[PATCH] x86/syscall: Avoid memcpy() for ia32 syscall_get_arguments()

[Kees Cook]

Modern (fortified) memcpy() prefers to avoid writing (or reading) beyond
the end of the addressed destination (or source) struct member:

In function ‘fortify_memcpy_chk’,
    inlined from ‘syscall_get_arguments’ at ./arch/x86/include/asm/syscall.h:85:2,
    inlined from ‘populate_seccomp_data’ at kernel/seccomp.c:258:2,
    inlined from ‘__seccomp_filter’ at kernel/seccomp.c:1231:3:
./include/linux/fortify-string.h:580:25: error: call to ‘__read_overflow2_field’ declared with attribute warning: detected read beyond size of field (2nd parameter); maybe use struct_group()? [-Werror=attribute-warning]
  580 |                         __read_overflow2_field(q_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As already done for x86_64 and compat mode, do not use memcpy() to
extract syscall arguments from struct pt_regs but rather just perform
direct assignments. Binary output differences are negligible, and actually
ends up using less stack space:

-       sub    $0x84,%esp
+       sub    $0x6c,%esp

and less text size:

   text    data     bss     dec     hex filename
  10794     252       0   11046    2b26 gcc-32b/kernel/seccomp.o.stock
  10714     252       0   10966    2ad6 gcc-32b/kernel/seccomp.o.after

Reported-by: Mirsad Todorovac <mtodorovac69@xxxxxxxxx>
Closes: https://lore.kernel.org/lkml/9b69fb14-df89-4677-9c82-056ea9e706f5@xxxxxxxxx/
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Cc: Peter Collingbourne <pcc@xxxxxxxxxx>
---
 arch/x86/include/asm/syscall.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/syscall.h b/arch/x86/include/asm/syscall.h
index 2fc7bc3863ff..7c488ff0c764 100644
--- a/arch/x86/include/asm/syscall.h
+++ b/arch/x86/include/asm/syscall.h
@@ -82,7 +82,12 @@ static inline void syscall_get_arguments(struct task_struct *task,
 					 struct pt_regs *regs,
 					 unsigned long *args)
 {
-	memcpy(args, &regs->bx, 6 * sizeof(args[0]));
+	args[0] = regs->bx;
+	args[1] = regs->cx;
+	args[2] = regs->dx;
+	args[3] = regs->si;
+	args[4] = regs->di;
+	args[5] = regs->bp;
 }
 
 static inline int syscall_get_arch(struct task_struct *task)
-- 
2.34.1