linux-kernel - arch/arm64/kernel/compat_alignment.c:333 do_compat_alignment_fixup() error: uninitialized symbol 'tinst2'.

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Date:   Mon, 3 Apr 2023 09:28:55 +0300
From:   Dan Carpenter <error27@...il.com>
To:     oe-kbuild@...ts.linux.dev, Ard Biesheuvel <ardb@...nel.org>
Cc:     lkp@...el.com, oe-kbuild-all@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Arnd Bergmann <arnd@...db.de>
Subject: arch/arm64/kernel/compat_alignment.c:333 do_compat_alignment_fixup()
 error: uninitialized symbol 'tinst2'.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   00c7b5f4ddc5b346df62b757ec73f9357bb452af
commit: 3fc24ef32d3b9368f4c103dcd21d6a3f959b4870 arm64: compat: Implement misalignment fixups for multiword loads
config: arm64-randconfig-m041-20230329 (https://download.01.org/0day-ci/archive/20230402/202304021214.gekJ8yRc-lkp@intel.com/config)
compiler: aarch64-linux-gcc (GCC) 12.1.0

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@...el.com>
| Reported-by: Dan Carpenter <error27@...il.com>
| Link: https://lore.kernel.org/r/202304021214.gekJ8yRc-lkp@intel.com/

smatch warnings:
arch/arm64/kernel/compat_alignment.c:333 do_compat_alignment_fixup() error: uninitialized symbol 'tinst2'.

vim +/tinst2 +333 arch/arm64/kernel/compat_alignment.c

3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  310  int do_compat_alignment_fixup(unsigned long addr, struct pt_regs *regs)
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  311  {
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  312  	union offset_union offset;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  313  	unsigned long instrptr;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  314  	int (*handler)(unsigned long addr, u32 instr, struct pt_regs *regs);
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  315  	unsigned int type;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  316  	u32 instr = 0;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  317  	u16 tinstr = 0;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  318  	int isize = 4;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  319  	int thumb2_32b = 0;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  320  	int fault;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  321  
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  322  	instrptr = instruction_pointer(regs);
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  323  
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  324  	if (compat_thumb_mode(regs)) {
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  325  		__le16 __user *ptr = (__le16 __user *)(instrptr & ~1);
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  326  
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  327  		fault = alignment_get_thumb(regs, ptr, &tinstr);
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  328  		if (!fault) {
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  329  			if (IS_T32(tinstr)) {
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  330  				/* Thumb-2 32-bit */
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  331  				u16 tinst2;
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01  332  				fault = alignment_get_thumb(regs, ptr + 1, &tinst2);
3fc24ef32d3b93 Ard Biesheuvel 2022-07-01 @333  				instr = ((u32)tinstr << 16) | tinst2;

Smatch is complaining that there is no error checking to see if the
copy_from_user() fails in alignment_get_thumb.  Eventually the syzbot
will learn to detect this as well.

Most distro kernels are going to automatically zero out stack variables
like tinst2 to prevent undefined behavior.

Presumably this is a fast path.  So setting "u16 tinst2 = 0;" does not
affect runtime speed for distro kernels and it might be the best
solution.

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests