linux-hardening - How large is the xtensa pt_regs::areg array supposed to be?

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Date:   Wed, 2 Feb 2022 15:03:51 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Chris Zankel <chris@...kel.net>, Max Filippov <jcmvbkbc@...il.com>
Cc:     linux-xtensa@...ux-xtensa.org, linux-hardening@...r.kernel.org
Subject: How large is the xtensa pt_regs::areg array supposed to be?

Hi,

When building with -Warray-bounds, I see this:

In file included from ./include/linux/uaccess.h:11,
                 from ./include/linux/sched/task.h:11,
                 from arch/xtensa/kernel/process.c:21:
arch/xtensa/kernel/process.c: In function 'copy_thread':
arch/xtensa/kernel/process.c:262:52: warning: array subscript 53 is above array bounds of 'long unsigned int[16]' [-Warray-bounds]
  262 |                                 put_user(regs->areg[caller_ars+1],
./arch/xtensa/include/asm/uaccess.h:171:18: note: in definition of macro '__put_user_asm'
  171 |         :[x] "r"(x_), [efault] "i"(-EFAULT))
      |                  ^~
./arch/xtensa/include/asm/uaccess.h:89:17: note: in expansion of macro '__put_user_size'
   89 |                 __put_user_size((x), __pu_addr, (size), __pu_err);      \
      |                 ^~~~~~~~~~~~~~~
./arch/xtensa/include/asm/uaccess.h:62:33: note: in expansion of macro '__put_user_check'
   62 | #define put_user(x, ptr)        __put_user_check((x), (ptr), sizeof(*(ptr)))
      |                                 ^~~~~~~~~~~~~~~~
arch/xtensa/kernel/process.c:262:33: note: in expansion of macro 'put_user'
  262 |                                 put_user(regs->areg[caller_ars+1],
      |                                 ^~~~~~~~
In file included from ./arch/xtensa/include/asm/processor.h:17,
                 from ./arch/xtensa/include/asm/thread_info.h:20,
                 from ./arch/xtensa/include/asm/current.h:14,
                 from ./include/linux/sched.h:12,
                 from arch/xtensa/kernel/process.c:19:
./arch/xtensa/include/asm/ptrace.h:80:23: note: while referencing 'areg'
   80 |         unsigned long areg[16];
      |                       ^~~~


The code is:
                                int callinc = (regs->areg[0] >> 30) & 3;
                                int caller_ars = XCHAL_NUM_AREGS - callinc * 4;
                                put_user(regs->areg[caller_ars+1],
                                         (unsigned __user*)(usp - 12));

It looks like XCHAL_NUM_AREGS is larger than "16", though?

struct pt_regs {
...
        unsigned long areg[16];

What should be happening here?

Thanks!

-- 
Kees Cook