Re: [f2fs-dev] [PATCH] f2fs: fix uninit-value in f2fs_lookup

[Chao Yu]

On 2020-9-26 1:14, Eric Biggers wrote:
> On Fri, Sep 25, 2020 at 11:19:26PM +0800, Chao Yu wrote:
> > From: Chao Yu <yuchao0@xxxxxxxxxx>
> >
> > As syzbot reported:
> >
> > Call Trace:
> >  __dump_stack lib/dump_stack.c:77 [inline]
> >  dump_stack+0x21c/0x280 lib/dump_stack.c:118
> >  kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:122
> >  __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:219
> >  f2fs_lookup+0xe05/0x1a80 fs/f2fs/namei.c:503
> >  lookup_open fs/namei.c:3082 [inline]
> >  open_last_lookups fs/namei.c:3177 [inline]
> >  path_openat+0x2729/0x6a90 fs/namei.c:3365
> >  do_filp_open+0x2b8/0x710 fs/namei.c:3395
> >  do_sys_openat2+0xa88/0x1140 fs/open.c:1168
> >  do_sys_open fs/open.c:1184 [inline]
> >  __do_compat_sys_openat fs/open.c:1242 [inline]
> >  __se_compat_sys_openat+0x2a4/0x310 fs/open.c:1240
> >  __ia32_compat_sys_openat+0x56/0x70 fs/open.c:1240
> >  do_syscall_32_irqs_on arch/x86/entry/common.c:80 [inline]
> >  __do_fast_syscall_32+0x129/0x180 arch/x86/entry/common.c:139
> >  do_fast_syscall_32+0x6a/0xc0 arch/x86/entry/common.c:162
> >  do_SYSENTER_32+0x73/0x90 arch/x86/entry/common.c:205
> >  entry_SYSENTER_compat_after_hwframe+0x4d/0x5c
> >
> > In f2fs_lookup(), @res_page could be used before being initialized,
> > because in __f2fs_find_entry(), once F2FS_I(dir)->i_current_depth was
> > been fuzzed to zero, then @res_page will never be initialized, causing
> > this kmsan warning, relocating @res_page initialization place to fix
> > this bug.
> >
> > Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
> > ---
> >  fs/f2fs/dir.c | 7 +++----
> >  1 file changed, 3 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
> > index 703cf8e21fc0..83630341ffa3 100644
> > --- a/fs/f2fs/dir.c
> > +++ b/fs/f2fs/dir.c
> > @@ -357,16 +357,15 @@ struct f2fs_dir_entry *__f2fs_find_entry(struct inode *dir,
> >  	unsigned int max_depth;
> >  	unsigned int level;
> >
> > +	*res_page = NULL;
> > +
> >  	if (f2fs_has_inline_dentry(dir)) {
> > -		*res_page = NULL;
> >  		de = f2fs_find_in_inline_dir(dir, fname, res_page);
> >  		goto out;
> >  	}
> >
> > -	if (npages == 0) {
> > -		*res_page = NULL;
> > +	if (npages == 0)
> >  		goto out;
> > -	}
> >
> >  	max_depth = F2FS_I(dir)->i_current_depth;
> >  	if (unlikely(max_depth > MAX_DIR_HASH_DEPTH)) {
>
> Can't the assignment to *res_page below be removed too?

Now I checked all branches in find_in_level(), it looks safe to remove *res_page
assignment in the loop.

Thanks,

>         for (level = 0; level < max_depth; level++) {
>                 *res_page = NULL;
>                 de = find_in_level(dir, level, fname, res_page);
>                 if (de || IS_ERR(*res_page))
>                         break;
>         }