[PATCH] Yama: use READ_ONCE() when reading ptrace_scope

[Jann Horn <jannh@google.com>]

ptrace_scope can be modified concurrently. To ensure that the compiler only
reads ptrace_scope once, use READ_ONCE() here. (In practice, at least the
version of gcc on my machine only generates a single read anyway, and it
seems unlikely that a compiler would do something different.)

This also serves as documentation for the reader.

Signed-off-by: Jann Horn <jannh@google.com>
---
 security/yama/yama_lsm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
index ffda91a4a1aa..3b18e4455f53 100644
--- a/security/yama/yama_lsm.c
+++ b/security/yama/yama_lsm.c
@@ -362,7 +362,7 @@ static int yama_ptrace_access_check(struct task_struct *child,
 
 	/* require ptrace target be a child of ptracer on attach */
 	if (mode & PTRACE_MODE_ATTACH) {
-		switch (ptrace_scope) {
+		switch (READ_ONCE(ptrace_scope)) {
 		case YAMA_SCOPE_DISABLED:
 			/* No additional restrictions. */
 			break;
@@ -404,7 +404,7 @@ int yama_ptrace_traceme(struct task_struct *parent)
 	int rc = 0;
 
 	/* Only disallow PTRACE_TRACEME on more aggressive settings. */
-	switch (ptrace_scope) {
+	switch (READ_ONCE(ptrace_scope)) {
 	case YAMA_SCOPE_CAPABILITY:
 		if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
 			rc = -EPERM;
-- 
2.19.0.rc1.350.ge57e33dbd1-goog