[RESEND][PATCH 3/3] rseq: Ensure SIGBUS delivered on memory failure

[Andrew Zaborowski <andrew.zaborowski@intel.com>]

Uncorrected memory errors for user pages are signaled to processes
using SIGBUS or, if the error happens in a syscall, an error retval
from the syscall.  The SIGBUS is documented in
Documentation/mm/hwpoison.rst#failure-recovery-modes

Once a user task sets t->rseq in the rseq() syscall, if the kernel
cannot access the memory pointed to by t->rseq->rseq_cs, that initial
rseq() and all future syscalls should return an error so understandably
the code just kills the task.

To ensure that SIGBUS is used set the new t->kill_on_efault flag and
run queued task work on rseq_get_rseq_cs() errors to give memory_failure
the chance to run.

Note: the rseq checks run inside resume_user_mode_work() so whenever
_TIF_NOTIFY_RESUME is set.  They do not run on every syscall exit so
I'm not concerned that these extra flag operations are in a hot path,
except with CONFIG_DEBUG_RSEQ.

Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
---
 kernel/rseq.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 9de6e35fe..c5809cd13 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -13,6 +13,7 @@
 #include <linux/syscalls.h>
 #include <linux/rseq.h>
 #include <linux/types.h>
+#include <linux/task_work.h>
 #include <asm/ptrace.h>
 
 #define CREATE_TRACE_POINTS
@@ -320,6 +321,8 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 	if (unlikely(t->flags & PF_EXITING))
 		return;
 
+	t->kill_on_efault = true;
+
 	/*
 	 * regs is NULL if and only if the caller is in a syscall path.  Skip
 	 * fixup and leave rseq_cs as is so that rseq_sycall() will detect and
@@ -330,13 +333,18 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 		if (unlikely(ret < 0))
 			goto error;
 	}
-	if (unlikely(rseq_update_cpu_node_id(t)))
-		goto error;
-	return;
+	if (likely(!rseq_update_cpu_node_id(t)))
+		goto out;
 
 error:
+	/* Allow task work to override signr */
+	task_work_run();
+
 	sig = ksig ? ksig->sig : 0;
 	force_sigsegv(sig);
+
+out:
+	t->kill_on_efault = false;
 }
 
 #ifdef CONFIG_DEBUG_RSEQ
@@ -353,8 +361,17 @@ void rseq_syscall(struct pt_regs *regs)
 
 	if (!t->rseq)
 		return;
-	if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
+
+	t->kill_on_efault = true;
+
+	if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) {
+		/* Allow task work to override signr */
+		task_work_run();
+
 		force_sig(SIGSEGV);
+	}
+
+	t->kill_on_efault = false;
 }
 
 #endif
-- 
2.43.0