[PATCH] um: fix stub location calculation

[Johannes Berg]

From: Johannes Berg <johannes.berg at intel.com>

In commit 9f0b4807a44f ("um: rework userspace stubs to not hard-code
stub location") I changed stub_segv_handler() to do a calculation with
a pointer to a stack variable to find the data page that we're using
for the stack and the rest of the data. This same commit was meant to
do it as well for stub_clone_handler(), but the change inadvertently
went into commit 84b2789d6115 ("um: separate child and parent errors
in clone stub") instead.

This was reported to not be compiled correctly by gcc 5, causing the
code to crash here. I'm not sure why, perhaps it's UB because the var
isn't initialized? In any case, this trick always seemed bad, so just
create a new inline function that does the calculation in assembly.

Reported-by: subashab at codeaurora.org
Fixes: 9f0b4807a44f ("um: rework userspace stubs to not hard-code stub location")
Fixes: 84b2789d6115 ("um: separate child and parent errors in clone stub")
Signed-off-by: Johannes Berg <johannes.berg at intel.com>
---
 arch/um/kernel/skas/clone.c         |  3 +--
 arch/x86/um/shared/sysdep/stub_32.h | 12 ++++++++++++
 arch/x86/um/shared/sysdep/stub_64.h | 12 ++++++++++++
 arch/x86/um/stub_segv.c             |  3 +--
 4 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/arch/um/kernel/skas/clone.c b/arch/um/kernel/skas/clone.c
index 592cdb138441..c0d2d592b31b 100644
--- a/arch/um/kernel/skas/clone.c
+++ b/arch/um/kernel/skas/clone.c
@@ -24,8 +24,7 @@
 void __attribute__ ((__section__ (".__syscall_stub")))
 stub_clone_handler(void)
 {
-	int stack;
-	struct stub_data *data = (void *) ((unsigned long)&stack & ~(UM_KERN_PAGE_SIZE - 1));
+	struct stub_data *data = get_stub_page();
 	long err;
 
 	err = stub_syscall2(__NR_clone, CLONE_PARENT | CLONE_FILES | SIGCHLD,
diff --git a/arch/x86/um/shared/sysdep/stub_32.h b/arch/x86/um/shared/sysdep/stub_32.h
index b95db9daf0e8..4c6c2be0c899 100644
--- a/arch/x86/um/shared/sysdep/stub_32.h
+++ b/arch/x86/um/shared/sysdep/stub_32.h
@@ -101,4 +101,16 @@ static inline void remap_stack_and_trap(void)
 		"memory");
 }
 
+static __always_inline void *get_stub_page(void)
+{
+	unsigned long ret;
+
+	asm volatile (
+		"movl %%esp,%0 ;"
+		"andl %1,%0"
+		: "=a" (ret)
+		: "g" (~(UM_KERN_PAGE_SIZE - 1)));
+
+	return (void *)ret;
+}
 #endif
diff --git a/arch/x86/um/shared/sysdep/stub_64.h b/arch/x86/um/shared/sysdep/stub_64.h
index 6e2626b77a2e..e9c4b2b38803 100644
--- a/arch/x86/um/shared/sysdep/stub_64.h
+++ b/arch/x86/um/shared/sysdep/stub_64.h
@@ -108,4 +108,16 @@ static inline void remap_stack_and_trap(void)
 		__syscall_clobber, "r10", "r8", "r9");
 }
 
+static __always_inline void *get_stub_page(void)
+{
+	unsigned long ret;
+
+	asm volatile (
+		"movq %%rsp,%0 ;"
+		"andq %1,%0"
+		: "=a" (ret)
+		: "g" (~(UM_KERN_PAGE_SIZE - 1)));
+
+	return (void *)ret;
+}
 #endif
diff --git a/arch/x86/um/stub_segv.c b/arch/x86/um/stub_segv.c
index 21836eaf1725..f7eefba034f9 100644
--- a/arch/x86/um/stub_segv.c
+++ b/arch/x86/um/stub_segv.c
@@ -11,9 +11,8 @@
 void __attribute__ ((__section__ (".__syscall_stub")))
 stub_segv_handler(int sig, siginfo_t *info, void *p)
 {
-	int stack;
+	struct faultinfo *f = get_stub_page();
 	ucontext_t *uc = p;
-	struct faultinfo *f = (void *)(((unsigned long)&stack) & ~(UM_KERN_PAGE_SIZE - 1));
 
 	GET_FAULTINFO_FROM_MC(*f, &uc->uc_mcontext);
 	trap_myself();
-- 
2.31.1