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[PATCH v4 01/17] perf: Protect perf_guest_cbs with RCU



Protect perf_guest_cbs with RCU to fix multiple possible errors.  Luckily,
all paths that read perf_guest_cbs already require RCU protection, e.g. to
protect the callback chains, so only the direct perf_guest_cbs touchpoints
need to be modified.

Bug #1 is a simple lack of WRITE_ONCE/READ_ONCE behavior to ensure
perf_guest_cbs isn't reloaded between a !NULL check and a dereference.
Fixed via the READ_ONCE() in rcu_dereference().

Bug #2 is that on weakly-ordered architectures, updates to the callbacks
themselves are not guaranteed to be visible before the pointer is made
visible to readers.  Fixed by the smp_store_release() in
rcu_assign_pointer() when the new pointer is non-NULL.

Bug #3 is that, because the callbacks are global, it's possible for
readers to run in parallel with an unregisters, and thus a module
implementing the callbacks can be unloaded while readers are in flight,
resulting in a use-after-free.  Fixed by a synchronize_rcu() call when
unregistering callbacks.

Bug #1 escaped notice because it's extremely unlikely a compiler will
reload perf_guest_cbs in this sequence.  perf_guest_cbs does get reloaded
for future derefs, e.g. for ->is_user_mode(), but the ->is_in_guest()
guard all but guarantees the consumer will win the race, e.g. to nullify
perf_guest_cbs, KVM has to completely exit the guest and teardown down
all VMs before KVM start its module unload / unregister sequence.  This
also makes it all but impossible to encounter bug #3.

Bug #2 has not been a problem because all architectures that register
callbacks are strongly ordered and/or have a static set of callbacks.

But with help, unloading kvm_intel can trigger bug #1 e.g. wrapping
perf_guest_cbs with READ_ONCE in perf_misc_flags() while spamming
kvm_intel module load/unload leads to:

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] PREEMPT SMP
  CPU: 6 PID: 1825 Comm: stress Not tainted 5.14.0-rc2+ #459
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:perf_misc_flags+0x1c/0x70
  Call Trace:
   perf_prepare_sample+0x53/0x6b0
   perf_event_output_forward+0x67/0x160
   __perf_event_overflow+0x52/0xf0
   handle_pmi_common+0x207/0x300
   intel_pmu_handle_irq+0xcf/0x410
   perf_event_nmi_handler+0x28/0x50
   nmi_handle+0xc7/0x260
   default_do_nmi+0x6b/0x170
   exc_nmi+0x103/0x130
   asm_exc_nmi+0x76/0xbf

Fixes: 39447b386c84 ("perf: Enhance perf to allow for guest statistic 
collection from host")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/arm/kernel/perf_callchain.c   | 17 +++++++++++------
 arch/arm64/kernel/perf_callchain.c | 18 ++++++++++++------
 arch/csky/kernel/perf_callchain.c  |  6 ++++--
 arch/nds32/kernel/perf_event_cpu.c | 17 +++++++++++------
 arch/riscv/kernel/perf_callchain.c |  7 +++++--
 arch/x86/events/core.c             | 17 +++++++++++------
 arch/x86/events/intel/core.c       |  9 ++++++---
 include/linux/perf_event.h         | 13 ++++++++++++-
 kernel/events/core.c               | 13 ++++++++++---
 9 files changed, 82 insertions(+), 35 deletions(-)

diff --git a/arch/arm/kernel/perf_callchain.c b/arch/arm/kernel/perf_callchain.c
index 3b69a76d341e..1626dfc6f6ce 100644
--- a/arch/arm/kernel/perf_callchain.c
+++ b/arch/arm/kernel/perf_callchain.c
@@ -62,9 +62,10 @@ user_backtrace(struct frame_tail __user *tail,
 void
 perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct pt_regs 
*regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        struct frame_tail __user *tail;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                /* We don't support guest os callchain now */
                return;
        }
@@ -98,9 +99,10 @@ callchain_trace(struct stackframe *fr,
 void
 perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs 
*regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        struct stackframe fr;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                /* We don't support guest os callchain now */
                return;
        }
@@ -111,18 +113,21 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx 
*entry, struct pt_regs *re
 
 unsigned long perf_instruction_pointer(struct pt_regs *regs)
 {
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
-               return perf_guest_cbs->get_guest_ip();
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
+
+       if (guest_cbs && guest_cbs->is_in_guest())
+               return guest_cbs->get_guest_ip();
 
        return instruction_pointer(regs);
 }
 
 unsigned long perf_misc_flags(struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        int misc = 0;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
-               if (perf_guest_cbs->is_user_mode())
+       if (guest_cbs && guest_cbs->is_in_guest()) {
+               if (guest_cbs->is_user_mode())
                        misc |= PERF_RECORD_MISC_GUEST_USER;
                else
                        misc |= PERF_RECORD_MISC_GUEST_KERNEL;
diff --git a/arch/arm64/kernel/perf_callchain.c 
b/arch/arm64/kernel/perf_callchain.c
index 4a72c2727309..86d9f2013172 100644
--- a/arch/arm64/kernel/perf_callchain.c
+++ b/arch/arm64/kernel/perf_callchain.c
@@ -102,7 +102,9 @@ compat_user_backtrace(struct compat_frame_tail __user *tail,
 void perf_callchain_user(struct perf_callchain_entry_ctx *entry,
                         struct pt_regs *regs)
 {
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
+
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                /* We don't support guest os callchain now */
                return;
        }
@@ -147,9 +149,10 @@ static bool callchain_trace(void *data, unsigned long pc)
 void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
                           struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        struct stackframe frame;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                /* We don't support guest os callchain now */
                return;
        }
@@ -160,18 +163,21 @@ void perf_callchain_kernel(struct 
perf_callchain_entry_ctx *entry,
 
 unsigned long perf_instruction_pointer(struct pt_regs *regs)
 {
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
-               return perf_guest_cbs->get_guest_ip();
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
+
+       if (guest_cbs && guest_cbs->is_in_guest())
+               return guest_cbs->get_guest_ip();
 
        return instruction_pointer(regs);
 }
 
 unsigned long perf_misc_flags(struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        int misc = 0;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
-               if (perf_guest_cbs->is_user_mode())
+       if (guest_cbs && guest_cbs->is_in_guest()) {
+               if (guest_cbs->is_user_mode())
                        misc |= PERF_RECORD_MISC_GUEST_USER;
                else
                        misc |= PERF_RECORD_MISC_GUEST_KERNEL;
diff --git a/arch/csky/kernel/perf_callchain.c 
b/arch/csky/kernel/perf_callchain.c
index ab55e98ee8f6..35318a635a5f 100644
--- a/arch/csky/kernel/perf_callchain.c
+++ b/arch/csky/kernel/perf_callchain.c
@@ -86,10 +86,11 @@ static unsigned long user_backtrace(struct 
perf_callchain_entry_ctx *entry,
 void perf_callchain_user(struct perf_callchain_entry_ctx *entry,
                         struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        unsigned long fp = 0;
 
        /* C-SKY does not support virtualization. */
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
+       if (guest_cbs && guest_cbs->is_in_guest())
                return;
 
        fp = regs->regs[4];
@@ -110,10 +111,11 @@ void perf_callchain_user(struct perf_callchain_entry_ctx 
*entry,
 void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
                           struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        struct stackframe fr;
 
        /* C-SKY does not support virtualization. */
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                pr_warn("C-SKY does not support perf in guest mode!");
                return;
        }
diff --git a/arch/nds32/kernel/perf_event_cpu.c 
b/arch/nds32/kernel/perf_event_cpu.c
index 0ce6f9f307e6..f38791960781 100644
--- a/arch/nds32/kernel/perf_event_cpu.c
+++ b/arch/nds32/kernel/perf_event_cpu.c
@@ -1363,6 +1363,7 @@ void
 perf_callchain_user(struct perf_callchain_entry_ctx *entry,
                    struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        unsigned long fp = 0;
        unsigned long gp = 0;
        unsigned long lp = 0;
@@ -1371,7 +1372,7 @@ perf_callchain_user(struct perf_callchain_entry_ctx 
*entry,
 
        leaf_fp = 0;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                /* We don't support guest os callchain now */
                return;
        }
@@ -1479,9 +1480,10 @@ void
 perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
                      struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        struct stackframe fr;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                /* We don't support guest os callchain now */
                return;
        }
@@ -1493,20 +1495,23 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx 
*entry,
 
 unsigned long perf_instruction_pointer(struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
+
        /* However, NDS32 does not support virtualization */
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
-               return perf_guest_cbs->get_guest_ip();
+       if (guest_cbs && guest_cbs->is_in_guest())
+               return guest_cbs->get_guest_ip();
 
        return instruction_pointer(regs);
 }
 
 unsigned long perf_misc_flags(struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        int misc = 0;
 
        /* However, NDS32 does not support virtualization */
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
-               if (perf_guest_cbs->is_user_mode())
+       if (guest_cbs && guest_cbs->is_in_guest()) {
+               if (guest_cbs->is_user_mode())
                        misc |= PERF_RECORD_MISC_GUEST_USER;
                else
                        misc |= PERF_RECORD_MISC_GUEST_KERNEL;
diff --git a/arch/riscv/kernel/perf_callchain.c 
b/arch/riscv/kernel/perf_callchain.c
index 0bb1854dce83..8ecfc4c128bc 100644
--- a/arch/riscv/kernel/perf_callchain.c
+++ b/arch/riscv/kernel/perf_callchain.c
@@ -56,10 +56,11 @@ static unsigned long user_backtrace(struct 
perf_callchain_entry_ctx *entry,
 void perf_callchain_user(struct perf_callchain_entry_ctx *entry,
                         struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        unsigned long fp = 0;
 
        /* RISC-V does not support perf in guest mode. */
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
+       if (guest_cbs && guest_cbs->is_in_guest())
                return;
 
        fp = regs->s0;
@@ -78,8 +79,10 @@ static bool fill_callchain(void *entry, unsigned long pc)
 void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry,
                           struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
+
        /* RISC-V does not support perf in guest mode. */
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                pr_warn("RISC-V does not support perf in guest mode!");
                return;
        }
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 38b2c779146f..32cec290d3ad 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -2768,10 +2768,11 @@ static bool perf_hw_regs(struct pt_regs *regs)
 void
 perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs 
*regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        struct unwind_state state;
        unsigned long addr;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                /* TODO: We don't support guest os callchain now */
                return;
        }
@@ -2871,10 +2872,11 @@ perf_callchain_user32(struct pt_regs *regs, struct 
perf_callchain_entry_ctx *ent
 void
 perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct pt_regs 
*regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        struct stack_frame frame;
        const struct stack_frame __user *fp;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
+       if (guest_cbs && guest_cbs->is_in_guest()) {
                /* TODO: We don't support guest os callchain now */
                return;
        }
@@ -2951,18 +2953,21 @@ static unsigned long code_segment_base(struct pt_regs 
*regs)
 
 unsigned long perf_instruction_pointer(struct pt_regs *regs)
 {
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
-               return perf_guest_cbs->get_guest_ip();
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
+
+       if (guest_cbs && guest_cbs->is_in_guest())
+               return guest_cbs->get_guest_ip();
 
        return regs->ip + code_segment_base(regs);
 }
 
 unsigned long perf_misc_flags(struct pt_regs *regs)
 {
+       struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
        int misc = 0;
 
-       if (perf_guest_cbs && perf_guest_cbs->is_in_guest()) {
-               if (perf_guest_cbs->is_user_mode())
+       if (guest_cbs && guest_cbs->is_in_guest()) {
+               if (guest_cbs->is_user_mode())
                        misc |= PERF_RECORD_MISC_GUEST_USER;
                else
                        misc |= PERF_RECORD_MISC_GUEST_KERNEL;
diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 603964408d2d..cde29bd5bb53 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -2837,6 +2837,7 @@ static int handle_pmi_common(struct pt_regs *regs, u64 
status)
 {
        struct perf_sample_data data;
        struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+       struct perf_guest_info_callbacks *guest_cbs;
        int bit;
        int handled = 0;
        u64 intel_ctrl = hybrid(cpuc->pmu, intel_ctrl);
@@ -2903,9 +2904,11 @@ static int handle_pmi_common(struct pt_regs *regs, u64 
status)
         */
        if (__test_and_clear_bit(GLOBAL_STATUS_TRACE_TOPAPMI_BIT, (unsigned 
long *)&status)) {
                handled++;
-               if (unlikely(perf_guest_cbs && perf_guest_cbs->is_in_guest() &&
-                       perf_guest_cbs->handle_intel_pt_intr))
-                       perf_guest_cbs->handle_intel_pt_intr();
+
+               guest_cbs = perf_get_guest_cbs();
+               if (unlikely(guest_cbs && guest_cbs->is_in_guest() &&
+                            guest_cbs->handle_intel_pt_intr))
+                       guest_cbs->handle_intel_pt_intr();
                else
                        intel_pt_interrupt();
        }
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 0dcfd265beed..318c489b735b 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1240,7 +1240,18 @@ extern void perf_event_bpf_event(struct bpf_prog *prog,
                                 enum perf_bpf_event_type type,
                                 u16 flags);
 
-extern struct perf_guest_info_callbacks *perf_guest_cbs;
+extern struct perf_guest_info_callbacks __rcu *perf_guest_cbs;
+static inline struct perf_guest_info_callbacks *perf_get_guest_cbs(void)
+{
+       /*
+        * Callbacks are RCU-protected and must be READ_ONCE to avoid reloading
+        * the callbacks between a !NULL check and dereferences, to ensure
+        * pending stores/changes to the callback pointers are visible before a
+        * non-NULL perf_guest_cbs is visible to readers, and to prevent a
+        * module from unloading callbacks while readers are active.
+        */
+       return rcu_dereference(perf_guest_cbs);
+}
 extern int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks 
*callbacks);
 extern int perf_unregister_guest_info_callbacks(struct 
perf_guest_info_callbacks *callbacks);
 
diff --git a/kernel/events/core.c b/kernel/events/core.c
index f2253ea729a2..0cc775f702f8 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6526,18 +6526,25 @@ static void perf_pending_event(struct irq_work *entry)
  * Later on, we might change it to a list if there is
  * another virtualization implementation supporting the callbacks.
  */
-struct perf_guest_info_callbacks *perf_guest_cbs;
+struct perf_guest_info_callbacks __rcu *perf_guest_cbs;
 
 int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
 {
-       perf_guest_cbs = cbs;
+       if (WARN_ON_ONCE(rcu_access_pointer(perf_guest_cbs)))
+               return -EBUSY;
+
+       rcu_assign_pointer(perf_guest_cbs, cbs);
        return 0;
 }
 EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
 
 int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
 {
-       perf_guest_cbs = NULL;
+       if (WARN_ON_ONCE(rcu_access_pointer(perf_guest_cbs) != cbs))
+               return -EINVAL;
+
+       rcu_assign_pointer(perf_guest_cbs, NULL);
+       synchronize_rcu();
        return 0;
 }
 EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
-- 
2.34.0.rc0.344.g81b53c2807-goog




 


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