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* [PATCH V2] powerpc/perf: Fix handling of privilege level checks in perf interrupt context
@ 2021-02-25 10:10 Athira Rajeev
  2021-02-26  9:35 ` Peter Zijlstra
  2021-03-14 10:01 ` Michael Ellerman
  0 siblings, 2 replies; 4+ messages in thread
From: Athira Rajeev @ 2021-02-25 10:10 UTC (permalink / raw)
  To: mpe; +Cc: maddy, peterz, omosnace, acme, jolsa, linuxppc-dev, kan.liang

Running "perf mem record" in powerpc platforms with selinux enabled
resulted in soft lockup's. Below call-trace was seen in the logs:

CPU: 58 PID: 3751 Comm: sssd_nss Not tainted 5.11.0-rc7+ #2
NIP:  c000000000dff3d4 LR: c000000000dff3d0 CTR: 0000000000000000
REGS: c000007fffab7d60 TRAP: 0100   Not tainted  (5.11.0-rc7+)
<<>>
NIP [c000000000dff3d4] _raw_spin_lock_irqsave+0x94/0x120
LR [c000000000dff3d0] _raw_spin_lock_irqsave+0x90/0x120
Call Trace:
[c00000000fd471a0] [c00000000fd47260] 0xc00000000fd47260 (unreliable)
[c00000000fd471e0] [c000000000b5fbbc] skb_queue_tail+0x3c/0x90
[c00000000fd47220] [c000000000296edc] audit_log_end+0x6c/0x180
[c00000000fd47260] [c0000000006a3f20] common_lsm_audit+0xb0/0xe0
[c00000000fd472a0] [c00000000066c664] slow_avc_audit+0xa4/0x110
[c00000000fd47320] [c00000000066cff4] avc_has_perm+0x1c4/0x260
[c00000000fd47430] [c00000000066e064] selinux_perf_event_open+0x74/0xd0
[c00000000fd47450] [c000000000669888] security_perf_event_open+0x68/0xc0
[c00000000fd47490] [c00000000013d788] record_and_restart+0x6e8/0x7f0
[c00000000fd476c0] [c00000000013dabc] perf_event_interrupt+0x22c/0x560
[c00000000fd477d0] [c00000000002d0fc] performance_monitor_exception0x4c/0x60
[c00000000fd477f0] [c00000000000b378] performance_monitor_common_virt+0x1c8/0x1d0
interrupt: f00 at _raw_spin_lock_irqsave+0x38/0x120
NIP:  c000000000dff378 LR: c000000000b5fbbc CTR: c0000000007d47f0
REGS: c00000000fd47860 TRAP: 0f00   Not tainted  (5.11.0-rc7+)
<<>>
NIP [c000000000dff378] _raw_spin_lock_irqsave+0x38/0x120
LR [c000000000b5fbbc] skb_queue_tail+0x3c/0x90
interrupt: f00
[c00000000fd47b00] [0000000000000038] 0x38 (unreliable)
[c00000000fd47b40] [c00000000aae6200] 0xc00000000aae6200
[c00000000fd47b80] [c000000000296edc] audit_log_end+0x6c/0x180
[c00000000fd47bc0] [c00000000029f494] audit_log_exit+0x344/0xf80
[c00000000fd47d10] [c0000000002a2b00] __audit_syscall_exit+0x2c0/0x320
[c00000000fd47d60] [c000000000032878] do_syscall_trace_leave+0x148/0x200
[c00000000fd47da0] [c00000000003d5b4] syscall_exit_prepare+0x324/0x390
[c00000000fd47e10] [c00000000000d76c] system_call_common+0xfc/0x27c

The above trace shows that while the CPU was handling a performance
monitor exception, there was a call to "security_perf_event_open"
function. In powerpc core-book3s, this function is called from
'perf_allow_kernel' check during recording of data address in the sample
via perf_get_data_addr().

Commit da97e18458fb ("perf_event: Add support for LSM and SELinux checks")
introduced security enhancements to perf. As part of this commit, the new
security hook for perf_event_open was added in all places where perf
paranoid check was previously used. In powerpc core-book3s code, originally
had paranoid checks in 'perf_get_data_addr' and 'power_pmu_bhrb_read'. So
'perf_paranoid_kernel' checks were replaced with 'perf_allow_kernel' in
these pmu helper functions as well.

The intention of paranoid checks in core-book3s was to verify privilege
access before capturing some of the sample data. Along with paranoid
checks, 'perf_allow_kernel' also does a 'security_perf_event_open'. Since
these functions are accessed while recording sample, we end up in calling
selinux_perf_event_open in PMI context. Some of the security functions
use spinlock like sidtab_sid2str_put(). If a perf interrupt hits under
a spin lock and if we end up in calling selinux hook functions in PMI
handler, this could cause a dead lock.

Since the purpose of this security hook is to control access to
perf_event_open, it is not right to call this in interrupt context.
The paranoid checks in powerpc core-book3s were done at interrupt
time which is also not correct.
Reference commits:
Commit cd1231d7035f ("powerpc/perf: Prevent kernel address leak via
perf_get_data_addr()")
Commit bb19af816025 ("powerpc/perf: Prevent kernel address leak to
userspace via BHRB buffer")

We only allow creation of events that has already passed the privilege
checks in perf_event_open. So these paranoid checks are not needed at
event time. As a fix, patch uses 'event->attr.exclude_kernel' check
to prevent exposing kernel address for userspace only sampling.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
---
Changes in v2:
- Addressed review comments from Ondrej Mosnacek and Peter Zijlstra.
  Changed the approach to use 'event->attr.exclude_kernel'
  check to prevent exposing kernel address for userspace only
  sampling as suggested by Ondrej Mosnacek.

 arch/powerpc/perf/core-book3s.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 4b4319d8..c8be44c 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -222,7 +222,7 @@ static inline void perf_get_data_addr(struct perf_event *event, struct pt_regs *
 	if (!(mmcra & MMCRA_SAMPLE_ENABLE) || sdar_valid)
 		*addrp = mfspr(SPRN_SDAR);
 
-	if (is_kernel_addr(mfspr(SPRN_SDAR)) && perf_allow_kernel(&event->attr) != 0)
+	if (is_kernel_addr(mfspr(SPRN_SDAR)) && event->attr.exclude_kernel)
 		*addrp = 0;
 }
 
@@ -507,7 +507,7 @@ static void power_pmu_bhrb_read(struct perf_event *event, struct cpu_hw_events *
 			 * addresses, hence include a check before filtering code
 			 */
 			if (!(ppmu->flags & PPMU_ARCH_31) &&
-				is_kernel_addr(addr) && perf_allow_kernel(&event->attr) != 0)
+			    is_kernel_addr(addr) && event->attr.exclude_kernel)
 				continue;
 
 			/* Branches are read most recent first (ie. mfbhrb 0 is
-- 
1.8.3.1


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2021-02-25 10:10 [PATCH V2] powerpc/perf: Fix handling of privilege level checks in perf interrupt context Athira Rajeev
2021-02-26  9:35 ` Peter Zijlstra
2021-03-01  2:22   ` Athira Rajeev
2021-03-14 10:01 ` Michael Ellerman

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