[PATCH] powerpc/perf: Fix kernel address leaks via Sampling registers

From: Michael Ellerman <mpe@ellerman.id.au>

Current code in power_pmu_disable() does not clear the sampling
registers like Sampling Instruction Address Register (SAIR) and
Sampling Data Address Register (SDAR) after disabling the PMU.
Since these are userspace readable and could contain kernel
address, add code to explicitly clear the content of these registers.
Patch also adds a "context synchronizing instruction" to enforce
no further updates to these registers as mandated by PowerISA.

"If an mtspr instruction is executed that changes the
value of a Performance Monitor register other than
SIAR, SDAR, and SIER, the change is not guaranteed
to have taken effect until after a subsequent context
synchronizing instruction has been executed (see
Chapter 11. "Synchronization Requirements for Con-
text Alterations" on page 1133)."

Tested-by: Madhavan Srinivasan <redacted>
Reviewed-by: Madhavan Srinivasan <redacted>
Signed-off-by: Madhavan Srinivasan <redacted>
---
 arch/powerpc/perf/core-book3s.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 73f997b84d19..e8754243f3cb 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c

@@ -1235,6 +1235,7 @@ static void power_pmu_disable(struct pmu *pmu)
 		 */
 		write_mmcr0(cpuhw, val);
 		mb();
+		isync();
 
 		/*
 		 * Disable instruction sampling if it was enabled

@@ -1243,12 +1244,22 @@ static void power_pmu_disable(struct pmu *pmu)
 			mtspr(SPRN_MMCRA,
 			      cpuhw->mmcr[2] & ~MMCRA_SAMPLE_ENABLE);
 			mb();
+			isync();
 		}
 
 		cpuhw->disabled = 1;
 		cpuhw->n_added = 0;
 
 		ebb_switch_out(mmcr0);
+
+		/*
+		 * These are readable by userspace, may contain kernel
+		 * addresses and are not switched by context switch, so clear
+		 * them now to avoid leaking anything to userspace in general
+		 * including to another process.
+		 */
+		mtspr(SPRN_SDAR, 0);
+		mtspr(SPRN_SIAR, 0);
 	}
 
 	local_irq_restore(flags);

-- 
2.7.4

Hi Michael,

I love your patch! Yet something to improve:

[auto build test ERROR on v4.16-rc1]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Madhavan-Srinivasan/powerpc-perf-Fix-kernel-address-leaks-via-Sampling-registers/20180306-041036
config: powerpc-pmac32_defconfig (attached as .config)
compiler: powerpc-linux-gnu-gcc (Debian 7.2.0-11) 7.2.0
reproduce:
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        make.cross ARCH=powerpc 

All errors (new ones prefixed by >>):

   {standard input}: Assembler messages:

quoted

{standard input}:2476: Error: unsupported relocation against SPRN_SDAR

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

On Tuesday 06 March 2018 08:36 PM, kbuild test robot wrote:

Hi Michael,

I love your patch! Yet something to improve:

[auto build test ERROR on v4.16-rc1]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Madhavan-Srinivasan/powerpc-perf-Fix-kernel-address-leaks-via-Sampling-registers/20180306-041036
config: powerpc-pmac32_defconfig (attached as .config)
compiler: powerpc-linux-gnu-gcc (Debian 7.2.0-11) 7.2.0
reproduce:
         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
         chmod +x ~/bin/make.cross
         # save the attached .config to linux build tree
         make.cross ARCH=powerpc

My bad should have added the ppmu->flag or config check to avoid this
in 32bit case. Will respin it.

Maddy

All errors (new ones prefixed by >>):

    {standard input}: Assembler messages:

quoted

quoted

{standard input}:2476: Error: unsupported relocation against SPRN_SDAR

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation