread_info.h>
#include <asm/v7m.h>
#include <asm/mpu.h>
+#include <asm/page.h>
/*
* Kernel startup entry point.
@@ -188,6 +189,7 @@ ENDPROC(__after_proc_init)
* Region 0: Use this for probing the MPU details, so leave disabled.
* Region 1: Background region - covers the whole of RAM as strongly ordered
* Region 2: Normal, Shared, cacheable for RAM. From PHYS_OFFSET, size from r6
+ * Region 3: Normal, shared, inaccessible from PL0 to protect the vectors page
*
* r6: Value to be written to DRSR (and IRSR if required) for MPU_RAM_REGION
*/
@@ -232,6 +234,20 @@ ENTRY(__setup_mpu)
setup_region r0, r5, r6, MPU_INSTR_SIDE @ 0x0, BG region, enabled
2: isb
+ /* Vectors region */
+ set_region_nr r0, #MPU_VECTORS_REGION
+ isb
+ /* Shared, inaccessible to PL0, rw PL1 */
+ mov r0, #CONFIG_VECTORS_BASE @ Cover from VECTORS_BASE
+ ldr r5,=(MPU_AP_PL1RW_PL0NA | MPU_RGN_NORMAL)
+ /* Writing N to bits 5:1 (RSR_SZ) --> region size 2^N+1 */
+ mov r6, #(((PAGE_SHIFT - 1) << MPU_RSR_SZ) | 1 << MPU_RSR_EN)
+
+ setup_region r0, r5, r6, MPU_DATA_SIDE @ VECTORS_BASE, PL0 NA, enabled
+ beq 3f @ Memory-map not unified
+ setup_region r0, r5, r6, MPU_INSTR_SIDE @ VECTORS_BASE, PL0 NA, enabled
+3: isb
+
/* Enable the MPU */
mrc p15, 0, r0, c1, c0, 0 @ Read SCTLR
bic r0, r0, #CR_BR @ Disable the 'default mem-map'
diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 296786bdbb73..1c16c35c271a 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -392,14 +392,19 @@ setup_return(struct pt_regs *regs, struct ksignal *ksig,
if (ksig->ka.sa.sa_flags & SA_SIGINFO)
idx += 3;
+ /*
+ * Put the sigreturn code on the stack no matter which return
+ * mechanism we use in order to remain ABI compliant
+ */
if (__put_user(sigreturn_codes[idx], rc) ||
__put_user(sigreturn_codes[idx+1], rc+1))
return 1;
- if (cpsr & MODE32_BIT) {
+ if ((cpsr & MODE32_BIT) && !IS_ENABLED(CONFIG_ARM_MPU)) {
/*
* 32-bit code can use the new high-page
- * signal return code support.
+ * signal return code support except when the MPU has
+ * protected the vectors page from PL0
*/
retcode = KERN_SIGRETURN_CODE + (idx << 2) + thumb;
} else {