Re: [linus:master] [x86/entry] be5341eb0d: WARNING:CPU:#PID:#at_int80_emulation

[Sami Tolvanen <samitolvanen@xxxxxxxxxx>]

On Tue, Dec 19, 2023 at 10:21 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 19 Dec 2023 at 01:58, Borislav Petkov <bp@xxxxxxxxx> wrote:
> >
> > Looking at the dmesg, I think you missed the most important part - the
> > preceding line:
> >
> > [   13.480504][   T48] CFI failure at int80_emulation+0x67/0xb0 (target: 
> > sys_ni_posix_timers+0x0/0x70; expected type: 0xb02b34d9)
> >                         ^^^^^^^^^^^
>
> So I think the issue here is that sys_ni_posix_timers is just linker
> alias that is used for any non-implemented posix timer system call.
>
> See:
>
>   #define __SYS_NI(abi, name)                                             \
>         SYSCALL_ALIAS(__##abi##_##name, sys_ni_posix_timers);
>
> and this all worked fine when the actual call to this was done in
> assembly code that happily just called that function directly and
> didn't care about any argument types.

Yes, that's exactly the issue.

> But commit be5341eb0d43 ("x86/entry: Convert INT 0x80 emulation to
> IDTENTRY") moved that call from assembly into C, and in the process
> ended up enabling CFI for it all, and now the compiler will check that
> the function types match. Which they don't, because we use that dummy
> function (I don't think they do in general).
>
> I don't know what the best fix is. Either CFI should be turned off for
> that call, or we should make sure to generate those NI system calls
> with the proper types.

Probably the easiest fix would be to use SYSCALL_DEFINE0 for
sys_ni_posix_timers, and for architectures that implement syscall
wrappers, change sys_ni_posix_timers references to
__<abi>_sys_ni_posix_timers.

Something like this should fix the issue for x86, but it looks like
arm64, riscv, and s390 would need similar syscall wrapper changes:

diff --git a/arch/x86/include/asm/syscall_wrapper.h
b/arch/x86/include/asm/syscall_wrapper.h
index fd2669b1cb2d..ed38265cad27 100644
--- a/arch/x86/include/asm/syscall_wrapper.h
+++ b/arch/x86/include/asm/syscall_wrapper.h
@@ -87,7 +87,7 @@ extern long __ia32_sys_ni_syscall(const struct pt_regs *regs);
        }

 #define __SYS_NI(abi, name)                                            \
-       SYSCALL_ALIAS(__##abi##_##name, sys_ni_posix_timers);
+       SYSCALL_ALIAS(__##abi##_##name, __##abi##_sys_ni_posix_timers);

 #ifdef CONFIG_X86_64
 #define __X64_SYS_STUB0(name)                                          \
diff --git a/kernel/time/posix-stubs.c b/kernel/time/posix-stubs.c
index 828aeecbd1e8..d58f976ec926 100644
--- a/kernel/time/posix-stubs.c
+++ b/kernel/time/posix-stubs.c
@@ -22,7 +22,7 @@
 #include <asm/syscall_wrapper.h>
 #endif

-asmlinkage long sys_ni_posix_timers(void)
+SYSCALL_DEFINE0(ni_posix_timers)
 {
        pr_err_once("process %d (%s) attempted a POSIX timer syscall "
                    "while CONFIG_POSIX_TIMERS is not set\n",

Sami