[PATCH v9 06/10] um: Calculate stub data address relative to stub code

[Benjamin Berg]

From: Benjamin Berg <benjamin.berg at intel.com>

Instead of using the current stack pointer, we can also use the current
instruction to calculate where the stub data is. With this the stub data
only needs to be aligned to a full page boundary.

Changing this has the advantage that we do not have a hole in the memory
space above the stub data (which would need to be explicitly cleared).

Another motivation to do this is that with the planned addition of a
SECCOMP based userspace the stack pointer may not be fully trustworthy.

Signed-off-by: Benjamin Berg <benjamin.berg at intel.com>
---
 arch/um/kernel/um_arch.c            |  6 ++----
 arch/x86/um/shared/sysdep/stub_32.h | 10 +++++++---
 arch/x86/um/shared/sysdep/stub_64.h |  8 +++++---
 3 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/arch/um/kernel/um_arch.c b/arch/um/kernel/um_arch.c
index 8e594cda6d77..44589cbd4174 100644
--- a/arch/um/kernel/um_arch.c
+++ b/arch/um/kernel/um_arch.c
@@ -325,10 +325,8 @@ int __init linux_main(int argc, char **argv)
 		add_arg(DEFAULT_COMMAND_LINE_CONSOLE);
 
 	host_task_size = os_get_top_address();
-	/* reserve a few pages for the stubs (taking care of data alignment) */
-	/* align the data portion */
-	BUILD_BUG_ON(!is_power_of_2(STUB_DATA_PAGES));
-	stub_start = (host_task_size - 1) & ~(STUB_DATA_PAGES * PAGE_SIZE - 1);
+	/* reserve a few pages for the stubs */
+	stub_start = host_task_size - STUB_DATA_PAGES * PAGE_SIZE;
 	/* another page for the code portion */
 	stub_start -= PAGE_SIZE;
 	host_task_size = stub_start;
diff --git a/arch/x86/um/shared/sysdep/stub_32.h b/arch/x86/um/shared/sysdep/stub_32.h
index 0b44a86dd346..631a18d0ff44 100644
--- a/arch/x86/um/shared/sysdep/stub_32.h
+++ b/arch/x86/um/shared/sysdep/stub_32.h
@@ -112,10 +112,14 @@ static __always_inline void *get_stub_data(void)
 	unsigned long ret;
 
 	asm volatile (
-		"movl %%esp,%0 ;"
-		"andl %1,%0"
+		"call _here_%=;"
+		"_here_%=:"
+		"popl %0;"
+		"andl %1, %0 ;"
+		"addl %2, %0 ;"
 		: "=a" (ret)
-		: "g" (~(STUB_DATA_PAGES * UM_KERN_PAGE_SIZE - 1)));
+		: "g" (~(UM_KERN_PAGE_SIZE - 1)),
+		  "g" (UM_KERN_PAGE_SIZE));
 
 	return (void *)ret;
 }
diff --git a/arch/x86/um/shared/sysdep/stub_64.h b/arch/x86/um/shared/sysdep/stub_64.h
index 8e4ff39dcade..17153dfd780a 100644
--- a/arch/x86/um/shared/sysdep/stub_64.h
+++ b/arch/x86/um/shared/sysdep/stub_64.h
@@ -117,10 +117,12 @@ static __always_inline void *get_stub_data(void)
 	unsigned long ret;
 
 	asm volatile (
-		"movq %%rsp,%0 ;"
-		"andq %1,%0"
+		"lea 0(%%rip), %0;"
+		"andq %1, %0 ;"
+		"addq %2, %0 ;"
 		: "=a" (ret)
-		: "g" (~(STUB_DATA_PAGES * UM_KERN_PAGE_SIZE - 1)));
+		: "g" (~(UM_KERN_PAGE_SIZE - 1)),
+		  "g" (UM_KERN_PAGE_SIZE));
 
 	return (void *)ret;
 }
-- 
2.46.0