signal: Deliver SIGTRAP on perf event asynchronously if blocked

With SIGTRAP on perf events, we have encountered termination of
processes due to user space attempting to block delivery of SIGTRAP.
Consider this case:

    <set up SIGTRAP on a perf event>
    ...
    sigset_t s;
    sigemptyset(&s);
    sigaddset(&s, SIGTRAP | <and others>);
    sigprocmask(SIG_BLOCK, &s, ...);
    ...
    <perf event triggers>

When the perf event triggers, while SIGTRAP is blocked, force_sig_perf()
will force the signal, but revert back to the default handler, thus
terminating the task.

This makes sense for error conditions, but not so much for explicitly
requested monitoring. However, the expectation is still that signals
generated by perf events are synchronous, which will no longer be the
case if the signal is blocked and delivered later.

To give user space the ability to clearly distinguish synchronous from
asynchronous signals, introduce siginfo_t::si_perf_flags and
TRAP_PERF_FLAG_ASYNC (opted for flags in case more binary information is
required in future).

The resolution to the problem is then to (a) no longer force the signal
(avoiding the terminations), but (b) tell user space via si_perf_flags
if the signal was synchronous or not, so that such signals can be
handled differently (e.g. let user space decide to ignore or consider
the data imprecise).

The alternative of making the kernel ignore SIGTRAP on perf events if
the signal is blocked may work for some usecases, but likely causes
issues in others that then have to revert back to interception of
sigprocmask() (which we want to avoid). [ A concrete example: when using
breakpoint perf events to track data-flow, in a region of code where
signals are blocked, data-flow can no longer be tracked accurately.
When a relevant asynchronous signal is received after unblocking the
signal, the data-flow tracking logic needs to know its state is
imprecise. ]

Fixes: 97ba62b27867 ("perf: Add support for SIGTRAP on perf events")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://lore.kernel.org/r/20220404111204.935357-1-elver@google.com

12 files changed, 35 insertions, 5 deletions

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index 459abc5d1819..ea128e32e8ca 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c
@@ -708,6 +708,7 @@ static_assert(offsetof(siginfo_t, si_upper)	== 0x18);
 static_assert(offsetof(siginfo_t, si_pkey)	== 0x14);
 static_assert(offsetof(siginfo_t, si_perf_data)	== 0x10);
 static_assert(offsetof(siginfo_t, si_perf_type)	== 0x14);
+static_assert(offsetof(siginfo_t, si_perf_flags) == 0x18);
 static_assert(offsetof(siginfo_t, si_band)	== 0x0c);
 static_assert(offsetof(siginfo_t, si_fd)	== 0x10);
 static_assert(offsetof(siginfo_t, si_call_addr)	== 0x0c);
diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
index 4a4122ef6f39..41b5d9d3672a 100644
--- a/arch/arm64/kernel/signal.c
+++ b/arch/arm64/kernel/signal.c
@@ -1011,6 +1011,7 @@ static_assert(offsetof(siginfo_t, si_upper)	== 0x28);
 static_assert(offsetof(siginfo_t, si_pkey)	== 0x20);
 static_assert(offsetof(siginfo_t, si_perf_data)	== 0x18);
 static_assert(offsetof(siginfo_t, si_perf_type)	== 0x20);
+static_assert(offsetof(siginfo_t, si_perf_flags) == 0x24);
 static_assert(offsetof(siginfo_t, si_band)	== 0x10);
 static_assert(offsetof(siginfo_t, si_fd)	== 0x18);
 static_assert(offsetof(siginfo_t, si_call_addr)	== 0x10);
diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
index d984282b979f..4700f8522d27 100644
--- a/arch/arm64/kernel/signal32.c
+++ b/arch/arm64/kernel/signal32.c
@@ -487,6 +487,7 @@ static_assert(offsetof(compat_siginfo_t, si_upper)	== 0x18);
 static_assert(offsetof(compat_siginfo_t, si_pkey)	== 0x14);
 static_assert(offsetof(compat_siginfo_t, si_perf_data)	== 0x10);
 static_assert(offsetof(compat_siginfo_t, si_perf_type)	== 0x14);
+static_assert(offsetof(compat_siginfo_t, si_perf_flags)	== 0x18);
 static_assert(offsetof(compat_siginfo_t, si_band)	== 0x0c);
 static_assert(offsetof(compat_siginfo_t, si_fd)		== 0x10);
 static_assert(offsetof(compat_siginfo_t, si_call_addr)	== 0x0c);
diff --git a/arch/m68k/kernel/signal.c b/arch/m68k/kernel/signal.c
index 49533f65958a..b9f6908a31bc 100644
--- a/arch/m68k/kernel/signal.c
+++ b/arch/m68k/kernel/signal.c
@@ -625,6 +625,7 @@ static inline void siginfo_build_tests(void)
 	/* _sigfault._perf */
 	BUILD_BUG_ON(offsetof(siginfo_t, si_perf_data) != 0x10);
 	BUILD_BUG_ON(offsetof(siginfo_t, si_perf_type) != 0x14);
+	BUILD_BUG_ON(offsetof(siginfo_t, si_perf_flags) != 0x18);
 
 	/* _sigpoll */
 	BUILD_BUG_ON(offsetof(siginfo_t, si_band)   != 0x0c);
diff --git a/arch/sparc/kernel/signal32.c b/arch/sparc/kernel/signal32.c
index f9fe502b81c6..dad38960d1a8 100644
--- a/arch/sparc/kernel/signal32.c
+++ b/arch/sparc/kernel/signal32.c
@@ -779,5 +779,6 @@ static_assert(offsetof(compat_siginfo_t, si_upper)	== 0x18);
 static_assert(offsetof(compat_siginfo_t, si_pkey)	== 0x14);
 static_assert(offsetof(compat_siginfo_t, si_perf_data)	== 0x10);
 static_assert(offsetof(compat_siginfo_t, si_perf_type)	== 0x14);
+static_assert(offsetof(compat_siginfo_t, si_perf_flags)	== 0x18);
 static_assert(offsetof(compat_siginfo_t, si_band)	== 0x0c);
 static_assert(offsetof(compat_siginfo_t, si_fd)		== 0x10);
diff --git a/arch/sparc/kernel/signal_64.c b/arch/sparc/kernel/signal_64.c
index 8b9fc76cd3e0..570e43e6fda5 100644
--- a/arch/sparc/kernel/signal_64.c
+++ b/arch/sparc/kernel/signal_64.c
@@ -590,5 +590,6 @@ static_assert(offsetof(siginfo_t, si_upper)	== 0x28);
 static_assert(offsetof(siginfo_t, si_pkey)	== 0x20);
 static_assert(offsetof(siginfo_t, si_perf_data)	== 0x18);
 static_assert(offsetof(siginfo_t, si_perf_type)	== 0x20);
+static_assert(offsetof(siginfo_t, si_perf_flags) == 0x24);
 static_assert(offsetof(siginfo_t, si_band)	== 0x10);
 static_assert(offsetof(siginfo_t, si_fd)	== 0x14);
diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
index b52407c56000..879ef8c72f5c 100644
--- a/arch/x86/kernel/signal_compat.c
+++ b/arch/x86/kernel/signal_compat.c
@@ -149,8 +149,10 @@ static inline void signal_compat_build_tests(void)
 
 	BUILD_BUG_ON(offsetof(siginfo_t, si_perf_data) != 0x18);
 	BUILD_BUG_ON(offsetof(siginfo_t, si_perf_type) != 0x20);
+	BUILD_BUG_ON(offsetof(siginfo_t, si_perf_flags) != 0x24);
 	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_perf_data) != 0x10);
 	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_perf_type) != 0x14);
+	BUILD_BUG_ON(offsetof(compat_siginfo_t, si_perf_flags) != 0x18);
 
 	CHECK_CSI_OFFSET(_sigpoll);
 	CHECK_CSI_SIZE  (_sigpoll, 2*sizeof(int));
diff --git a/include/linux/compat.h b/include/linux/compat.h
index 1c758b0e0359..01fddf72a81f 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -235,6 +235,7 @@ typedef struct compat_siginfo {
 				struct {
 					compat_ulong_t _data;
 					u32 _type;
+					u32 _flags;
 				} _perf;
 			};
 		} _sigfault;
diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 3c8b34876744..bab7cc56b13a 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -320,7 +320,7 @@ int send_sig_mceerr(int code, void __user *, short, struct task_struct *);
 
 int force_sig_bnderr(void __user *addr, void __user *lower, void __user *upper);
 int force_sig_pkuerr(void __user *addr, u32 pkey);
-int force_sig_perf(void __user *addr, u32 type, u64 sig_data);
+int send_sig_perf(void __user *addr, u32 type, u64 sig_data);
 
 int force_sig_ptrace_errno_trap(int errno, void __user *addr);
 int force_sig_fault_trapno(int sig, int code, void __user *addr, int trapno);
diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index 3ba180f550d7..ffbe4cec9f32 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -99,6 +99,7 @@ union __sifields {
 			struct {
 				unsigned long _data;
 				__u32 _type;
+				__u32 _flags;
 			} _perf;
 		};
 	} _sigfault;
@@ -164,6 +165,7 @@ typedef struct siginfo {
 #define si_pkey		_sifields._sigfault._addr_pkey._pkey
 #define si_perf_data	_sifields._sigfault._perf._data
 #define si_perf_type	_sifields._sigfault._perf._type
+#define si_perf_flags	_sifields._sigfault._perf._flags
 #define si_band		_sifields._sigpoll._band
 #define si_fd		_sifields._sigpoll._fd
 #define si_call_addr	_sifields._sigsys._call_addr
@@ -271,6 +273,11 @@ typedef struct siginfo {
  */
 
 /*
+ * Flags for si_perf_flags if SIGTRAP si_code is TRAP_PERF.
+ */
+#define TRAP_PERF_FLAG_ASYNC (1u << 0)
+
+/*
  * SIGCHLD si_codes
  */
 #define CLD_EXITED	1	/* child has exited */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index cfde994ce61c..6eafb1b0ad4a 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6533,8 +6533,8 @@ static void perf_sigtrap(struct perf_event *event)
 	if (current->flags & PF_EXITING)
 		return;
 
-	force_sig_perf((void __user *)event->pending_addr,
-		       event->attr.type, event->attr.sig_data);
+	send_sig_perf((void __user *)event->pending_addr,
+		      event->attr.type, event->attr.sig_data);
 }
 
 static void perf_pending_event_disable(struct perf_event *event)
diff --git a/kernel/signal.c b/kernel/signal.c
index 30cd1ca43bcd..e43bc2a692f5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1805,7 +1805,7 @@ int force_sig_pkuerr(void __user *addr, u32 pkey)
 }
 #endif
 
-int force_sig_perf(void __user *addr, u32 type, u64 sig_data)
+int send_sig_perf(void __user *addr, u32 type, u64 sig_data)
 {
 	struct kernel_siginfo info;
 
@@ -1817,7 +1817,18 @@ int force_sig_perf(void __user *addr, u32 type, u64 sig_data)
 	info.si_perf_data = sig_data;
 	info.si_perf_type = type;
 
-	return force_sig_info(&info);
+	/*
+	 * Signals generated by perf events should not terminate the whole
+	 * process if SIGTRAP is blocked, however, delivering the signal
+	 * asynchronously is better than not delivering at all. But tell user
+	 * space if the signal was asynchronous, so it can clearly be
+	 * distinguished from normal synchronous ones.
+	 */
+	info.si_perf_flags = sigismember(&current->blocked, info.si_signo) ?
+				     TRAP_PERF_FLAG_ASYNC :
+				     0;
+
+	return send_sig_info(info.si_signo, &info, current);
 }
 
 /**
@@ -3432,6 +3443,7 @@ void copy_siginfo_to_external32(struct compat_siginfo *to,
 		to->si_addr = ptr_to_compat(from->si_addr);
 		to->si_perf_data = from->si_perf_data;
 		to->si_perf_type = from->si_perf_type;
+		to->si_perf_flags = from->si_perf_flags;
 		break;
 	case SIL_CHLD:
 		to->si_pid = from->si_pid;
@@ -3509,6 +3521,7 @@ static int post_copy_siginfo_from_user32(kernel_siginfo_t *to,
 		to->si_addr = compat_ptr(from->si_addr);
 		to->si_perf_data = from->si_perf_data;
 		to->si_perf_type = from->si_perf_type;
+		to->si_perf_flags = from->si_perf_flags;
 		break;
 	case SIL_CHLD:
 		to->si_pid    = from->si_pid;
@@ -4722,6 +4735,7 @@ static inline void siginfo_buildtime_checks(void)
 	CHECK_OFFSET(si_pkey);
 	CHECK_OFFSET(si_perf_data);
 	CHECK_OFFSET(si_perf_type);
+	CHECK_OFFSET(si_perf_flags);
 
 	/* sigpoll */
 	CHECK_OFFSET(si_band);