Re: [PATCH] mm: shmem: Fix UAF bug in shmem_show_options()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2023/5/25 11:16, Tu Jinjiang wrote:
> shmem_show_options() uses sbinfo->mpol without adding it's refcnt. This
> may lead to race with replacement of the mpol by remount. The execution
> sequence is as follows.
> 
>         CPU0                                   CPU1
> shmem_show_options()                        shmem_reconfigure()
>      shmem_show_mpol(seq, sbinfo->mpol)          mpol = sbinfo->mpol
>                                                  mpol_put(mpol)
>          mpol->mode
> 
> The KASAN report is as follows.
> 
> BUG: KASAN: slab-use-after-free in shmem_show_options+0x21b/0x340
> Read of size 2 at addr ffff888124324004 by task mount/2388
> 
> CPU: 2 PID: 2388 Comm: mount Not tainted 6.4.0-rc3-00017-g9d646009f65d-dirty #8
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
> Call Trace:
>   <TASK>
>   dump_stack_lvl+0x37/0x50
>   print_report+0xd0/0x620
>   ? shmem_show_options+0x21b/0x340
>   ? __virt_addr_valid+0xf4/0x180
>   ? shmem_show_options+0x21b/0x340
>   kasan_report+0xb8/0xe0
>   ? shmem_show_options+0x21b/0x340
>   shmem_show_options+0x21b/0x340
>   ? __pfx_shmem_show_options+0x10/0x10
>   ? strchr+0x2c/0x50
>   ? strlen+0x23/0x40
>   ? seq_puts+0x7d/0x90
>   show_vfsmnt+0x1e6/0x260
>   ? __pfx_show_vfsmnt+0x10/0x10
>   ? __kasan_kmalloc+0x7f/0x90
>   seq_read_iter+0x57a/0x740
>   vfs_read+0x2e2/0x4a0
>   ? __pfx_vfs_read+0x10/0x10
>   ? down_write_killable+0xb8/0x140
>   ? __pfx_down_write_killable+0x10/0x10
>   ? __fget_light+0xa9/0x1e0
>   ? up_write+0x3f/0x80
>   ksys_read+0xb8/0x150
>   ? __pfx_ksys_read+0x10/0x10
>   ? fpregs_assert_state_consistent+0x55/0x60
>   ? exit_to_user_mode_prepare+0x2d/0x120
>   do_syscall_64+0x3c/0x90
>   entry_SYSCALL_64_after_hwframe+0x72/0xdc
> 
>   </TASK>
> 

Maybe drop the unreliable stack, not mandatory, the patch is look good 
to me,

[snip]

> 
> To fix the bug, shmem_get_sbmpol() / mpol_put() needs to be called
> before / after shmem_show_mpol() call.
> 
> Signed-off-by: Tu Jinjiang <tujinjiang@huawei.com>

Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>

> ---
>   mm/shmem.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index e40a08c5c6d7..5e54ab5f61f2 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3726,6 +3726,7 @@ static int shmem_reconfigure(struct fs_context *fc)
>   static int shmem_show_options(struct seq_file *seq, struct dentry *root)
>   {
>   	struct shmem_sb_info *sbinfo = SHMEM_SB(root->d_sb);
> +	struct mempolicy *mpol;
>   
>   	if (sbinfo->max_blocks != shmem_default_max_blocks())
>   		seq_printf(seq, ",size=%luk",
> @@ -3768,7 +3769,9 @@ static int shmem_show_options(struct seq_file *seq, struct dentry *root)
>   	if (sbinfo->huge)
>   		seq_printf(seq, ",huge=%s", shmem_format_huge(sbinfo->huge));
>   #endif
> -	shmem_show_mpol(seq, sbinfo->mpol);
> +	mpol = shmem_get_sbmpol(sbinfo);
> +	shmem_show_mpol(seq, mpol);
> +	mpol_put(mpol);
>   	if (sbinfo->noswap)
>   		seq_printf(seq, ",noswap");
>   	return 0;