[PATCH v2] um: Fix stack pointer alignment

[YiFei Zhu]

GCC assumes that stack is aligned to 16-byte on call sites [1].
Since GCC 8, GCC began using 16-byte aligned SSE instructions to
implement assignments to structs on stack. This affects
os-Linux/sigio.c, write_sigio_thread:

  struct pollfds *fds, tmp;
  tmp = current_poll;

Note that struct pollfds is exactly 16 bytes in size.
GCC 8+ generates assembly similar to:

  movdqa (%rdi),%xmm0
  movaps %xmm0,-0x50(%rbp)

This is an issue, because movaps will #GP if -0x50(%rbp) is not
aligned to 16 bytes [2], and how rbp gets assigned to is via glibc
clone thread_start, then function prologue, going though execution
trace similar to (showing only relevant instructions):

  sub    $0x10,%rsi
  mov    %rcx,0x8(%rsi)
  mov    %rdi,(%rsi)
  syscall
  pop    %rax
  pop    %rdi
  callq  *%rax
  push   %rbp
  mov    %rsp,%rbp

The stack pointer always points to the topmost element on stack,
rather then the space right above the topmost. On push, the
pointer decrements first before writing to the memory pointed to
by it. Therefore, there is no need to have the stack pointer
pointer always point to valid memory unless the stack is poped;
so the `- sizeof(void *)` in the code is unnecessary.

On the other hand, glibc reserves the 16 bytes it needs on stack
and pops itself, so by the call instruction the stack pointer
is exactly the caller-supplied sp. It then push the 16 bytes of
the return address and the saved stack pointer, so the base
pointer will be 16-byte aligned if and only if the caller
supplied sp is 16-byte aligned. Therefore, the caller must supply
a 16-byte aligned pointer, which `stack + UM_KERN_PAGE_SIZE`
already satisfies.

On a side note, musl is unaffected by this issue because it forces
16 byte alignment via `and $-16,%rsi` in its clone wrapper.
Similarly, glibc i386 is also uneffected because it has
`andl $0xfffffff0, %ecx`.

To reproduce this bug, enable CONFIG_UML_RTC. uml_rtc will call
add_sigio_fd which will then cause write_sigio_thread to go
into segfault loop.

Similarly, signal stacks will be aligned by the host kernel upon
signal delivery. `- sizeof(void *)` to sigaltstack is
unconventional and extraneous.

On a related note, initialization of longjmp buffers do require
`- sizeof(void *)`. This is to account for the return address
that would have been pushed to the stack at the call site.

The reason for uml to respect 16-byte alignment, rather than
telling GCC to assume 8-byte alignment like the host kernel since
commit d9b0cde91c60 ("x86-64, gcc: Use
-mpreferred-stack-boundary=3 if supported"), is because uml links
against libc. There is no reason to assume libc is also compiled
with that flag and assumes 8-byte alignment rather than 16-byte.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838
[2] https://c9x.me/x86/html/file_module_x86_id_180.html

Signed-off-by: YiFei Zhu <zhuyifei1999 at gmail.com>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
---
 arch/um/drivers/ubd_kern.c      | 3 +--
 arch/um/kernel/skas/clone.c     | 2 +-
 arch/um/os-Linux/helper.c       | 4 ++--
 arch/um/os-Linux/signal.c       | 2 +-
 arch/um/os-Linux/skas/process.c | 2 +-
 5 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index 8e0b43cf089f..cbd4f00fe77e 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -1242,8 +1242,7 @@ static int __init ubd_driver_init(void){
 		 * enough. So use anyway the io thread. */
 	}
 	stack = alloc_stack(0, 0);
-	io_pid = start_io_thread(stack + PAGE_SIZE - sizeof(void *),
-				 &thread_fd);
+	io_pid = start_io_thread(stack + PAGE_SIZE, &thread_fd);
 	if(io_pid < 0){
 		printk(KERN_ERR
 		       "ubd : Failed to start I/O thread (errno = %d) - "
diff --git a/arch/um/kernel/skas/clone.c b/arch/um/kernel/skas/clone.c
index 592cdb138441..5afac0fef24e 100644
--- a/arch/um/kernel/skas/clone.c
+++ b/arch/um/kernel/skas/clone.c
@@ -29,7 +29,7 @@ stub_clone_handler(void)
 	long err;
 
 	err = stub_syscall2(__NR_clone, CLONE_PARENT | CLONE_FILES | SIGCHLD,
-			    (unsigned long)data + UM_KERN_PAGE_SIZE / 2 - sizeof(void *));
+			    (unsigned long)data + UM_KERN_PAGE_SIZE / 2);
 	if (err) {
 		data->parent_err = err;
 		goto done;
diff --git a/arch/um/os-Linux/helper.c b/arch/um/os-Linux/helper.c
index 9fa6e4187d4f..32e88baf18dd 100644
--- a/arch/um/os-Linux/helper.c
+++ b/arch/um/os-Linux/helper.c
@@ -64,7 +64,7 @@ int run_helper(void (*pre_exec)(void *), void *pre_data, char **argv)
 		goto out_close;
 	}
 
-	sp = stack + UM_KERN_PAGE_SIZE - sizeof(void *);
+	sp = stack + UM_KERN_PAGE_SIZE;
 	data.pre_exec = pre_exec;
 	data.pre_data = pre_data;
 	data.argv = argv;
@@ -120,7 +120,7 @@ int run_helper_thread(int (*proc)(void *), void *arg, unsigned int flags,
 	if (stack == 0)
 		return -ENOMEM;
 
-	sp = stack + UM_KERN_PAGE_SIZE - sizeof(void *);
+	sp = stack + UM_KERN_PAGE_SIZE;
 	pid = clone(proc, (void *) sp, flags, arg);
 	if (pid < 0) {
 		err = -errno;
diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c
index 96f511d1aabe..e283f130aadc 100644
--- a/arch/um/os-Linux/signal.c
+++ b/arch/um/os-Linux/signal.c
@@ -129,7 +129,7 @@ void set_sigstack(void *sig_stack, int size)
 	stack_t stack = {
 		.ss_flags = 0,
 		.ss_sp = sig_stack,
-		.ss_size = size - sizeof(void *)
+		.ss_size = size
 	};
 
 	if (sigaltstack(&stack, NULL) != 0)
diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index fba674fac8b7..43d6ec2a4bea 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -327,7 +327,7 @@ int start_userspace(unsigned long stub_stack)
 	}
 
 	/* set stack pointer to the end of the stack page, so it can grow downwards */
-	sp = (unsigned long) stack + UM_KERN_PAGE_SIZE - sizeof(void *);
+	sp = (unsigned long)stack + UM_KERN_PAGE_SIZE;
 
 	flags = CLONE_FILES | SIGCHLD;
 
-- 
2.31.1