Currently restore_user_regs deallocates the SVC stack early in its execution and relies on no exception being taken between the deallocation and the registers being restored. The introduction of a default FIQ handler that also uses the SVC stack breaks this assumption and can result in corrupted register state.
This patch works around the problem by removing the early stack deallocation and using r2 as a temporary instead. I have not found a way to do this without introducing an extra mov instruction to the macro.
Signed-off-by: Daniel Thompson daniel.thompson@linaro.org ---
Notes: I have recently started to hook up the PMU via FIQ (although its slightly hacky at present) and was seeing random userspace SEGVs when perf was running (after ~100,000 or so FIQs).
Instrumenting the code eventually revealed that in almost all cases the last FIQ handler to run prior the SEGV had interrupted ret_to_user_from_irq or ret_fast_syscall. Very occasionally it was in the fault handling code (because that code runs as part of SEGV handling and the PMU is instrumenting that too).
No SEGV problems have been observed since fixing the issue. This version of the patch has seen >7M FIQs and an older version (based on cpsid f) ran overnight.
arch/arm/kernel/entry-header.S | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S index 4176df721bf0..1a0045abead7 100644 --- a/arch/arm/kernel/entry-header.S +++ b/arch/arm/kernel/entry-header.S @@ -253,21 +253,22 @@ .endm
.macro restore_user_regs, fast = 0, offset = 0 - ldr r1, [sp, #\offset + S_PSR] @ get calling cpsr - ldr lr, [sp, #\offset + S_PC]! @ get pc + mov r2, sp + ldr r1, [r2, #\offset + S_PSR] @ get calling cpsr + ldr lr, [r2, #\offset + S_PC]! @ get pc msr spsr_cxsf, r1 @ save in spsr_svc #if defined(CONFIG_CPU_V6) || defined(CONFIG_CPU_32v6K) @ We must avoid clrex due to Cortex-A15 erratum #830321 - strex r1, r2, [sp] @ clear the exclusive monitor + strex r1, r2, [r2] @ clear the exclusive monitor #endif .if \fast - ldmdb sp, {r1 - lr}^ @ get calling r1 - lr + ldmdb r2, {r1 - lr}^ @ get calling r1 - lr .else - ldmdb sp, {r0 - lr}^ @ get calling r0 - lr + ldmdb r2, {r0 - lr}^ @ get calling r0 - lr .endif mov r0, r0 @ ARMv5T and earlier require a nop @ after ldm {}^ - add sp, sp, #S_FRAME_SIZE - S_PC + add sp, sp, #\offset + S_FRAME_SIZE movs pc, lr @ return & move spsr_svc into cpsr .endm
-- 1.9.3
Currently restore_user_regs deallocates the SVC stack early in its execution and relies on no exception being taken between the deallocation and the registers being restored. The introduction of a default FIQ handler that also uses the SVC stack breaks this assumption and can result in corrupted register state.
This patch works around the problem by removing the early stack deallocation and using r2 as a temporary instead. I have not found a way to do this without introducing an extra mov instruction to the macro.
Signed-off-by: Daniel Thompson daniel.thompson@linaro.org ---
Notes: [This patch has not been modified since its original posting as an RFC].
I have recently started to hook up the PMU via FIQ (although it's slightly hacky at present) and was seeing random userspace SEGVs when perf was running (after ~100,000 or so FIQs).
Instrumenting the code eventually revealed that in almost all cases the last FIQ handler to run prior the SEGV had interrupted ret_to_user_from_irq or ret_fast_syscall. Very occasionally it was in the fault handling code (because that code runs as part of SEGV handling and the PMU is instrumenting that too).
No SEGV problems have been observed since fixing the issue. This version of the patch has seen >7M FIQs and an older version (based on cpsid f) ran overnight.
arch/arm/kernel/entry-header.S | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S index 4176df721bf0..1a0045abead7 100644 --- a/arch/arm/kernel/entry-header.S +++ b/arch/arm/kernel/entry-header.S @@ -253,21 +253,22 @@ .endm
.macro restore_user_regs, fast = 0, offset = 0 - ldr r1, [sp, #\offset + S_PSR] @ get calling cpsr - ldr lr, [sp, #\offset + S_PC]! @ get pc + mov r2, sp + ldr r1, [r2, #\offset + S_PSR] @ get calling cpsr + ldr lr, [r2, #\offset + S_PC]! @ get pc msr spsr_cxsf, r1 @ save in spsr_svc #if defined(CONFIG_CPU_V6) || defined(CONFIG_CPU_32v6K) @ We must avoid clrex due to Cortex-A15 erratum #830321 - strex r1, r2, [sp] @ clear the exclusive monitor + strex r1, r2, [r2] @ clear the exclusive monitor #endif .if \fast - ldmdb sp, {r1 - lr}^ @ get calling r1 - lr + ldmdb r2, {r1 - lr}^ @ get calling r1 - lr .else - ldmdb sp, {r0 - lr}^ @ get calling r0 - lr + ldmdb r2, {r0 - lr}^ @ get calling r0 - lr .endif mov r0, r0 @ ARMv5T and earlier require a nop @ after ldm {}^ - add sp, sp, #S_FRAME_SIZE - S_PC + add sp, sp, #\offset + S_FRAME_SIZE movs pc, lr @ return & move spsr_svc into cpsr .endm
-- 1.9.3
On Mon, Jan 05, 2015 at 03:12:38PM +0000, Daniel Thompson wrote:
Currently restore_user_regs deallocates the SVC stack early in its execution and relies on no exception being taken between the deallocation and the registers being restored. The introduction of a default FIQ handler that also uses the SVC stack breaks this assumption and can result in corrupted register state.
This patch works around the problem by removing the early stack deallocation and using r2 as a temporary instead. I have not found a way to do this without introducing an extra mov instruction to the macro.
Signed-off-by: Daniel Thompson daniel.thompson@linaro.org
Please put it in the patch system, thanks. I think we should queue this one for stable too, as I think we need this for v3.18 (as a result of c0e7f7ee717e2b4c5791e7422424c96b5008c39e, ARM: 8150/3: fiq: Replace default FIQ handler)?
On 09/01/15 16:46, Russell King - ARM Linux wrote:
On Mon, Jan 05, 2015 at 03:12:38PM +0000, Daniel Thompson wrote:
Currently restore_user_regs deallocates the SVC stack early in its execution and relies on no exception being taken between the deallocation and the registers being restored. The introduction of a default FIQ handler that also uses the SVC stack breaks this assumption and can result in corrupted register state.
This patch works around the problem by removing the early stack deallocation and using r2 as a temporary instead. I have not found a way to do this without introducing an extra mov instruction to the macro.
Signed-off-by: Daniel Thompson daniel.thompson@linaro.org
Please put it in the patch system, thanks.
Will do.
I think we should queue this one for stable too, as I think we need this for v3.18 (as a result of c0e7f7ee717e2b4c5791e7422424c96b5008c39e, ARM: 8150/3: fiq: Replace default FIQ handler)?
It's a close call.
Before 8150/3 the system would probably crash if the default FIQ handler ran. After 8150/3 the system is also likely to crash since there's no code hooked into the handler in v3.18 that can clear the source of FIQ leaving us stuck re-entering the FIQ handler.
Nevertheless, this is a nasty gremlin to leave for backporters (it wasn't easy to find) so I'd be very happy to Cc: stable and see what they think.
Daniel.
On Fri, Jan 09, 2015 at 05:06:54PM +0000, Daniel Thompson wrote:
On 09/01/15 16:46, Russell King - ARM Linux wrote:
On Mon, Jan 05, 2015 at 03:12:38PM +0000, Daniel Thompson wrote:
Currently restore_user_regs deallocates the SVC stack early in its execution and relies on no exception being taken between the deallocation and the registers being restored. The introduction of a default FIQ handler that also uses the SVC stack breaks this assumption and can result in corrupted register state.
This patch works around the problem by removing the early stack deallocation and using r2 as a temporary instead. I have not found a way to do this without introducing an extra mov instruction to the macro.
Signed-off-by: Daniel Thompson daniel.thompson@linaro.org
Please put it in the patch system, thanks.
Will do.
I think we should queue this one for stable too, as I think we need this for v3.18 (as a result of c0e7f7ee717e2b4c5791e7422424c96b5008c39e, ARM: 8150/3: fiq: Replace default FIQ handler)?
It's a close call.
I agree.
Before 8150/3 the system would probably crash if the default FIQ handler ran. After 8150/3 the system is also likely to crash since there's no code hooked into the handler in v3.18 that can clear the source of FIQ leaving us stuck re-entering the FIQ handler.
Nevertheless, this is a nasty gremlin to leave for backporters (it wasn't easy to find) so I'd be very happy to Cc: stable and see what they think.
Well, we could ask Greg now. It's not a regression (as nothing makes use of the previously merged changes yet), but it is a nasty latent bug which we could easily solve.
Having thought about it some more, I'm tempted to say... leave the stable tag off it, and we can make the decision later - after it's had a chance to really prove itself to a much wider audience. That'd be the lowest risk for the 3.18 stable tree.
On Fri, Jan 09, 2015 at 05:20:35PM +0000, Russell King - ARM Linux wrote:
On Fri, Jan 09, 2015 at 05:06:54PM +0000, Daniel Thompson wrote:
On 09/01/15 16:46, Russell King - ARM Linux wrote:
On Mon, Jan 05, 2015 at 03:12:38PM +0000, Daniel Thompson wrote:
Currently restore_user_regs deallocates the SVC stack early in its execution and relies on no exception being taken between the deallocation and the registers being restored. The introduction of a default FIQ handler that also uses the SVC stack breaks this assumption and can result in corrupted register state.
This patch works around the problem by removing the early stack deallocation and using r2 as a temporary instead. I have not found a way to do this without introducing an extra mov instruction to the macro.
Signed-off-by: Daniel Thompson daniel.thompson@linaro.org
Please put it in the patch system, thanks.
Will do.
I think we should queue this one for stable too, as I think we need this for v3.18 (as a result of c0e7f7ee717e2b4c5791e7422424c96b5008c39e, ARM: 8150/3: fiq: Replace default FIQ handler)?
It's a close call.
I agree.
Before 8150/3 the system would probably crash if the default FIQ handler ran. After 8150/3 the system is also likely to crash since there's no code hooked into the handler in v3.18 that can clear the source of FIQ leaving us stuck re-entering the FIQ handler.
Nevertheless, this is a nasty gremlin to leave for backporters (it wasn't easy to find) so I'd be very happy to Cc: stable and see what they think.
Well, we could ask Greg now. It's not a regression (as nothing makes use of the previously merged changes yet), but it is a nasty latent bug which we could easily solve.
Having thought about it some more, I'm tempted to say... leave the stable tag off it, and we can make the decision later - after it's had a chance to really prove itself to a much wider audience. That'd be the lowest risk for the 3.18 stable tree.
That sounds like a good idea, you can always email stable@ to let us know of a patch we should add at a later time.
thanks,
greg k-h
linaro-kernel@lists.linaro.org
-
Daniel Thompson -
Greg KH -
Russell King - ARM Linux