If an application registers rseq, and ever switches to another pkey
protection (such that the rseq becomes inaccessible), then any
context switch will cause failure in __rseq_handle_notify_resume()
attempting to read/write struct rseq and/or rseq_cs. Since context
switches are asynchronous and are outside of the application control
(not part of the restricted code scope), temporarily switch to
pkey value that allows access to the 0 (default) PKEY.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
Fixes: d7822b1e24f2 ("rseq: Introduce restartable sequences system call")

---
Changes in v4:
 - Added Fixes tag

Changes in v3:
 - simplify control flow to always enable access to 0 pkey

Changes in v2:
 - fixed typos and reworded the comment
---
 kernel/rseq.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 2cb16091ec0ae..9d9c976d3b78c 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -10,6 +10,7 @@
 
 #include <linux/sched.h>
 #include <linux/uaccess.h>
+#include <linux/pkeys.h>
 #include <linux/syscalls.h>
 #include <linux/rseq.h>
 #include <linux/types.h>
@@ -402,11 +403,19 @@ static int rseq_ip_fixup(struct pt_regs *regs)
 void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 {
 	struct task_struct *t = current;
+	pkey_reg_t saved_pkey;
 	int ret, sig;
 
 	if (unlikely(t->flags & PF_EXITING))
 		return;
 
+	/*
+	 * Enable access to the default (0) pkey in case the thread has
+	 * currently disabled access to it and struct rseq/rseq_cs has
+	 * 0 pkey assigned (the only supported value for now).
+	 */
+	saved_pkey = enable_zero_pkey_val();
+
 	/*
 	 * 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
@@ -419,9 +428,11 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
 	}
 	if (unlikely(rseq_update_cpu_node_id(t)))
 		goto error;
+	write_pkey_val(saved_pkey);
 	return;
 
 error:
+	write_pkey_val(saved_pkey);
 	sig = ksig ? ksig->sig : 0;
 	force_sigsegv(sig);
 }
-- 
2.48.1.658.g4767266eb4-goog