(CC'ing LSM and SELinux lists; the initial message can be found here: https://lore.kernel.org/linuxppc-dev/1614061909-1734-1-git-send-email-atrajeev@xxxxxxxxxxxxxxxxxx/T/) On Tue, Feb 23, 2021 at 7:32 AM Athira Rajeev <atrajeev@xxxxxxxxxxxxxxxxxx> wrote: > > 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_exception+0x4c/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 is 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. > But in case of powerpc PMU, we need the privilege checks for specific > samples from branch history ring buffer and sampling register values. > 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") > > As a fix, patch caches 'perf_allow_kernel' value in event_init in > 'pmu_private' field of perf_event. The cached value is used in the > PMI code path. > > Suggested-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > Signed-off-by: Athira Rajeev <atrajeev@xxxxxxxxxxxxxxxxxx> > --- > arch/powerpc/perf/core-book3s.c | 16 ++++++++++++++-- > 1 file changed, 14 insertions(+), 2 deletions(-) > > diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c > index 4b4319d8..9e9f67f 100644 > --- a/arch/powerpc/perf/core-book3s.c > +++ b/arch/powerpc/perf/core-book3s.c > @@ -189,6 +189,11 @@ static inline unsigned long perf_ip_adjust(struct pt_regs *regs) > return 0; > } > > +static bool event_allow_kernel(struct perf_event *event) > +{ > + return (bool)event->pmu_private; > +} > + > /* > * The user wants a data address recorded. > * If we're not doing instruction sampling, give them the SDAR > @@ -222,7 +227,7 @@ 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_allow_kernel(event)) > *addrp = 0; > } > > @@ -507,7 +512,7 @@ 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_allow_kernel(event)) > continue; > > /* Branches are read most recent first (ie. mfbhrb 0 is > @@ -2049,6 +2054,13 @@ static int power_pmu_event_init(struct perf_event *event) > if (err) > return -EINVAL; > > + /* > + * We (ab)use pmu_private to cache the result of perf_allow_kernel(). We > + * need access to that result at interrupt time, but can't call > + * perf_allow_kernel() directly from interrupt context. > + */ > + event->pmu_private = (void *)(long)(perf_allow_kernel(&event->attr) == 0); I don't think you need this. Unless I'm missing something, you can simply use "event->attr.exclude_kernel" in place of "!event_allow_kernel(event)". If it is set, then there must have been a successful perf_allow_kernel() check in perf_event_open(2) before the event was created. power_pmu_event_init() would be called shortly after via perf_event_alloc() -> perf_init_event(), so I don't think this additional check would add much value. > + > event->hw.config = events[n]; > event->hw.event_base = cflags[n]; > event->hw.last_period = event->hw.sample_period; > -- > 1.8.3.1 > -- Ondrej Mosnacek Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.
