[RFC PATCH 2/2] x86/cmdline: Use strscpy to initialize boot_command_line

[Arvind Sankar]

The x86 boot protocol requires the kernel command line to be a
NUL-terminated string of length at most COMMAND_LINE_SIZE (including the
terminating NUL). In case the bootloader messed up and the command line
is too long (hence not NUL-terminated), use strscpy to copy the command
line into boot_command_line. This ensures that boot_command_line is
NUL-terminated, and it also avoids accessing beyond the actual end of
the command line if it was properly NUL-terminated.

Note that setup_arch() will already force command_line to be
NUL-terminated by using strlcpy(), as well as boot_command_line if a
builtin command line is configured. If boot_command_line was not
initially NUL-terminated, the strlen() inside of strlcpy()/strlcat()
will run beyond boot_command_line, but this is almost certainly
harmless in practice.

Signed-off-by: Arvind Sankar <nived...@alum.mit.edu>
---
 arch/x86/kernel/head64.c  |  2 +-
 arch/x86/kernel/head_32.S | 11 +++++------
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index cbb71c1b574f..740dd05b9462 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -410,7 +410,7 @@ static void __init copy_bootdata(char *real_mode_data)
        cmd_line_ptr = get_cmd_line_ptr();
        if (cmd_line_ptr) {
                command_line = __va(cmd_line_ptr);
-               memcpy(boot_command_line, command_line, COMMAND_LINE_SIZE);
+               strscpy(boot_command_line, command_line, COMMAND_LINE_SIZE);
        }
 
        /*
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 7ed84c282233..2a7ced159d6b 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -102,13 +102,12 @@ SYM_CODE_START(startup_32)
        cld
        rep
        movsl
-       movl pa(boot_params) + NEW_CL_POINTER,%esi
-       andl %esi,%esi
+       movl pa(boot_params) + NEW_CL_POINTER,%edx
+       testl %edx,%edx
        jz 1f                   # No command line
-       movl $pa(boot_command_line),%edi
-       movl $(COMMAND_LINE_SIZE/4),%ecx
-       rep
-       movsl
+       movl $pa(boot_command_line),%eax
+       movl $COMMAND_LINE_SIZE,%ecx
+       call strscpy
 1:
 
 #ifdef CONFIG_OLPC
-- 
2.26.2