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

From: Gustavo A. R. Silva
Date: Mon Jul 08 2024 - 19:45:07 EST



On 7/8/24 14:22, Kees Cook wrote:
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>

Reviewed-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>

Thanks
--
Gustavo

---
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)