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

[Hugh Dickins]

On Thu, 25 May 2023, 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>
> 
> Allocated by task 2387:
>  kasan_save_stack+0x22/0x50
>  kasan_set_track+0x25/0x30
>  __kasan_slab_alloc+0x59/0x70
>  kmem_cache_alloc+0xdd/0x220
>  mpol_new+0x83/0x150
>  mpol_parse_str+0x280/0x4a0
>  shmem_parse_one+0x364/0x520
>  vfs_parse_fs_param+0xf8/0x1a0
>  vfs_parse_fs_string+0xc9/0x130
>  shmem_parse_options+0xb2/0x110
>  path_mount+0x597/0xdf0
>  do_mount+0xcd/0xf0
>  __x64_sys_mount+0xbd/0x100
>  do_syscall_64+0x3c/0x90
>  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> 
> Freed by task 2389:
>  kasan_save_stack+0x22/0x50
>  kasan_set_track+0x25/0x30
>  kasan_save_free_info+0x2e/0x50
>  __kasan_slab_free+0x10e/0x1a0
>  kmem_cache_free+0x9c/0x350
>  shmem_reconfigure+0x278/0x370
>  reconfigure_super+0x383/0x450
>  path_mount+0xcc5/0xdf0
>  do_mount+0xcd/0xf0
>  __x64_sys_mount+0xbd/0x100
>  do_syscall_64+0x3c/0x90
>  entry_SYSCALL_64_after_hwframe+0x72/0xdc
> 
> The buggy address belongs to the object at ffff888124324000
>  which belongs to the cache numa_policy of size 32
> The buggy address is located 4 bytes inside of
>  freed 32-byte region [ffff888124324000, ffff888124324020)
> ==================================================================
> 
> 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@xxxxxxxxxx>

Thank you -
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

> ---
>  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;
> -- 
> 2.25.1