Re: [PATCH -next] hugetlbfs: fix null-ptr-deref in hugetlbfs_parse_param()

[Hawkins Jiawei <yin31149@gmail.com>]

Hi Mike,

On Fri, 21 Oct 2022 at 07:38, Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 10/21/22 07:16, Hawkins Jiawei wrote:
> > Syzkaller reports a null-ptr-deref bug as follows:
> > ======================================================
> > KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> > RIP: 0010:hugetlbfs_parse_param+0x1dd/0x8e0 fs/hugetlbfs/inode.c:1380
> > [...]
> > Call Trace:
> >  <TASK>
> >  vfs_parse_fs_param fs/fs_context.c:148 [inline]
> >  vfs_parse_fs_param+0x1f9/0x3c0 fs/fs_context.c:129
> >  vfs_parse_fs_string+0xdb/0x170 fs/fs_context.c:191
> >  generic_parse_monolithic+0x16f/0x1f0 fs/fs_context.c:231
> >  do_new_mount fs/namespace.c:3036 [inline]
> >  path_mount+0x12de/0x1e20 fs/namespace.c:3370
> >  do_mount fs/namespace.c:3383 [inline]
> >  __do_sys_mount fs/namespace.c:3591 [inline]
> >  __se_sys_mount fs/namespace.c:3568 [inline]
> >  __x64_sys_mount+0x27f/0x300 fs/namespace.c:3568
> >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> >  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> >  [...]
> >  </TASK>
> > ======================================================
> >
> > According to commit
> > ac369cdd9448("vfs: parse: deal with zero length string value"),
> > kernel will sets the param->string to null pointer in
> > vfs_parse_fs_string() if fs string has zero length.
> >
> > Yet the problem is that, hugetlbfs_parse_param() will
> > dereference the param->string, without checking whether it
> > is a null pointer. To be more specific, if hugetlbfs_parse_param()
> > parses an illegal mount parameter, such as "size=,", kernel will
> > constructs struct fs_parameter with null pointer in
> > vfs_parse_fs_string(), then passes this struct fs_parameter to
> > hugetlbfs_parse_param(), which triggers the above
> > null-ptr-deref bug.
> >
> > This patch solves it by adding sanity check on param->string
> > in hugetlbfs_parse_param().
> >
> > Reported-by: syzbot+a3e6acd85ded5c16a709@syzkaller.appspotmail.com
> > Tested-by: syzbot+a3e6acd85ded5c16a709@syzkaller.appspotmail.com
> > Link: https://lore.kernel.org/all/0000000000005ad00405eb7148c6@google.com/
> > Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
>
> Thank you!
>
> This was on my list of things to look at.
>
> Do you know if the issue existed before commit ac369cdd9448?  Just
> wondering if we need a Fixes tag and stable backports.
This issue did not exist before commit ac369cdd9448, because kernel will
sets param->string a string(although a zero length string) by kmemdup_nul()
when fs string has zero length in vfs_parse_fs_string(). So it will ensures
that param->string is not a null pointer in hugetlbfs_parse_param().

I also revert commit ac369cdd9448 or reset to ac369cdd9448~, and test
the syzkaller reproducer. The reproducer does not trigger any issue
in both situations.

>
> I'll take a closer look at the patch a bit later, unless someone beats
> me to it.
> --
> Mike Kravetz
>
> > ---
> >  fs/hugetlbfs/inode.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> > index 7f836f8f9db1..3ee84604e36d 100644
> > --- a/fs/hugetlbfs/inode.c
> > +++ b/fs/hugetlbfs/inode.c
> > @@ -1377,7 +1377,7 @@ static int hugetlbfs_parse_param(struct fs_context *fc, struct fs_parameter *par
> >
> >       case Opt_size:
> >               /* memparse() will accept a K/M/G without a digit */
> > -             if (!isdigit(param->string[0]))
> > +             if (!param->string || !isdigit(param->string[0]))
> >                       goto bad_val;
> >               ctx->max_size_opt = memparse(param->string, &rest);
> >               ctx->max_val_type = SIZE_STD;
> > @@ -1387,7 +1387,7 @@ static int hugetlbfs_parse_param(struct fs_context *fc, struct fs_parameter *par
> >
> >       case Opt_nr_inodes:
> >               /* memparse() will accept a K/M/G without a digit */
> > -             if (!isdigit(param->string[0]))
> > +             if (!param->string || !isdigit(param->string[0]))
> >                       goto bad_val;
> >               ctx->nr_inodes = memparse(param->string, &rest);
> >               return 0;
> > @@ -1403,7 +1403,7 @@ static int hugetlbfs_parse_param(struct fs_context *fc, struct fs_parameter *par
> >
> >       case Opt_min_size:
> >               /* memparse() will accept a K/M/G without a digit */
> > -             if (!isdigit(param->string[0]))
> > +             if (!param->string || !isdigit(param->string[0]))
> >                       goto bad_val;
> >               ctx->min_size_opt = memparse(param->string, &rest);
> >               ctx->min_val_type = SIZE_STD;
> > --
> > 2.25.1
> >