linux-kernel - Re: [linus:master] [x86/entry] be5341eb0d: WARNING:CPU:#PID:#at_int80

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Message-ID: <87013f74-5828-4bef-848b-ffedde8e86f5@citrix.com>
Date: Tue, 19 Dec 2023 19:15:31 +0000
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Linus Torvalds <torvalds@...uxfoundation.org>,
 Borislav Petkov <bp@...en8.de>
Cc: kernel test robot <oliver.sang@...el.com>,
 Thomas Gleixner <tglx@...utronix.de>, oe-lkp@...ts.linux.dev, lkp@...el.com,
 linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 xen-devel@...ts.xenproject.org
Subject: Re: [linus:master] [x86/entry] be5341eb0d:
 WARNING:CPU:#PID:#at_int80_emulation

On 19/12/2023 6:20 pm, Linus Torvalds wrote:
> On Tue, 19 Dec 2023 at 01:58, Borislav Petkov <bp@...en8.de> 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.

My reading of the original report is that there used to be:

    dmesg.WARNING:CPU:#PID:#at_do_int80_syscall_32

and now there's:

    dmesg.WARNING:CPU:#PID:#at_int80_emulation

i.e. kCFI was broken before, and all we did with be5341eb0d43 was change
the inlining, and therefore how the error was rendered.

> See:
>
>   #define __SYS_NI(abi, name)                                             \
>         SYSCALL_ALIAS(__##abi##_##name, sys_ni_posix_timers);

AFAICT, this is the problem, but it was preexiting too.

This is stuffing a function of type void into an array of function
pointers wanting pt_regs * which is indeed a kCFI violation.

Isn't the fix simply this?

~Andrew

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index fd9d12de7e92..12195164d5a4 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1171,7 +1171,7 @@ asmlinkage long sys_ni_syscall(void);

 #endif /* CONFIG_ARCH_HAS_SYSCALL_WRAPPER */

-asmlinkage long sys_ni_posix_timers(void);
+asmlinkage long sys_ni_posix_timers(const struct pt_regs *regs);

 /*
  * Kernel code should not call syscalls (i.e., sys_xyzyyz()) directly.
diff --git a/kernel/time/posix-stubs.c b/kernel/time/posix-stubs.c
index 828aeecbd1e8..29692354a908 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)
+asmlinkage long sys_ni_posix_timers(const struct pt_regs *regs)
 {
        pr_err_once("process %d (%s) attempted a POSIX timer syscall "
                    "while CONFIG_POSIX_TIMERS is not set\n",